[DebugInfo][InstrRef] Use correct (and existing) PHI placement utilities for machine locations

[jmorse]

Summary

Over in D102158 I outlined some flaws in my LiveDebugValues-rewrite, and pointed out that in reality it was SSA-construction with value propagation. I've been re-formulating the implementation to be defined in terms of SSA construction, which is what this patch does. This patch addresses the first "problem" that's solved, finding which value numbers are in which locations in the MachineFunction. The existing LLVM iterated-dominance-frontier tooling is used for PHI placement. This eliminates the lattice arrangement I was trying to work with before.

This patch also adds a dependency on MachineDominatorTree for LiveDebugValues -- it's needed for the IDF calculator.

I find worked examples to be the best way of demonstrating how this works, thus this patch also adds a bunch of unit tests: different patterns of register definitions are tested over five or six weird control flow forms. They're all of the form:

• Set-up where the definitions (or copies) of registers are in the function,
• Run the PHI-placement / value-propagation function,
• Test that the correct values are found in the correct locations.

I can't promise that they exhaustively covers all input patterns, but they illustrate what this code is _supposed_ to do, and it'll help check future changes.

Over on the excellent compile-time-tracker pages, this patch [0] causes a roughly 1% slowdown across CTMark, possibly because my previous implementation wasn't doing PHI placement correctly, but it's quite likely that the IDF calculator is slower than what I was doing. I reckon that this performance can be recovered by only running the PHI placement algorithm for register units, then placing PHIs for all aliasing registers too. I haven't tried this yet though.

I have a similar patch coming that does this again, but for the other "problem", what values should variables have in each block. (Still writing unit tests for that).

[0] http://llvm-compile-time-tracker.com/compare.php?from=be06359638e470418d57c1c2592d15c4e468daf1&to=42e0868898a534d4772ee5e369ef29dfb89c95bc&stat=instructions

[StephenTozer]

First pass looking through this, haven't gone through the unit test yet - I think I understand this roughly, and don't have any complaints, but will continue to review - 1 nit and 1 question enclosed.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

1757–1759

My understanding of this is incomplete, under what circumstances would we have eliminated the PHI without having already assigned InLocs[Idx.asU64()] = FirstVal?

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.h

750

Return value documentation needs updating

[jmorse]

In D110173#3017893, @StephenTozer wrote:

First pass looking through this, haven't gone through the unit test yet - I think I understand this roughly, and don't have any complaints, but will continue to review - 1 nit and 1 question enclosed.

Huzzah!,

Reviewing the whole set of unit tests might be beyond the call of duty, but would be appreciated,

I'm trying to piece back together what I'd intended in the unit tests, and it looks like it'd be clearer if the transfer function was completely re-constructed before each call to buildMLocValueMap. Right now, I'm relying on earlier state, which makes it harder to localise and understand a single block of test.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

1757–1759

Great question; the elimination of one PHI might enable the elimination of additional PHIs, possibly higher up the CFG. That could cause the live-in value on entry to a block to change more than once. If we had for example:

     |
  loop head-------\
     |            ^
     A            ^
   /   \          ^
  /     \         ^  
assign   B        ^ 
  \     /         ^    
   \   /          ^     
     C------------/
     |
   exit

Assume that we're dealing with $rax, which has some value prior to the loop head that I'll call "foo", and $rax is modified in the block labelled "assign". The PHI placement code isn't aware of what concrete values the register has, only that it's modified in the "assign" block. Therefore it installs two PHIs: at the 'C' block, and the loop head.

During value propagation, lets say that the "assign" block is found to not actually change the value of $rax: perhaps it modifies it, but then restores it from a stack spill slot. This means the PHI in block C can be eliminated: the live-in value is determined to be the PHI from "loop head"

Subsequently, we can identify that the PHI at "loop head" just feeds the value it generates in $rax back into itself on the backedge: that PHI can be eliminated too. We can then propagate the "foo" value into every block in the function. When we come to "C", there's no longer a PHI on entry to that block, but we still need to set "foo" as its live-in value for $rax. That's what this portion of code seeks to do.

[Orlando]

haven't gone through the unit test yet

I focused my review on the unittest parts. I reviewed the tests closely up to and including MLocSimpleLoop, after which I started skimming a little (see the inline nit/comment about improving test readability).

Once both of our nits have been addressed, assuming @StephenTozer is happy with the code changes he reviewed, I think we can consider both halves of this patch reviewed. So, LGTM.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

55

👍

llvm/unittests/CodeGen/InstrRefLDVTest.cpp

110

nit: missing a full stop.

117

Can you add an assert / llvm_unreachable?

230

nit: full stop.

286

LiveInRax is unused in this test. Is that intentional?

374

👍

425

nit: could you fix this comment?

478

nit: wobbly comment

485

👍

546

I think that the tests, especially this one, would be easier to read if there was a comment mapping the block numbers to the block names used in the CFG comment at the start of each test. Or, slightly more radically, replace the integer literals with a name. E.g.

const uint64_t Entry = 0, Loop1 = ...;
LiveInRsp(Entry, 0, RspLoc);

[jmorse]

Mashed review comments into this patch

[jmorse]

NB: I've also fiddled with the mloc tests so that they clear and rebuild the transfer function between each call to buildMLocValueMap, that way you don't have to keep "what's in the transfer function" in mind throughout.

llvm/unittests/CodeGen/InstrRefLDVTest.cpp

117

I think this is probably discouraged in unit tests, as it blocks all the other CodeGen tests from running; on the other hand, the test configuration is completely broken if it fires, so SGTM

286

Ah, yeah, this is a copy+paste error

546

Probably yes; although then it probably wouldn't all nicely align with fixed-width fonts then :(

I'll try to do this more in future tests, would rather not rewrite this one though.

[jmorse]

NB, I did a little shuffling in the landed version: rather than asking for a MachineDominatorTree analysis, I'm keeping the object in the LiveDebugValues object, and building the dominator tree whenever LiveDebugValues is run in instr-ref mode.

The reason is that registering our use of the MachineDominatorTree analysis means it'll run whether or not we're in instruction referencing mode; needlessly putting an extra compile time cost on people not using instruction referencing. There's also (currently) no benefit from sharing analysis.

This could be changed in the future for efficiency, particularly when the new pass manager gets to the CodeGen backend.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.h

private:
 
   /// Type for a live-in value: the predecessor block, and its value.
   using InValueT = std::pair<MachineBasicBlock *, DbgValue *>;
 
   /// Vector (per block) of a collection (inner smallvector) of live-ins.
   /// Used as the result type for the variable value dataflow problem.
   using LiveInsT = SmallVector<SmallVector<VarAndLoc, 8>, 8>;
 
+  MachineDominatorTree *DomTree;
   const TargetRegisterInfo *TRI;
   const TargetInstrInfo *TII;
   const TargetFrameLowering *TFI;
   const MachineFrameInfo *MFI;
   BitVector CalleeSavedRegs;
   LexicalScopes LS;
   TargetPassConfig *TPC;
 
produceMLocTransferFunction(MachineFunction &MF,
                               SmallVectorImpl<MLocTransferMap> &MLocTransfer,
                               unsigned MaxNumBlocks);
 
   /// Solve the machine value location dataflow problem. Takes as input the
   /// transfer functions in \p MLocTransfer. Writes the output live-in and
   /// live-out arrays to the (initialized to zero) multidimensional arrays in
   /// \p MInLocs and \p MOutLocs. The outer dimension is indexed by block
   /// number, the inner by LocIdx.
-  void mlocDataflow(ValueIDNum **MInLocs, ValueIDNum **MOutLocs,
+  void buildMLocValueMap(MachineFunction &MF, ValueIDNum **MInLocs,
+                         ValueIDNum **MOutLocs,
                          SmallVectorImpl<MLocTransferMap> &MLocTransfer);
 
+  /// Calculate the iterated-dominance-frontier for a set of defs, using the
+  /// existing LLVM facilities for this. Works for a single "value" or
+  /// machine/variable location.
+  /// \p AllBlocks Set of blocks where we might consume the value.
+  /// \p DefBlocks Set of blocks where the value/location is defined.
+  /// \p PHIBlocks Output set of blocks where PHIs must be placed.
+  void BlockPHIPlacement(const SmallPtrSetImpl<MachineBasicBlock *> &AllBlocks,
+                         const SmallPtrSetImpl<MachineBasicBlock *> &DefBlocks,
+                         SmallVectorImpl<MachineBasicBlock *> &PHIBlocks);
+
   /// Perform a control flow join (lattice value meet) of the values in machine
   /// locations at \p MBB. Follows the algorithm described in the file-comment,
   /// reading live-outs of predecessors from \p OutLocs, the current live ins
   /// from \p InLocs, and assigning the newly computed live ins back into
   /// \p InLocs. \returns two bools -- the first indicates whether a change
   /// was made, the second whether a lattice downgrade occurred. If the latter
   /// is true, revisiting this block is necessary.
-  std::tuple<bool, bool>
-  mlocJoin(MachineBasicBlock &MBB,
+  bool mlocJoin(MachineBasicBlock &MBB,
                 SmallPtrSet<const MachineBasicBlock *, 16> &Visited,
                 ValueIDNum **OutLocs, ValueIDNum *InLocs);
 
   /// Solve the variable value dataflow problem, for a single lexical scope.
   /// Uses the algorithm from the file comment to resolve control flow joins,
   /// although there are extra hacks, see vlocJoin. Reads the
   /// locations of values from the \p MInLocs and \p MOutLocs arrays (see
-  /// mlocDataflow) and reads the variable values transfer function from
+  /// buildMLocValueMap) and reads the variable values transfer function from
   /// \p AllTheVlocs. Live-in and Live-out variable values are stored locally,
   /// with the live-ins permanently stored to \p Output once the fixedpoint is
   /// reached.
   /// \p VarsWeCareAbout contains a collection of the variables in \p Scope
   /// that we should be tracking.
   /// \p AssignBlocks contains the set of blocks that aren't in \p Scope, but
   /// which do contain DBG_VALUEs, which VarLocBasedImpl tracks locations
   /// through.
void emitLocations(MachineFunction &MF, LiveInsT SavedLiveIns,
                      ValueIDNum **MOutLocs, ValueIDNum **MInLocs,
                      DenseMap<DebugVariable, unsigned> &AllVarsNumbering,
                      const TargetPassConfig &TPC);
 
   /// Boilerplate computation of some initial sets, artifical blocks and
   /// RPOT block ordering.
   void initialSetup(MachineFunction &MF);
 
-  bool ExtendRanges(MachineFunction &MF, TargetPassConfig *TPC,
-                    unsigned InputBBLimit, unsigned InputDbgValLimit) override;
+  bool ExtendRanges(MachineFunction &MF, MachineDominatorTree *DomTree,
+                    TargetPassConfig *TPC, unsigned InputBBLimit,
+                    unsigned InputDbgValLimit) override;
 
 public:
   /// Default construct and initialize the pass.
   InstrRefBasedLDV();
 
   LLVM_DUMP_METHOD
   void dump_mloc_transfer(const MLocTransferMap &mloc_transfer) const;
 
   bool isCalleeSaved(LocIdx L) const;
 };
 
 } // namespace LiveDebugValues
 
 #endif /* LLVM_LIB_CODEGEN_LIVEDEBUGVALUES_INSTRREFBASEDLDV_H */

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

 //===- InstrRefBasedImpl.cpp - Tracking Debug Value MIs -------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 /// \file InstrRefBasedImpl.cpp
 ///
 /// This is a separate implementation of LiveDebugValues, see
 /// LiveDebugValues.cpp and VarLocBasedImpl.cpp for more information.
 ///
 /// This pass propagates variable locations between basic blocks, resolving
-/// control flow conflicts between them. The problem is much like SSA
-/// construction, where each DBG_VALUE instruction assigns the *value* that
-/// a variable has, and every instruction where the variable is in scope uses
-/// that variable. The resulting map of instruction-to-value is then translated
-/// into a register (or spill) location for each variable over each instruction.
+/// control flow conflicts between them. The problem is SSA construction, where
+/// each debug instruction assigns the *value* that a variable has, and every
+/// instruction where the variable is in scope uses that variable. The resulting
+/// map of instruction-to-value is then translated into a register (or spill)
+/// location for each variable over each instruction.
+///
+/// The primary difference from normal SSA construction is that we cannot
+/// _create_ PHI values that contain variable values. CodeGen has already
+/// completed, and we can't alter it just to make debug-info complete. Thus:
+/// we can identify function positions where we would like a PHI value for a
+/// variable, but must search the MachineFunction to see whether such a PHI is
+/// available. If no such PHI exists, the variable location must be dropped.
 ///
-/// This pass determines which DBG_VALUE dominates which instructions, or if
-/// none do, where values must be merged (like PHI nodes). The added
-/// complication is that because codegen has already finished, a PHI node may
-/// be needed for a variable location to be correct, but no register or spill
-/// slot merges the necessary values. In these circumstances, the variable
-/// location is dropped.
-///
-/// What makes this analysis non-trivial is loops: we cannot tell in advance
-/// whether a variable location is live throughout a loop, or whether its
-/// location is clobbered (or redefined by another DBG_VALUE), without
-/// exploring all the way through.
-///
-/// To make this simpler we perform two kinds of analysis. First, we identify
+/// To achieve this, we perform two kinds of analysis. First, we identify
 /// every value defined by every instruction (ignoring those that only move
-/// another value), then compute a map of which values are available for each
-/// instruction. This is stronger than a reaching-def analysis, as we create
-/// PHI values where other values merge.
-///
-/// Secondly, for each variable, we effectively re-construct SSA using each
-/// DBG_VALUE as a def. The DBG_VALUEs read a value-number computed by the
-/// first analysis from the location they refer to. We can then compute the
-/// dominance frontiers of where a variable has a value, and create PHI nodes
-/// where they merge.
-/// This isn't precisely SSA-construction though, because the function shape
-/// is pre-defined. If a variable location requires a PHI node, but no
-/// PHI for the relevant values is present in the function (as computed by the
-/// first analysis), the location must be dropped.
-///
-/// Once both are complete, we can pass back over all instructions knowing:
-///  * What _value_ each variable should contain, either defined by an
-///    instruction or where control flow merges
-///  * What the location of that value is (if any).
-/// Allowing us to create appropriate live-in DBG_VALUEs, and DBG_VALUEs when
-/// a value moves location. After this pass runs, all variable locations within
-/// a block should be specified by DBG_VALUEs within that block, allowing
-/// DbgEntityHistoryCalculator to focus on individual blocks.
-///
-/// This pass is able to go fast because the size of the first
-/// reaching-definition analysis is proportional to the working-set size of
-/// the function, which the compiler tries to keep small. (It's also
-/// proportional to the number of blocks). Additionally, we repeatedly perform
-/// the second reaching-definition analysis with only the variables and blocks
-/// in a single lexical scope, exploiting their locality.
-///
-/// Determining where PHIs happen is trickier with this approach, and it comes
-/// to a head in the major problem for LiveDebugValues: is a value live-through
-/// a loop, or not? Your garden-variety dataflow analysis aims to build a set of
-/// facts about a function, however this analysis needs to generate new value
-/// numbers at joins.
-///
-/// To do this, consider a lattice of all definition values, from instructions
-/// and from PHIs. Each PHI is characterised by the RPO number of the block it
-/// occurs in. Each value pair A, B can be ordered by RPO(A) < RPO(B):
-/// with non-PHI values at the top, and any PHI value in the last block (by RPO
-/// order) at the bottom.
-///
-/// (Awkwardly: lower-down-the _lattice_ means a greater RPO _number_. Below,
-/// "rank" always refers to the former).
-///
-/// At any join, for each register, we consider:
-///  * All incoming values, and
-///  * The PREVIOUS live-in value at this join.
-/// If all incoming values agree: that's the live-in value. If they do not, the
-/// incoming values are ranked according to the partial order, and the NEXT
-/// LOWEST rank after the PREVIOUS live-in value is picked (multiple values of
-/// the same rank are ignored as conflicting). If there are no candidate values,
-/// or if the rank of the live-in would be lower than the rank of the current
-/// blocks PHIs, create a new PHI value.
-///
-/// Intuitively: if it's not immediately obvious what value a join should result
-/// in, we iteratively descend from instruction-definitions down through PHI
-/// values, getting closer to the current block each time. If the current block
-/// is a loop head, this ordering is effectively searching outer levels of
-/// loops, to find a value that's live-through the current loop.
-///
-/// If there is no value that's live-through this loop, a PHI is created for
-/// this location instead. We can't use a lower-ranked PHI because by definition
-/// it doesn't dominate the current block. We can't create a PHI value any
-/// earlier, because we risk creating a PHI value at a location where values do
-/// not in fact merge, thus misrepresenting the truth, and not making the true
-/// live-through value for variable locations.
-///
-/// This algorithm applies to both calculating the availability of values in
-/// the first analysis, and the location of variables in the second. However
-/// for the second we add an extra dimension of pain: creating a variable
-/// location PHI is only valid if, for each incoming edge,
-///  * There is a value for the variable on the incoming edge, and
-///  * All the edges have that value in the same register.
-/// Or put another way: we can only create a variable-location PHI if there is
-/// a matching machine-location PHI, each input to which is the variables value
-/// in the predecessor block.
-///
-/// To accommodate this difference, each point on the lattice is split in
-/// two: a "proposed" PHI and "definite" PHI. Any PHI that can immediately
-/// have a location determined are "definite" PHIs, and no further work is
-/// needed. Otherwise, a location that all non-backedge predecessors agree
-/// on is picked and propagated as a "proposed" PHI value. If that PHI value
-/// is truly live-through, it'll appear on the loop backedges on the next
-/// dataflow iteration, after which the block live-in moves to be a "definite"
-/// PHI. If it's not truly live-through, the variable value will be downgraded
-/// further as we explore the lattice, or remains "proposed" and is considered
-/// invalid once dataflow completes.
+/// another value), then re-compute an SSA-form representation of the
+/// MachineFunction, using value propagation to eliminate any un-necessary
+/// PHI values. This gives us a map of every value computed in the function,
+/// and its location within the register file / stack.
+///
+/// Secondly, for each variable we perform the same analysis, where each debug
+/// instruction is considered a def, and every instruction where the variable
+/// is in lexical scope as a use. Value propagation is used again to eliminate
+/// any un-necessary PHIs. This gives us a map of each variable to the value
+/// it should have in a block.
+///
+/// Once both are complete, we have two maps for each block:
+///  * Variables to the values they should have,
+///  * Values to the register / spill slot they are located in.
+/// After which we can marry-up variable values with a location, and emit
+/// DBG_VALUE instructions specifying those locations. Variable locations may
+/// be dropped in this process due to the desired variable value not being
+/// resident in any machine location, or because there is no PHI value in any
+/// location that accurately represents the desired value.  The building of
+/// location lists for each block is left to DbgEntityHistoryCalculator.
+///
+/// This pass is kept efficient because the size of the first SSA problem
+/// is proportional to the working-set size of the function, which the compiler
+/// tries to keep small. (It's also proportional to the number of blocks).
+/// Additionally, we repeatedly perform the second SSA problem analysis with
+/// only the variables and blocks in a single lexical scope, exploiting their
+/// locality.
 ///
 /// ### Terminology
 ///
 /// A machine location is a register or spill slot, a value is something that's
 /// defined by an instruction or PHI node, while a variable value is the value
 /// assigned to a variable. A variable location is a machine location, that must
 /// contain the appropriate variable value. A value that is a PHI node is
 /// occasionally called an mphi.
 ///
-/// The first dataflow problem is the "machine value location" problem,
+/// The first SSA problem is the "machine value location" problem,
 /// because we're determining which machine locations contain which values.
 /// The "locations" are constant: what's unknown is what value they contain.
 ///
-/// The second dataflow problem (the one for variables) is the "variable value
+/// The second SSA problem (the one for variables) is the "variable value
 /// problem", because it's determining what values a variable has, rather than
-/// what location those values are placed in. Unfortunately, it's not that
-/// simple, because producing a PHI value always involves picking a location.
-/// This is an imperfection that we just have to accept, at least for now.
+/// what location those values are placed in.
 ///
 /// TODO:
 ///   Overlapping fragments
 ///   Entry values
 ///   Add back DEBUG statements for debugging this
 ///   Collect statistics
 ///
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/IteratedDominanceFrontier.h"
 #include "llvm/CodeGen/LexicalScopes.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineInstrBundle.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
 #include "llvm/CodeGen/MachineOperand.h"
for (unsigned Bit : BV.set_bits()) {
         if (ValueID.getBlock() == I && ValueID.isPHI())
           // It was left as live-through. Set it to clobbered.
           ValueID = NotGeneratedNum;
       }
     }
   }
 }
 
-std::tuple<bool, bool>
-InstrRefBasedLDV::mlocJoin(MachineBasicBlock &MBB,
-                           SmallPtrSet<const MachineBasicBlock *, 16> &Visited,
-                           ValueIDNum **OutLocs, ValueIDNum *InLocs) {
+bool InstrRefBasedLDV::mlocJoin(
+    MachineBasicBlock &MBB, SmallPtrSet<const MachineBasicBlock *, 16> &Visited,
   LLVM_DEBUG(dbgs() << "join MBB: " << MBB.getNumber() << "\n");
   bool Changed = false;
-  bool DowngradeOccurred = false;
 
-  // Collect predecessors that have been visited. Anything that hasn't been
-  // visited yet is a backedge on the first iteration, and the meet of it's
-  // lattice value for all locations will be unaffected.
+  // Handle value-propagation when control flow merges on entry to a block. For
+  // any location without a PHI already placed, the location has the same value
+  // as its predecessors. If a PHI is placed, test to see whether it's now a
+  // redundant PHI that we can eliminate.
+
   SmallVector<const MachineBasicBlock *, 8> BlockOrders;
-  for (auto Pred : MBB.predecessors()) {
-    if (Visited.count(Pred)) {
-      BlockOrders.push_back(Pred);
-    }
-  }
+  for (auto Pred : MBB.predecessors())
 
   // Visit predecessors in RPOT order.
   auto Cmp = [&](const MachineBasicBlock *A, const MachineBasicBlock *B) {
     return BBToOrder.find(A)->second < BBToOrder.find(B)->second;
   };
   llvm::sort(BlockOrders, Cmp);
 
   // Skip entry block.
   if (BlockOrders.size() == 0)
-    return std::tuple<bool, bool>(false, false);
+    return false;
 
-  // Step through all machine locations, then look at each predecessor and
-  // detect disagreements.
-  unsigned ThisBlockRPO = BBToOrder.find(&MBB)->second;
+  // Step through all machine locations, look at each predecessor and test
+  // whether we can eliminate redundant PHIs.
   for (auto Location : MTracker->locations()) {
     LocIdx Idx = Location.Idx;
+
     // Pick out the first predecessors live-out value for this location. It's
-    // guaranteed to be not a backedge, as we order by RPO.
-    ValueIDNum BaseVal = OutLocs[BlockOrders[0]->getNumber()][Idx.asU64()];
+    // guaranteed to not be a backedge, as we order by RPO.
+    ValueIDNum FirstVal = OutLocs[BlockOrders[0]->getNumber()][Idx.asU64()];
 
-    // Some flags for whether there's a disagreement, and whether it's a
-    // disagreement with a backedge or not.
-    bool Disagree = false;
-    bool NonBackEdgeDisagree = false;
+    // If we've already eliminated a PHI here, do no further checking, just
+    // propagate the first live-in value into this block.
+    if (InLocs[Idx.asU64()] != ValueIDNum(MBB.getNumber(), 0, Idx)) {
+      if (InLocs[Idx.asU64()] != FirstVal) {
+        InLocs[Idx.asU64()] = FirstVal;
+        Changed |= true;
+      }
+      continue;
+    }
 
-    // Loop around everything that wasn't 'base'.
+    // We're now examining a PHI to see whether it's un-necessary. Loop around
+    // the other live-in values and test whether they're all the same.
+    bool Disagree = false;
     for (unsigned int I = 1; I < BlockOrders.size(); ++I) {
-      auto *MBB = BlockOrders[I];
-      if (BaseVal != OutLocs[MBB->getNumber()][Idx.asU64()]) {
-        // Live-out of a predecessor disagrees with the first predecessor.
-        Disagree = true;
+      const MachineBasicBlock *PredMBB = BlockOrders[I];
+      const ValueIDNum &PredLiveOut =
+          OutLocs[PredMBB->getNumber()][Idx.asU64()];
 
-        // Test whether it's a disagreemnt in the backedges or not.
-        if (BBToOrder.find(MBB)->second < ThisBlockRPO) // might be self b/e
-          NonBackEdgeDisagree = true;
-      }
-    }
+      // Incoming values agree, continue trying to eliminate this PHI.
+      if (FirstVal == PredLiveOut)
+        continue;
 
-    bool OverRide = false;
-    if (Disagree && !NonBackEdgeDisagree) {
-      // Only the backedges disagree. Consider demoting the livein
-      // lattice value, as per the file level comment. The value we consider
-      // demoting to is the value that the non-backedge predecessors agree on.
-      // The order of values is that non-PHIs are \top, a PHI at this block
-      // \bot, and phis between the two are ordered by their RPO number.
-      // If there's no agreement, or we've already demoted to this PHI value
-      // before, replace with a PHI value at this block.
-
-      // Calculate order numbers: zero means normal def, nonzero means RPO
-      // number.
-      unsigned BaseBlockRPONum = BBNumToRPO[BaseVal.getBlock()] + 1;
-      if (!BaseVal.isPHI())
-        BaseBlockRPONum = 0;
-
-      ValueIDNum &InLocID = InLocs[Idx.asU64()];
-      unsigned InLocRPONum = BBNumToRPO[InLocID.getBlock()] + 1;
-      if (!InLocID.isPHI())
-        InLocRPONum = 0;
-
-      // Should we ignore the disagreeing backedges, and override with the
-      // value the other predecessors agree on (in "base")?
-      unsigned ThisBlockRPONum = BBNumToRPO[MBB.getNumber()] + 1;
-      if (BaseBlockRPONum > InLocRPONum && BaseBlockRPONum < ThisBlockRPONum) {
-        // Override.
-        OverRide = true;
-        DowngradeOccurred = true;
-      }
+      // We can also accept a PHI value that feeds back into itself.
+      if (PredLiveOut == ValueIDNum(MBB.getNumber(), 0, Idx))
+        continue;
+
+      // Live-out of a predecessor disagrees with the first predecessor.
+      Disagree = true;
     }
-    // else: if we disagree in the non-backedges, then this is definitely
-    // a control flow merge where different values merge. Make it a PHI.
 
-    // Generate a phi...
-    ValueIDNum PHI = {(uint64_t)MBB.getNumber(), 0, Idx};
-    ValueIDNum NewVal = (Disagree && !OverRide) ? PHI : BaseVal;
-    if (InLocs[Idx.asU64()] != NewVal) {
+    // No disagreement? No PHI. Otherwise, leave the PHI in live-ins.
+    if (!Disagree) {
+      InLocs[Idx.asU64()] = FirstVal;
       Changed |= true;
-      InLocs[Idx.asU64()] = NewVal;
     }
   }
 
   // TODO: Reimplement NumInserted and NumRemoved.
-  return std::tuple<bool, bool>(Changed, DowngradeOccurred);
+  return Changed;
 }
 
-void InstrRefBasedLDV::mlocDataflow(
-    ValueIDNum **MInLocs, ValueIDNum **MOutLocs,
+void InstrRefBasedLDV::buildMLocValueMap(
+    MachineFunction &MF, ValueIDNum **MInLocs, ValueIDNum **MOutLocs,
     SmallVectorImpl<MLocTransferMap> &MLocTransfer) {
   std::priority_queue<unsigned int, std::vector<unsigned int>,
                       std::greater<unsigned int>>
       Worklist, Pending;
 
   // We track what is on the current and pending worklist to avoid inserting
   // the same thing twice. We could avoid this with a custom priority queue,
   // but this is probably not worth it.
   SmallPtrSet<MachineBasicBlock *, 16> OnPending, OnWorklist;
 
-  // Initialize worklist with every block to be visited.
+  // Initialize worklist with every block to be visited. Also produce list of
+  // all blocks.
+  SmallPtrSet<MachineBasicBlock *, 32> AllBlocks;
   for (unsigned int I = 0; I < BBToOrder.size(); ++I) {
     Worklist.push(I);
     OnWorklist.insert(OrderToBB[I]);
+    AllBlocks.insert(OrderToBB[I]);
   }
 
+  // Initialize entry block to PHIs. These represent arguments.
+  for (auto Location : MTracker->locations())
+    MInLocs[0][Location.Idx.asU64()] = ValueIDNum(0, 0, Location.Idx);
+
   MTracker->reset();
 
-  // Set inlocs for entry block -- each as a PHI at the entry block. Represents
-  // the incoming value to the function.
-  MTracker->setMPhis(0);
-  for (auto Location : MTracker->locations())
-    MInLocs[0][Location.Idx.asU64()] = Location.Value;
+  // Start by placing PHIs, using the usual SSA constructor algorithm. Consider
+  // any machine-location that isn't live-through a block to be def'd in that
+  // block.
+  for (auto Location : MTracker->locations()) {
+    // Collect the set of defs.
+    SmallPtrSet<MachineBasicBlock *, 32> DefBlocks;
+    for (unsigned int I = 0; I < OrderToBB.size(); ++I) {
+      MachineBasicBlock *MBB = OrderToBB[I];
+      const auto &TransferFunc = MLocTransfer[MBB->getNumber()];
+      if (TransferFunc.find(Location.Idx) != TransferFunc.end())
+        DefBlocks.insert(MBB);
+    }
+
+    // The entry block defs the location too: it's the live-in / argument value.
+    // Only insert if there are other defs though; everything is trivially live
+    // through otherwise.
+    if (!DefBlocks.empty())
+      DefBlocks.insert(&*MF.begin());
+
+    // Ask the SSA construction algorithm where we should put PHIs.
+    SmallVector<MachineBasicBlock *, 32> PHIBlocks;
+    BlockPHIPlacement(AllBlocks, DefBlocks, PHIBlocks);
+
+    // Install those PHI values into the live-in value array.
+    for (const MachineBasicBlock *MBB : PHIBlocks) {
+      MInLocs[MBB->getNumber()][Location.Idx.asU64()] =
+          ValueIDNum(MBB->getNumber(), 0, Location.Idx);
+    }
+  }
 
+  // Propagate values to eliminate redundant PHIs. At the same time, this
+  // produces the table of Block x Location => Value for the entry to each
+  // block.
+  // The kind of PHIs we can eliminate are, for example, where one path in a
+  // conditional spills and restores a register, and the register still has
+  // the same value once control flow joins, unbeknowns to the PHI placement
+  // code. Propagating values allows us to identify such un-necessary PHIs and
+  // remove them.
   SmallPtrSet<const MachineBasicBlock *, 16> Visited;
   while (!Worklist.empty() || !Pending.empty()) {
     // Vector for storing the evaluated block transfer function.
     SmallVector<std::pair<LocIdx, ValueIDNum>, 32> ToRemap;
 
     while (!Worklist.empty()) {
       MachineBasicBlock *MBB = OrderToBB[Worklist.top()];
       CurBB = MBB->getNumber();
       Worklist.pop();
 
       // Join the values in all predecessor blocks.
-      bool InLocsChanged, DowngradeOccurred;
-      std::tie(InLocsChanged, DowngradeOccurred) =
-          mlocJoin(*MBB, Visited, MOutLocs, MInLocs[CurBB]);
+      bool InLocsChanged;
+      InLocsChanged = mlocJoin(*MBB, Visited, MOutLocs, MInLocs[CurBB]);
       InLocsChanged |= Visited.insert(MBB).second;
 
-      // If a downgrade occurred, book us in for re-examination on the next
-      // iteration.
-      if (DowngradeOccurred && OnPending.insert(MBB).second)
-        Pending.push(BBToOrder[MBB]);
-
       // Don't examine transfer function if we've visited this loc at least
       // once, and inlocs haven't changed.
       if (!InLocsChanged)
         continue;
 
       // Load the current set of live-ins into MLocTracker.
       MTracker->loadFromArray(MInLocs[CurBB], CurBB);
 
while (!Worklist.empty()) {
 
       MTracker->reset();
 
       // No need to examine successors again if out-locs didn't change.
       if (!OLChanged)
         continue;
 
       // All successors should be visited: put any back-edges on the pending
-      // list for the next dataflow iteration, and any other successors to be
-      // visited this iteration, if they're not going to be already.
+      // list for the next pass-through, and any other successors to be
+      // visited this pass, if they're not going to be already.
       for (auto s : MBB->successors()) {
         // Does branching to this successor represent a back-edge?
         if (BBToOrder[s] > BBToOrder[MBB]) {
           // No: visit it during this dataflow iteration.
           if (OnWorklist.insert(s).second)
             Worklist.push(BBToOrder[s]);
         } else {
           // Yes: visit it on the next iteration.
           if (OnPending.insert(s).second)
             Pending.push(BBToOrder[s]);
         }
       }
     }
 
     Worklist.swap(Pending);
     std::swap(OnPending, OnWorklist);
     OnPending.clear();
     // At this point, pending must be empty, since it was just the empty
     // worklist
     assert(Pending.empty() && "Pending should be empty");
   }
 
-  // Once all the live-ins don't change on mlocJoin(), we've reached a
-  // fixedpoint.
+  // Once all the live-ins don't change on mlocJoin(), we've eliminated all
+  // redundant PHIs.
+}
+
+// Boilerplate for feeding MachineBasicBlocks into IDF calculator. Provide
+// template specialisations for graph traits and a successor enumerator.
+namespace llvm {
+template <> struct GraphTraits<MachineBasicBlock> {
+  using NodeRef = MachineBasicBlock *;
+  using ChildIteratorType = MachineBasicBlock::succ_iterator;
+
+  static NodeRef getEntryNode(MachineBasicBlock *BB) { return BB; }
+  static ChildIteratorType child_begin(NodeRef N) { return N->succ_begin(); }
+  static ChildIteratorType child_end(NodeRef N) { return N->succ_end(); }
+};
+
+template <> struct GraphTraits<const MachineBasicBlock> {
+  using NodeRef = const MachineBasicBlock *;
+  using ChildIteratorType = MachineBasicBlock::const_succ_iterator;
+
+  static NodeRef getEntryNode(const MachineBasicBlock *BB) { return BB; }
+  static ChildIteratorType child_begin(NodeRef N) { return N->succ_begin(); }
+  static ChildIteratorType child_end(NodeRef N) { return N->succ_end(); }
+};
+
+using MachineDomTreeBase = DomTreeBase<MachineBasicBlock>::NodeType;
+using MachineDomTreeChildGetter =
+    typename IDFCalculatorDetail::ChildrenGetterTy<MachineDomTreeBase, false>;
+
+template <>
+typename MachineDomTreeChildGetter::ChildrenTy
+MachineDomTreeChildGetter::get(const NodeRef &N) {
+  return {N->succ_begin(), N->succ_end()};
+}
+} // namespace llvm
+
+void InstrRefBasedLDV::BlockPHIPlacement(
+    const SmallPtrSetImpl<MachineBasicBlock *> &AllBlocks,
+    const SmallPtrSetImpl<MachineBasicBlock *> &DefBlocks,
+    SmallVectorImpl<MachineBasicBlock *> &PHIBlocks) {
+  // Apply IDF calculator to the designated set of location defs, storing
+  // required PHIs into PHIBlocks. Uses the dominator tree stored in the
+  // InstrRefBasedLDV object.
+  IDFCalculatorDetail::ChildrenGetterTy<MachineDomTreeBase, false> foo;
+  IDFCalculatorBase<MachineDomTreeBase, false> IDF(DomTree->getBase(), foo);
+
+  IDF.setLiveInBlocks(AllBlocks);
+  IDF.setDefiningBlocks(DefBlocks);
+  IDF.calculate(PHIBlocks);
 }
 
 bool InstrRefBasedLDV::vlocDowngradeLattice(
     const MachineBasicBlock &MBB, const DbgValue &OldLiveInLocation,
     const SmallVectorImpl<InValueT> &Values, unsigned CurBlockRPONum) {
   // Ranking value preference: see file level comment, the highest rank is
   // a plain def, followed by PHI values in reverse post-order. Numerically,
   // we assign all defs the rank '0', all PHIs their blocks RPO number plus
for (auto It = MF.DebugValueSubstitutions.begin();
                                               "substitution seen");
     }
   }
 #endif
 }
 
 /// Calculate the liveness information for the given machine function and
 /// extend ranges across basic blocks.
-bool InstrRefBasedLDV::ExtendRanges(MachineFunction &MF, TargetPassConfig *TPC,
+bool InstrRefBasedLDV::ExtendRanges(MachineFunction &MF,
+                                    MachineDominatorTree *DomTree,
+                                    TargetPassConfig *TPC,
                                     unsigned InputBBLimit,
                                     unsigned InputDbgValLimit) {
   // No subprogram means this function contains no debuginfo.
   if (!MF.getFunction().getSubprogram())
     return false;
 
   LLVM_DEBUG(dbgs() << "\nDebug Range Extension\n");
   this->TPC = TPC;
 
+  this->DomTree = DomTree;
   TRI = MF.getSubtarget().getRegisterInfo();
   TII = MF.getSubtarget().getInstrInfo();
   TFI = MF.getSubtarget().getFrameLowering();
   TFI->getCalleeSaves(MF, CalleeSavedRegs);
   MFI = &MF.getFrameInfo();
   LS.initialize(MF);
 
   MTracker =
bool InstrRefBasedLDV::ExtendRanges(MachineFunction &MF,
 
   // Allocate and initialize two array-of-arrays for the live-in and live-out
   // machine values. The outer dimension is the block number; while the inner
   // dimension is a LocIdx from MLocTracker.
   ValueIDNum **MOutLocs = new ValueIDNum *[MaxNumBlocks];
   ValueIDNum **MInLocs = new ValueIDNum *[MaxNumBlocks];
   unsigned NumLocs = MTracker->getNumLocs();
   for (int i = 0; i < MaxNumBlocks; ++i) {
+    // These all auto-initialize to ValueIDNum::EmptyValue
     MOutLocs[i] = new ValueIDNum[NumLocs];
     MInLocs[i] = new ValueIDNum[NumLocs];
   }
 
   // Solve the machine value dataflow problem using the MLocTransfer function,
   // storing the computed live-ins / live-outs into the array-of-arrays. We use
   // both live-ins and live-outs for decision making in the variable value
   // dataflow problem.
-  mlocDataflow(MInLocs, MOutLocs, MLocTransfer);
+  buildMLocValueMap(MF, MInLocs, MOutLocs, MLocTransfer);
 
   // Patch up debug phi numbers, turning unknown block-live-in values into
   // either live-through machine values, or PHIs.
   for (auto &DBG_PHI : DebugPHINumToValue) {
     // Identify unresolved block-live-ins.
     ValueIDNum &Num = DBG_PHI.ValueRead;
     if (!Num.isPHI())
       continue;

llvm/lib/CodeGen/LiveDebugValues/LiveDebugValues.h

 //===- LiveDebugValues.cpp - Tracking Debug Value MIs ---------*- C++ -*---===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIB_CODEGEN_LIVEDEBUGVALUES_LIVEDEBUGVALUES_H
 #define LLVM_LIB_CODEGEN_LIVEDEBUGVALUES_LIVEDEBUGVALUES_H
 
+#include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/TargetPassConfig.h"
 
 namespace llvm {
 
 // Inline namespace for types / symbols shared between different
 // LiveDebugValues implementations.
 inline namespace SharedLiveDebugValues {
 
 // Expose a base class for LiveDebugValues interfaces to inherit from. This
 // allows the generic LiveDebugValues pass handles to call into the
 // implementation.
 class LDVImpl {
 public:
-  virtual bool ExtendRanges(MachineFunction &MF, TargetPassConfig *TPC,
-                            unsigned InputBBLimit,
+  virtual bool ExtendRanges(MachineFunction &MF, MachineDominatorTree *DomTree,
+                            TargetPassConfig *TPC, unsigned InputBBLimit,
                             unsigned InputDbgValLimit) = 0;
   virtual ~LDVImpl() {}
 };
 
 } // namespace SharedLiveDebugValues
 
 // Factory functions for LiveDebugValues implementations.
 extern LDVImpl *makeVarLocBasedLiveDebugValues();
 extern LDVImpl *makeInstrRefBasedLiveDebugValues();
 } // namespace llvm
 
 #endif // LLVM_LIB_CODEGEN_LIVEDEBUGVALUES_LIVEDEBUGVALUES_H

llvm/lib/CodeGen/LiveDebugValues/LiveDebugValues.cpp

public:
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 
 private:
   LDVImpl *TheImpl;
   TargetPassConfig *TPC;
+  MachineDominatorTree MDT;
 };
 
 char LiveDebugValues::ID = 0;
 
 char &llvm::LiveDebugValuesID = LiveDebugValues::ID;
 
 INITIALIZE_PASS(LiveDebugValues, DEBUG_TYPE, "Live DEBUG_VALUE analysis", false,
                 false)
 
 /// Default construct and initialize the pass.
 LiveDebugValues::LiveDebugValues() : MachineFunctionPass(ID) {
   initializeLiveDebugValuesPass(*PassRegistry::getPassRegistry());
   TheImpl = nullptr;
 }
 
 bool LiveDebugValues::runOnMachineFunction(MachineFunction &MF) {
-  if (!TheImpl) {
-    TPC = getAnalysisIfAvailable<TargetPassConfig>();
-    bool InstrRefBased = MF.useDebugInstrRef();
-
-    // Allow the user to force selection of InstrRef LDV.
-    InstrRefBased |= ForceInstrRefLDV;
 
+  if (!TheImpl) {
+    TPC = getAnalysisIfAvailable<TargetPassConfig>();
+
     if (InstrRefBased)
       TheImpl = llvm::makeInstrRefBasedLiveDebugValues();
     else
       TheImpl = llvm::makeVarLocBasedLiveDebugValues();
   }
 
-  return TheImpl->ExtendRanges(MF, TPC, InputBBLimit, InputDbgValueLimit);
+  MachineDominatorTree *DomTree = nullptr;
+  if (InstrRefBased) {
+    DomTree = &MDT;
+    MDT.calculate(MF);
+  }
+
+  return TheImpl->ExtendRanges(MF, DomTree, TPC, InputBBLimit,
+                               InputDbgValueLimit);
 }

llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp

bool join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
             const VarLocMap &VarLocIDs,
             SmallPtrSet<const MachineBasicBlock *, 16> &Visited,
             SmallPtrSetImpl<const MachineBasicBlock *> &ArtificialBlocks);
 
   /// Create DBG_VALUE insts for inlocs that have been propagated but
   /// had their instruction creation deferred.
   void flushPendingLocs(VarLocInMBB &PendingInLocs, VarLocMap &VarLocIDs);
 
-  bool ExtendRanges(MachineFunction &MF, TargetPassConfig *TPC,
-                    unsigned InputBBLimit, unsigned InputDbgValLimit) override;
+  bool ExtendRanges(MachineFunction &MF, MachineDominatorTree *DomTree,
+                    TargetPassConfig *TPC, unsigned InputBBLimit,
+                    unsigned InputDbgValLimit) override;
 
 public:
   /// Default construct and initialize the pass.
   VarLocBasedLDV();
 
   ~VarLocBasedLDV();
 
   /// Print to ostream with a message.
DIExpression *NewExpr =
       DIExpression::prepend(MI.getDebugExpression(), DIExpression::EntryValue);
   VarLoc EntryValLocAsBackup = VarLoc::CreateEntryBackupLoc(MI, LS, NewExpr);
   LocIndices EntryValLocIDs = VarLocIDs.insert(EntryValLocAsBackup);
   OpenRanges.insert(EntryValLocIDs, EntryValLocAsBackup);
 }
 
 /// Calculate the liveness information for the given machine function and
 /// extend ranges across basic blocks.
-bool VarLocBasedLDV::ExtendRanges(MachineFunction &MF, TargetPassConfig *TPC,
-                                  unsigned InputBBLimit,
+bool VarLocBasedLDV::ExtendRanges(MachineFunction &MF,
+                                  MachineDominatorTree *DomTree,
+                                  TargetPassConfig *TPC, unsigned InputBBLimit,
                                   unsigned InputDbgValLimit) {
+  (void)DomTree;
   LLVM_DEBUG(dbgs() << "\nDebug Range Extension\n");
 
   if (!MF.getFunction().getSubprogram())
     // VarLocBaseLDV will already have removed all DBG_VALUEs.
     return false;
 
   // Skip functions from NoDebug compilation units.
   if (MF.getFunction().getSubprogram()->getUnit()->getEmissionKind() ==

llvm/unittests/CodeGen/InstrRefLDVTest.cpp

 using namespace llvm;
 using namespace LiveDebugValues;
 
 // Include helper functions to ease the manipulation of MachineFunctions
 #include "MFCommon.inc"
 
 class InstrRefLDVTest : public testing::Test {
 public:
+  friend class InstrRefBasedLDV;
+  using MLocTransferMap = InstrRefBasedLDV::MLocTransferMap;
+
   LLVMContext Ctx;
   Module Mod;
+  std::unique_ptr<TargetMachine> Machine;
   std::unique_ptr<MachineFunction> MF;
+  std::unique_ptr<MachineDominatorTree> DomTree;
   DICompileUnit *OurCU;
   DIFile *OurFile;
   DISubprogram *OurFunc;
   DILexicalBlock *OurBlock, *AnotherBlock;
   DISubprogram *ToInlineFunc;
   DILexicalBlock *ToInlineBlock;
 
   DebugLoc OutermostLoc, InBlockLoc, NotNestedBlockLoc, InlinedLoc;
 
-  MachineBasicBlock *MBB1, *MBB2, *MBB3, *MBB4;
+  MachineBasicBlock *MBB1, *MBB2, *MBB3, *MBB4, *MBB5;
+
+  std::unique_ptr<InstrRefBasedLDV> LDV;
+  std::unique_ptr<MLocTracker> MTracker;
 
   InstrRefLDVTest() : Ctx(), Mod("beehives", Ctx) {
+  }
+
+  void SetUp() {
     // Boilerplate that creates a MachineFunction and associated blocks.
-    MF = createMachineFunction(Ctx, Mod);
-    llvm::Function &F = const_cast<llvm::Function &>(MF->getFunction());
-    auto BB1 = BasicBlock::Create(Ctx, "a", &F);
-    auto BB2 = BasicBlock::Create(Ctx, "b", &F);
-    auto BB3 = BasicBlock::Create(Ctx, "c", &F);
-    auto BB4 = BasicBlock::Create(Ctx, "d", &F);
-    IRBuilder<> IRB1(BB1), IRB2(BB2), IRB3(BB3), IRB4(BB4);
-    IRB1.CreateBr(BB2);
-    IRB2.CreateBr(BB3);
-    IRB3.CreateBr(BB4);
-    IRB4.CreateRetVoid();
-    MBB1 = MF->CreateMachineBasicBlock(BB1);
-    MF->insert(MF->end(), MBB1);
-    MBB2 = MF->CreateMachineBasicBlock(BB2);
-    MF->insert(MF->end(), MBB2);
-    MBB3 = MF->CreateMachineBasicBlock(BB3);
-    MF->insert(MF->end(), MBB3);
-    MBB4 = MF->CreateMachineBasicBlock(BB4);
-    MF->insert(MF->end(), MBB4);
-    MBB1->addSuccessor(MBB2);
-    MBB1->addSuccessor(MBB3);
-    MBB2->addSuccessor(MBB4);
-    MBB3->addSuccessor(MBB4);
+
+    Mod.setDataLayout("e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-f64:32:64-f80:32-n8:16:32-S128");
+    Triple TargetTriple("x86_64--");
+    std::string Error;
+    const Target *T = TargetRegistry::lookupTarget("", TargetTriple, Error);
+    if (!T)
+      GTEST_SKIP();
+
+    TargetOptions Options;
+    Machine = std::unique_ptr<TargetMachine>(T->createTargetMachine(
+        "X86", "", "", Options, None, None, CodeGenOpt::Aggressive));
+
+    auto Type = FunctionType::get(Type::getVoidTy(Ctx), false);
+    auto F = Function::Create(Type, GlobalValue::ExternalLinkage, "Test", &Mod);
+
+    unsigned FunctionNum = 42;
+    MachineModuleInfo MMI((LLVMTargetMachine*)&*Machine);
+    const TargetSubtargetInfo &STI = *Machine->getSubtargetImpl(*F);
+
+    MF = std::make_unique<MachineFunction>(*F, (LLVMTargetMachine&)*Machine, STI, FunctionNum, MMI);
 
     // Create metadata: CU, subprogram, some blocks and an inline function
     // scope.
     DIBuilder DIB(Mod);
     OurFile = DIB.createFile("xyzzy.c", "/cave");
     OurCU =
         DIB.createCompileUnit(dwarf::DW_LANG_C99, OurFile, "nou", false, "", 0);
     auto OurSubT = DIB.createSubroutineType(DIB.getOrCreateTypeArray(None));
     OurFunc =
         DIB.createFunction(OurCU, "bees", "", OurFile, 1, OurSubT, 1,
                            DINode::FlagZero, DISubprogram::SPFlagDefinition);
-    F.setSubprogram(OurFunc);
+    F->setSubprogram(OurFunc);
     OurBlock = DIB.createLexicalBlock(OurFunc, OurFile, 2, 3);
     AnotherBlock = DIB.createLexicalBlock(OurFunc, OurFile, 2, 6);
     ToInlineFunc =
         DIB.createFunction(OurFile, "shoes", "", OurFile, 10, OurSubT, 10,
                            DINode::FlagZero, DISubprogram::SPFlagDefinition);
 
     // Make some nested scopes.
     OutermostLoc = DILocation::get(Ctx, 3, 1, OurFunc);
     InBlockLoc = DILocation::get(Ctx, 4, 1, OurBlock);
     InlinedLoc = DILocation::get(Ctx, 10, 1, ToInlineFunc, InBlockLoc.get());
 
     // Make a scope that isn't nested within the others.
     NotNestedBlockLoc = DILocation::get(Ctx, 4, 1, AnotherBlock);
 
     DIB.finalize();
   }
+
+  Register getRegByName(const char *WantedName) {
+    auto *TRI = MF->getRegInfo().getTargetRegisterInfo();
+    // Slow, but works.
+    for (unsigned int I = 1; I < TRI->getNumRegs(); ++I) {
+      const char *Name = TRI->getName(I);
+      if (strcmp(WantedName, Name) == 0)
+        return I;
+    }
+
+    // If this ever fails, something is very wrong with this unit test.
+    llvm_unreachable("Can't find register by name");
+  }
+
+  InstrRefBasedLDV *setupLDVObj() {
+    // Create a new LDV object, and plug some relevant object ptrs into it.
+    LDV = std::make_unique<InstrRefBasedLDV>();
+    const TargetSubtargetInfo &STI = MF->getSubtarget();
+    LDV->TII = STI.getInstrInfo();
+    LDV->TRI = STI.getRegisterInfo();
+    LDV->TFI = STI.getFrameLowering();
+    LDV->MFI = &MF->getFrameInfo();
+
+    DomTree = std::make_unique<MachineDominatorTree>(*MF);
+    LDV->DomTree = &*DomTree;
+
+    // Future work: unit tests for mtracker / vtracker / ttracker.
+
+    // Setup things like the artifical block map, and BlockNo <=> RPO Order
+    // mappings.
+    LDV->initialSetup(*MF);
+    addMTracker();
+    return &*LDV;
+  }
+
+  void addMTracker() {
+    ASSERT_TRUE(LDV);
+    // Add a machine-location-tracking object to LDV. Don't initialize any
+    // register locations within it though.
+    const TargetSubtargetInfo &STI = MF->getSubtarget();
+    MTracker = std::make_unique<MLocTracker>(
+          *MF, *LDV->TII, *LDV->TRI, *STI.getTargetLowering());
+    LDV->MTracker = &*MTracker;
+  }
+
+  // Some routines for bouncing into LDV,
+  void buildMLocValueMap(ValueIDNum **MInLocs, ValueIDNum **MOutLocs,
+                         SmallVectorImpl<MLocTransferMap> &MLocTransfer) {
+    LDV->buildMLocValueMap(*MF, MInLocs, MOutLocs, MLocTransfer);
+  }
+
+  void initValueArray(ValueIDNum **Nums, unsigned Blks, unsigned Locs) {
+    for (unsigned int I = 0; I < Blks; ++I)
+      for (unsigned int J = 0; J < Locs; ++J)
+        Nums[I][J] = ValueIDNum::EmptyValue;
+  }
 };
+
+TEST_F(InstrRefLDVTest, MLocSingleBlock) {
+  // Test some very simple properties about interpreting the transfer function.
+
+  // Add an entry block with nothing but 'ret void' in it.
+  Function &F = const_cast<llvm::Function &>(MF->getFunction());
+  auto *BB1 = BasicBlock::Create(Ctx, "entry", &F);
+  IRBuilder<> IRB(BB1);
+  IRB.CreateRetVoid();
+  MBB1 = MF->CreateMachineBasicBlock(BB1);
+  MF->insert(MF->end(), MBB1);
+  MF->RenumberBlocks();
+
+  setupLDVObj();
+  // We should start with a single location, the stack pointer.
+  ASSERT_TRUE(MTracker->getNumLocs() == 1);
+  LocIdx RspLoc(0);
+
+  // Set up live-in and live-out tables for this function: two locations (we
+  // add one later) in a single block.
+  ValueIDNum InLocs[2], OutLocs[2];
+  ValueIDNum *InLocsPtr[1] = {&InLocs[0]};
+  ValueIDNum *OutLocsPtr[1] = {&OutLocs[0]};
+
+  // Transfer function: nothing.
+  SmallVector<MLocTransferMap, 1> TransferFunc = {{}};
+
+  // Try and build value maps...
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+
+  // The result should be that RSP is marked as a live-in-PHI -- this represents
+  // an argument. And as there's no transfer function, the block live-out should
+  // be the same.
+  EXPECT_EQ(InLocs[0], ValueIDNum(0, 0, RspLoc));
+  EXPECT_EQ(OutLocs[0], ValueIDNum(0, 0, RspLoc));
+
+  // Try again, this time initialising the in-locs to be defined by an
+  // instruction. The entry block should always be re-assigned to be the
+  // arguments.
+  initValueArray(InLocsPtr, 1, 2);
+  initValueArray(OutLocsPtr, 1, 2);
+  InLocs[0] = ValueIDNum(0, 1, RspLoc);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0], ValueIDNum(0, 0, RspLoc));
+  EXPECT_EQ(OutLocs[0], ValueIDNum(0, 0, RspLoc));
+
+  // Now insert something into the transfer function to assign to the single
+  // machine location.
+  TransferFunc[0].insert({RspLoc, ValueIDNum(0, 1, RspLoc)});
+  initValueArray(InLocsPtr, 1, 2);
+  initValueArray(OutLocsPtr, 1, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0], ValueIDNum(0, 0, RspLoc));
+  EXPECT_EQ(OutLocs[0], ValueIDNum(0, 1, RspLoc));
+  TransferFunc[0].clear();
+
+  // Add a new register to be tracked, and insert it into the transfer function
+  // as a copy. The output of $rax should be the live-in value of $rsp.
+  Register RAX = getRegByName("RAX");
+  LocIdx RaxLoc = MTracker->lookupOrTrackRegister(RAX);
+  TransferFunc[0].insert({RspLoc, ValueIDNum(0, 1, RspLoc)});
+  TransferFunc[0].insert({RaxLoc, ValueIDNum(0, 0, RspLoc)});
+  initValueArray(InLocsPtr, 1, 2);
+  initValueArray(OutLocsPtr, 1, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0], ValueIDNum(0, 0, RspLoc));
+  EXPECT_EQ(InLocs[1], ValueIDNum(0, 0, RaxLoc));
+  EXPECT_EQ(OutLocs[0], ValueIDNum(0, 1, RspLoc));
+  EXPECT_EQ(OutLocs[1], ValueIDNum(0, 0, RspLoc)); // Rax contains RspLoc.
+  TransferFunc[0].clear();
+}
+
+TEST_F(InstrRefLDVTest, MLocDiamondBlocks) {
+  // Test that information flows from the entry block to two successors.
+
+  //        entry
+  //        /  \
+  //      br1  br2
+  //        \  /
+  //         ret
+  llvm::Function &F = const_cast<llvm::Function &>(MF->getFunction());
+  auto *BB1 = BasicBlock::Create(Ctx, "a", &F);
+  auto *BB2 = BasicBlock::Create(Ctx, "b", &F);
+  auto *BB3 = BasicBlock::Create(Ctx, "c", &F);
+  auto *BB4 = BasicBlock::Create(Ctx, "d", &F);
+  IRBuilder<> IRB1(BB1), IRB2(BB2), IRB3(BB3), IRB4(BB4);
+  IRB1.CreateBr(BB2);
+  IRB2.CreateBr(BB3);
+  IRB3.CreateBr(BB4);
+  IRB4.CreateRetVoid();
+  MBB1 = MF->CreateMachineBasicBlock(BB1);
+  MF->insert(MF->end(), MBB1);
+  MBB2 = MF->CreateMachineBasicBlock(BB2);
+  MF->insert(MF->end(), MBB2);
+  MBB3 = MF->CreateMachineBasicBlock(BB3);
+  MF->insert(MF->end(), MBB3);
+  MBB4 = MF->CreateMachineBasicBlock(BB4);
+  MF->insert(MF->end(), MBB4);
+  MBB1->addSuccessor(MBB2);
+  MBB1->addSuccessor(MBB3);
+  MBB2->addSuccessor(MBB4);
+  MBB3->addSuccessor(MBB4);
+  MF->RenumberBlocks();
+
+  setupLDVObj();
+
+  ASSERT_TRUE(MTracker->getNumLocs() == 1);
+  LocIdx RspLoc(0);
+  Register RAX = getRegByName("RAX");
+  LocIdx RaxLoc = MTracker->lookupOrTrackRegister(RAX);
+
+  ValueIDNum InLocs[4][2], OutLocs[4][2];
+  ValueIDNum *InLocsPtr[4] = {InLocs[0], InLocs[1], InLocs[2], InLocs[3]};
+  ValueIDNum *OutLocsPtr[4] = {OutLocs[0], OutLocs[1], OutLocs[2], OutLocs[3]};
+
+  // Transfer function: start with nothing.
+  SmallVector<MLocTransferMap, 1> TransferFunc;
+  TransferFunc.resize(4);
+
+  // Name some values.
+  ValueIDNum LiveInRsp(0, 0, RspLoc);
+  ValueIDNum RspDefInBlk0(0, 1, RspLoc);
+  ValueIDNum RspDefInBlk1(1, 1, RspLoc);
+  ValueIDNum RspDefInBlk2(2, 1, RspLoc);
+  ValueIDNum RspPHIInBlk3(3, 0, RspLoc);
+  ValueIDNum RaxLiveInBlk1(1, 0, RaxLoc);
+  ValueIDNum RaxLiveInBlk2(2, 0, RaxLoc);
+
+  // With no transfer function, the live-in values to the entry block should
+  // propagate to all live-outs and the live-ins to the two successor blocks.
+  // IN ADDITION: this checks that the exit block doesn't get a PHI put in it.
+  initValueArray(InLocsPtr, 4, 2);
+  initValueArray(OutLocsPtr, 4, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], LiveInRsp);
+  EXPECT_EQ(InLocs[2][0], LiveInRsp);
+  EXPECT_EQ(InLocs[3][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[3][0], LiveInRsp);
+  // (Skipped writing out locations for $rax).
+
+  // Check that a def of $rsp in the entry block will likewise reach all the
+  // successors.
+  TransferFunc[0].insert({RspLoc, RspDefInBlk0});
+  initValueArray(InLocsPtr, 4, 2);
+  initValueArray(OutLocsPtr, 4, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspDefInBlk0);
+  EXPECT_EQ(InLocs[2][0], RspDefInBlk0);
+  EXPECT_EQ(InLocs[3][0], RspDefInBlk0);
+  EXPECT_EQ(OutLocs[0][0], RspDefInBlk0);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk0);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk0);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk0);
+  TransferFunc[0].clear();
+
+  // Def in one branch of the diamond means that we need a PHI in the ret block
+  TransferFunc[0].insert({RspLoc, RspDefInBlk0});
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  initValueArray(InLocsPtr, 4, 2);
+  initValueArray(OutLocsPtr, 4, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  // This value map: like above, where RspDefInBlk0 is propagated through one
+  // branch of the diamond, but is def'ed in the live-outs of the other. The
+  // ret / merging block should have a PHI in its live-ins.
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspDefInBlk0);
+  EXPECT_EQ(InLocs[2][0], RspDefInBlk0);
+  EXPECT_EQ(InLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(OutLocs[0][0], RspDefInBlk0);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk0);
+  EXPECT_EQ(OutLocs[3][0], RspPHIInBlk3);
+  TransferFunc[0].clear();
+  TransferFunc[1].clear();
+
+  // If we have differeing defs in either side of the diamond, we should
+  // continue to produce a PHI,
+  TransferFunc[0].insert({RspLoc, RspDefInBlk0});
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  TransferFunc[2].insert({RspLoc, RspDefInBlk2});
+  initValueArray(InLocsPtr, 4, 2);
+  initValueArray(OutLocsPtr, 4, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspDefInBlk0);
+  EXPECT_EQ(InLocs[2][0], RspDefInBlk0);
+  EXPECT_EQ(InLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(OutLocs[0][0], RspDefInBlk0);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[3][0], RspPHIInBlk3);
+  TransferFunc[0].clear();
+  TransferFunc[1].clear();
+  TransferFunc[2].clear();
+
+  // If we have defs of the same value on either side of the branch, a PHI will
+  // initially be created, however value propagation should then eliminate it.
+  // Encode this by copying the live-in value to $rax, and copying it to $rsp
+  // from $rax in each branch of the diamond. We don't allow the definition of
+  // arbitary values in transfer functions.
+  TransferFunc[0].insert({RspLoc, RspDefInBlk0});
+  TransferFunc[0].insert({RaxLoc, LiveInRsp});
+  TransferFunc[1].insert({RspLoc, RaxLiveInBlk1});
+  TransferFunc[2].insert({RspLoc, RaxLiveInBlk2});
+  initValueArray(InLocsPtr, 4, 2);
+  initValueArray(OutLocsPtr, 4, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspDefInBlk0);
+  EXPECT_EQ(InLocs[2][0], RspDefInBlk0);
+  EXPECT_EQ(InLocs[3][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], RspDefInBlk0);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[3][0], LiveInRsp);
+  TransferFunc[0].clear();
+  TransferFunc[1].clear();
+  TransferFunc[2].clear();
+}
+
+TEST_F(InstrRefLDVTest, MLocSimpleLoop) {
+  //    entry
+  //     |
+  //     |/-----\
+  //    loopblk |
+  //     |\-----/
+  //     |
+  //     ret
+  llvm::Function &F = const_cast<llvm::Function &>(MF->getFunction());
+  auto *BB1 = BasicBlock::Create(Ctx, "entry", &F);
+  auto *BB2 = BasicBlock::Create(Ctx, "loop", &F);
+  auto *BB3 = BasicBlock::Create(Ctx, "ret", &F);
+  IRBuilder<> IRB1(BB1), IRB2(BB2), IRB3(BB3);
+  IRB1.CreateBr(BB2);
+  IRB2.CreateBr(BB3);
+  IRB3.CreateRetVoid();
+  MBB1 = MF->CreateMachineBasicBlock(BB1);
+  MF->insert(MF->end(), MBB1);
+  MBB2 = MF->CreateMachineBasicBlock(BB2);
+  MF->insert(MF->end(), MBB2);
+  MBB3 = MF->CreateMachineBasicBlock(BB3);
+  MF->insert(MF->end(), MBB3);
+  MBB1->addSuccessor(MBB2);
+  MBB2->addSuccessor(MBB3);
+  MBB2->addSuccessor(MBB2);
+  MF->RenumberBlocks();
+
+  setupLDVObj();
+
+  ASSERT_TRUE(MTracker->getNumLocs() == 1);
+  LocIdx RspLoc(0);
+  Register RAX = getRegByName("RAX");
+  LocIdx RaxLoc = MTracker->lookupOrTrackRegister(RAX);
+
+  ValueIDNum InLocs[3][2], OutLocs[3][2];
+  ValueIDNum *InLocsPtr[3] = {InLocs[0], InLocs[1], InLocs[2]};
+  ValueIDNum *OutLocsPtr[3] = {OutLocs[0], OutLocs[1], OutLocs[2]};
+
+  SmallVector<MLocTransferMap, 1> TransferFunc;
+  TransferFunc.resize(3);
+
+  // Name some values.
+  ValueIDNum LiveInRsp(0, 0, RspLoc);
+  ValueIDNum RspPHIInBlk1(1, 0, RspLoc);
+  ValueIDNum RspDefInBlk1(1, 1, RspLoc);
+  ValueIDNum LiveInRax(0, 0, RaxLoc);
+  ValueIDNum RaxPHIInBlk1(1, 0, RaxLoc);
+  ValueIDNum RaxPHIInBlk2(2, 0, RaxLoc);
+
+  // Begin test with all locations being live-through.
+  initValueArray(InLocsPtr, 3, 2);
+  initValueArray(OutLocsPtr, 3, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], LiveInRsp);
+  EXPECT_EQ(InLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+
+  // Add a def of $rsp to the loop block: it should be in the live-outs, but
+  // should cause a PHI to be placed in the live-ins. Test the transfer function
+  // by copying that PHI into $rax in the loop, then back to $rsp in the ret
+  // block.
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  TransferFunc[1].insert({RaxLoc, RspPHIInBlk1});
+  TransferFunc[2].insert({RspLoc, RaxPHIInBlk2});
+  initValueArray(InLocsPtr, 3, 2);
+  initValueArray(OutLocsPtr, 3, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspPHIInBlk1);
+  // Check rax as well,
+  EXPECT_EQ(InLocs[0][1], LiveInRax);
+  EXPECT_EQ(InLocs[1][1], RaxPHIInBlk1);
+  EXPECT_EQ(InLocs[2][1], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[0][1], LiveInRax);
+  EXPECT_EQ(OutLocs[1][1], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[2][1], RspPHIInBlk1);
+  TransferFunc[1].clear();
+  TransferFunc[2].clear();
+
+  // As with the diamond case, a PHI will be created if there's a (implicit)
+  // def in the entry block and loop block; but should be value propagated away
+  // if it copies in the same value. Copy live-in $rsp to $rax, then copy it
+  // into $rsp in the loop. Encoded as copying the live-in $rax value in block 1
+  // to $rsp.
+  TransferFunc[0].insert({RaxLoc, LiveInRsp});
+  TransferFunc[1].insert({RspLoc, RaxPHIInBlk1});
+  initValueArray(InLocsPtr, 3, 2);
+  initValueArray(OutLocsPtr, 3, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], LiveInRsp);
+  EXPECT_EQ(InLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+  // Check $rax's values.
+  EXPECT_EQ(InLocs[0][1], LiveInRax);
+  EXPECT_EQ(InLocs[1][1], LiveInRsp);
+  EXPECT_EQ(InLocs[2][1], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][1], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][1], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][1], LiveInRsp);
+  TransferFunc[0].clear();
+  TransferFunc[1].clear();
+}
+
+TEST_F(InstrRefLDVTest, MLocNestedLoop) {
+  //    entry
+  //     |
+  //    loop1
+  //     ^\
+  //     | \    /-\
+  //     |  loop2  |
+  //     |  /   \-/
+  //     ^ /
+  //     join
+  //     |
+  //     ret
+  llvm::Function &F = const_cast<llvm::Function &>(MF->getFunction());
+  auto *BB1 = BasicBlock::Create(Ctx, "entry", &F);
+  auto *BB2 = BasicBlock::Create(Ctx, "loop1", &F);
+  auto *BB3 = BasicBlock::Create(Ctx, "loop2", &F);
+  auto *BB4 = BasicBlock::Create(Ctx, "join", &F);
+  auto *BB5 = BasicBlock::Create(Ctx, "ret", &F);
+  IRBuilder<> IRB1(BB1), IRB2(BB2), IRB3(BB3), IRB4(BB4), IRB5(BB5);
+  IRB1.CreateBr(BB2);
+  IRB2.CreateBr(BB3);
+  IRB3.CreateBr(BB4);
+  IRB4.CreateBr(BB5);
+  IRB5.CreateRetVoid();
+  MBB1 = MF->CreateMachineBasicBlock(BB1);
+  MF->insert(MF->end(), MBB1);
+  MBB2 = MF->CreateMachineBasicBlock(BB2);
+  MF->insert(MF->end(), MBB2);
+  MBB3 = MF->CreateMachineBasicBlock(BB3);
+  MF->insert(MF->end(), MBB3);
+  MBB4 = MF->CreateMachineBasicBlock(BB4);
+  MF->insert(MF->end(), MBB4);
+  MBB5 = MF->CreateMachineBasicBlock(BB5);
+  MF->insert(MF->end(), MBB5);
+  MBB1->addSuccessor(MBB2);
+  MBB2->addSuccessor(MBB3);
+  MBB3->addSuccessor(MBB3);
+  MBB3->addSuccessor(MBB4);
+  MBB4->addSuccessor(MBB2);
+  MBB4->addSuccessor(MBB5);
+  MF->RenumberBlocks();
+
+  setupLDVObj();
+
+  ASSERT_TRUE(MTracker->getNumLocs() == 1);
+  LocIdx RspLoc(0);
+  Register RAX = getRegByName("RAX");
+  LocIdx RaxLoc = MTracker->lookupOrTrackRegister(RAX);
+
+  ValueIDNum InLocs[5][2], OutLocs[5][2];
+  ValueIDNum *InLocsPtr[5] = {InLocs[0], InLocs[1], InLocs[2], InLocs[3],
+                              InLocs[4]};
+  ValueIDNum *OutLocsPtr[5] = {OutLocs[0], OutLocs[1], OutLocs[2], OutLocs[3],
+                               OutLocs[4]};
+
+  SmallVector<MLocTransferMap, 1> TransferFunc;
+  TransferFunc.resize(5);
+
+  ValueIDNum LiveInRsp(0, 0, RspLoc);
+  ValueIDNum RspPHIInBlk1(1, 0, RspLoc);
+  ValueIDNum RspDefInBlk1(1, 1, RspLoc);
+  ValueIDNum RspPHIInBlk2(2, 0, RspLoc);
+  ValueIDNum RspDefInBlk2(2, 1, RspLoc);
+  ValueIDNum RspDefInBlk3(3, 1, RspLoc);
+  ValueIDNum LiveInRax(0, 0, RaxLoc);
+  ValueIDNum RaxPHIInBlk1(1, 0, RaxLoc);
+  ValueIDNum RaxPHIInBlk2(2, 0, RaxLoc);
+
+  // Like the other tests: first ensure that if there's nothing in the transfer
+  // function, then everything is live-through (check $rsp).
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], LiveInRsp);
+  EXPECT_EQ(InLocs[2][0], LiveInRsp);
+  EXPECT_EQ(InLocs[3][0], LiveInRsp);
+  EXPECT_EQ(InLocs[4][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[3][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[4][0], LiveInRsp);
+
+  // A def in the inner loop means we should get PHIs at the heads of both
+  // loops. Live-outs of the last three blocks will be the def, as it dominates
+  // those.
+  TransferFunc[2].insert({RspLoc, RspDefInBlk2});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], RspDefInBlk2);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk2);
+  TransferFunc[2].clear();
+
+  // Adding a def to the outer loop header shouldn't affect this much -- the
+  // live-out of block 1 changes.
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  TransferFunc[2].insert({RspLoc, RspDefInBlk2});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], RspDefInBlk2);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk2);
+  TransferFunc[1].clear();
+  TransferFunc[2].clear();
+
+  // Likewise, putting a def in the outer loop tail shouldn't affect where
+  // the PHIs go, and should propagate into the ret block.
+
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  TransferFunc[2].insert({RspLoc, RspDefInBlk2});
+  TransferFunc[3].insert({RspLoc, RspDefInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], RspDefInBlk2);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk3);
+  TransferFunc[1].clear();
+  TransferFunc[2].clear();
+  TransferFunc[3].clear();
+
+  // However: if we don't def in the inner-loop, then we just have defs in the
+  // head and tail of the outer loop. The inner loop should be live-through.
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  TransferFunc[3].insert({RspLoc, RspDefInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspDefInBlk1);
+  EXPECT_EQ(InLocs[3][0], RspDefInBlk1);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk3);
+  TransferFunc[1].clear();
+  TransferFunc[3].clear();
+
+  // Check that this still works if we copy RspDefInBlk1 to $rax and then
+  // copy it back into $rsp in the inner loop.
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  TransferFunc[1].insert({RaxLoc, RspDefInBlk1});
+  TransferFunc[2].insert({RspLoc, RaxPHIInBlk2});
+  TransferFunc[3].insert({RspLoc, RspDefInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspDefInBlk1);
+  EXPECT_EQ(InLocs[3][0], RspDefInBlk1);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk3);
+  // Look at raxes value in the relevant blocks,
+  EXPECT_EQ(InLocs[2][1], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[1][1], RspDefInBlk1);
+  TransferFunc[1].clear();
+  TransferFunc[2].clear();
+  TransferFunc[3].clear();
+
+  // If we have a single def in the tail of the outer loop, that should produce
+  // a PHI at the loop head, and be live-through the inner loop.
+  TransferFunc[3].insert({RspLoc, RspDefInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[3][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk3);
+  TransferFunc[3].clear();
+
+  // And if we copy from $rsp to $rax in block 2, it should resolve to the PHI
+  // in block 1, and we should keep that value in rax until the ret block.
+  // There'll be a PHI in block 1 and 2, because we're putting a def in the
+  // inner loop.
+  TransferFunc[2].insert({RaxLoc, RspPHIInBlk2});
+  TransferFunc[3].insert({RspLoc, RspDefInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  // Examining the values of rax,
+  EXPECT_EQ(InLocs[0][1], LiveInRax);
+  EXPECT_EQ(InLocs[1][1], RaxPHIInBlk1);
+  EXPECT_EQ(InLocs[2][1], RaxPHIInBlk2);
+  EXPECT_EQ(InLocs[3][1], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[4][1], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[0][1], LiveInRax);
+  EXPECT_EQ(OutLocs[1][1], RaxPHIInBlk1);
+  EXPECT_EQ(OutLocs[2][1], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[3][1], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[4][1], RspPHIInBlk1);
+  TransferFunc[2].clear();
+  TransferFunc[3].clear();
+}
+
+TEST_F(InstrRefLDVTest, MLocNoDominatingLoop) {
+  //           entry
+  //            / \
+  //           /   \
+  //          /     \
+  //        head1   head2
+  //        ^  \   /   ^
+  //        ^   \ /    ^
+  //        \-joinblk -/
+  //             |
+  //            ret
+  llvm::Function &F = const_cast<llvm::Function &>(MF->getFunction());
+  auto *BB1 = BasicBlock::Create(Ctx, "entry", &F);
+  auto *BB2 = BasicBlock::Create(Ctx, "head1", &F);
+  auto *BB3 = BasicBlock::Create(Ctx, "head2", &F);
+  auto *BB4 = BasicBlock::Create(Ctx, "joinblk", &F);
+  auto *BB5 = BasicBlock::Create(Ctx, "ret", &F);
+  IRBuilder<> IRB1(BB1), IRB2(BB2), IRB3(BB3), IRB4(BB4), IRB5(BB5);
+  IRB1.CreateBr(BB2);
+  IRB2.CreateBr(BB3);
+  IRB3.CreateBr(BB4);
+  IRB4.CreateBr(BB5);
+  IRB5.CreateRetVoid();
+  MBB1 = MF->CreateMachineBasicBlock(BB1);
+  MF->insert(MF->end(), MBB1);
+  MBB2 = MF->CreateMachineBasicBlock(BB2);
+  MF->insert(MF->end(), MBB2);
+  MBB3 = MF->CreateMachineBasicBlock(BB3);
+  MF->insert(MF->end(), MBB3);
+  MBB4 = MF->CreateMachineBasicBlock(BB4);
+  MF->insert(MF->end(), MBB4);
+  MBB5 = MF->CreateMachineBasicBlock(BB5);
+  MF->insert(MF->end(), MBB5);
+  MBB1->addSuccessor(MBB2);
+  MBB1->addSuccessor(MBB3);
+  MBB2->addSuccessor(MBB4);
+  MBB3->addSuccessor(MBB4);
+  MBB4->addSuccessor(MBB2);
+  MBB4->addSuccessor(MBB3);
+  MBB4->addSuccessor(MBB5);
+  MF->RenumberBlocks();
+
+  setupLDVObj();
+
+  ASSERT_TRUE(MTracker->getNumLocs() == 1);
+  LocIdx RspLoc(0);
+  Register RAX = getRegByName("RAX");
+  LocIdx RaxLoc = MTracker->lookupOrTrackRegister(RAX);
+
+  ValueIDNum InLocs[5][2], OutLocs[5][2];
+  ValueIDNum *InLocsPtr[5] = {InLocs[0], InLocs[1], InLocs[2], InLocs[3],
+                              InLocs[4]};
+  ValueIDNum *OutLocsPtr[5] = {OutLocs[0], OutLocs[1], OutLocs[2], OutLocs[3],
+                               OutLocs[4]};
+
+  SmallVector<MLocTransferMap, 1> TransferFunc;
+  TransferFunc.resize(5);
+
+  ValueIDNum LiveInRsp(0, 0, RspLoc);
+  ValueIDNum RspPHIInBlk1(1, 0, RspLoc);
+  ValueIDNum RspDefInBlk1(1, 1, RspLoc);
+  ValueIDNum RspPHIInBlk2(2, 0, RspLoc);
+  ValueIDNum RspDefInBlk2(2, 1, RspLoc);
+  ValueIDNum RspPHIInBlk3(3, 0, RspLoc);
+  ValueIDNum RspDefInBlk3(3, 1, RspLoc);
+  ValueIDNum RaxPHIInBlk1(1, 0, RaxLoc);
+  ValueIDNum RaxPHIInBlk2(2, 0, RaxLoc);
+
+  // As ever, test that everything is live-through if there are no defs.
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], LiveInRsp);
+  EXPECT_EQ(InLocs[2][0], LiveInRsp);
+  EXPECT_EQ(InLocs[3][0], LiveInRsp);
+  EXPECT_EQ(InLocs[4][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[3][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[4][0], LiveInRsp);
+
+  // Putting a def in the 'join' block will cause us to have two distinct
+  // PHIs in each loop head, then on entry to the join block.
+  TransferFunc[3].insert({RspLoc, RspDefInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk3);
+  TransferFunc[3].clear();
+
+  // We should get the same behaviour if we put the def in either of the
+  // loop heads -- it should force the other head to be a PHI.
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(InLocs[4][0], RspPHIInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(OutLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspPHIInBlk3);
+  TransferFunc[1].clear();
+
+  // Check symmetry,
+  TransferFunc[2].insert({RspLoc, RspDefInBlk2});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(InLocs[4][0], RspPHIInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk2);
+  EXPECT_EQ(OutLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspPHIInBlk3);
+  TransferFunc[2].clear();
+
+  // Test some scenarios where there _shouldn't_ be any PHIs created at heads.
+  // These are those PHIs are created, but value propagation eliminates them.
+  // For example, lets copy rsp-livein to $rsp inside each loop head, so that
+  // there's no need for a PHI in the join block. Put a def of $rsp in block 3
+  // to force PHIs elsewhere.
+  TransferFunc[0].insert({RaxLoc, LiveInRsp});
+  TransferFunc[1].insert({RspLoc, RaxPHIInBlk1});
+  TransferFunc[2].insert({RspLoc, RaxPHIInBlk2});
+  TransferFunc[3].insert({RspLoc, RspDefInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], LiveInRsp);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk3);
+  TransferFunc[0].clear();
+  TransferFunc[1].clear();
+  TransferFunc[2].clear();
+  TransferFunc[3].clear();
+
+  // In fact, if we eliminate the def in block 3, none of those PHIs are
+  // necessary, as we're just repeatedly copying LiveInRsp into $rsp. They
+  // should all be value propagated out.
+  TransferFunc[0].insert({RaxLoc, LiveInRsp});
+  TransferFunc[1].insert({RspLoc, RaxPHIInBlk1});
+  TransferFunc[2].insert({RspLoc, RaxPHIInBlk2});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], LiveInRsp);
+  EXPECT_EQ(InLocs[2][0], LiveInRsp);
+  EXPECT_EQ(InLocs[3][0], LiveInRsp);
+  EXPECT_EQ(InLocs[4][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[3][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[4][0], LiveInRsp);
+  TransferFunc[0].clear();
+  TransferFunc[1].clear();
+  TransferFunc[2].clear();
+}
+
+TEST_F(InstrRefLDVTest, MLocBadlyNestedLoops) {
+  //           entry
+  //             |
+  //           loop1 -o
+  //             | ^
+  //             | ^
+  //           loop2 -o
+  //             | ^
+  //             | ^
+  //           loop3 -o
+  //             |
+  //            ret
+  //
+  // NB: the loop blocks self-loop, which is a bit too fiddly to draw on
+  // accurately.
+  llvm::Function &F = const_cast<llvm::Function &>(MF->getFunction());
+  auto *BB1 = BasicBlock::Create(Ctx, "entry", &F);
+  auto *BB2 = BasicBlock::Create(Ctx, "loop1", &F);
+  auto *BB3 = BasicBlock::Create(Ctx, "loop2", &F);
+  auto *BB4 = BasicBlock::Create(Ctx, "loop3", &F);
+  auto *BB5 = BasicBlock::Create(Ctx, "ret", &F);
+  IRBuilder<> IRB1(BB1), IRB2(BB2), IRB3(BB3), IRB4(BB4), IRB5(BB5);
+  IRB1.CreateBr(BB2);
+  IRB2.CreateBr(BB3);
+  IRB3.CreateBr(BB4);
+  IRB4.CreateBr(BB5);
+  IRB5.CreateRetVoid();
+  MBB1 = MF->CreateMachineBasicBlock(BB1);
+  MF->insert(MF->end(), MBB1);
+  MBB2 = MF->CreateMachineBasicBlock(BB2);
+  MF->insert(MF->end(), MBB2);
+  MBB3 = MF->CreateMachineBasicBlock(BB3);
+  MF->insert(MF->end(), MBB3);
+  MBB4 = MF->CreateMachineBasicBlock(BB4);
+  MF->insert(MF->end(), MBB4);
+  MBB5 = MF->CreateMachineBasicBlock(BB5);
+  MF->insert(MF->end(), MBB5);
+  MBB1->addSuccessor(MBB2);
+  MBB2->addSuccessor(MBB2);
+  MBB2->addSuccessor(MBB3);
+  MBB3->addSuccessor(MBB2);
+  MBB3->addSuccessor(MBB3);
+  MBB3->addSuccessor(MBB4);
+  MBB4->addSuccessor(MBB3);
+  MBB4->addSuccessor(MBB4);
+  MBB4->addSuccessor(MBB5);
+  MF->RenumberBlocks();
+
+  setupLDVObj();
+
+  ASSERT_TRUE(MTracker->getNumLocs() == 1);
+  LocIdx RspLoc(0);
+  Register RAX = getRegByName("RAX");
+  LocIdx RaxLoc = MTracker->lookupOrTrackRegister(RAX);
+
+  ValueIDNum InLocs[5][2], OutLocs[5][2];
+  ValueIDNum *InLocsPtr[5] = {InLocs[0], InLocs[1], InLocs[2], InLocs[3],
+                              InLocs[4]};
+  ValueIDNum *OutLocsPtr[5] = {OutLocs[0], OutLocs[1], OutLocs[2], OutLocs[3],
+                               OutLocs[4]};
+
+  SmallVector<MLocTransferMap, 1> TransferFunc;
+  TransferFunc.resize(5);
+
+  ValueIDNum LiveInRsp(0, 0, RspLoc);
+  ValueIDNum RspPHIInBlk1(1, 0, RspLoc);
+  ValueIDNum RspDefInBlk1(1, 1, RspLoc);
+  ValueIDNum RspPHIInBlk2(2, 0, RspLoc);
+  ValueIDNum RspPHIInBlk3(3, 0, RspLoc);
+  ValueIDNum RspDefInBlk3(3, 1, RspLoc);
+  ValueIDNum LiveInRax(0, 0, RaxLoc);
+  ValueIDNum RaxPHIInBlk3(3, 0, RaxLoc);
+
+  // As ever, test that everything is live-through if there are no defs.
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], LiveInRsp);
+  EXPECT_EQ(InLocs[2][0], LiveInRsp);
+  EXPECT_EQ(InLocs[3][0], LiveInRsp);
+  EXPECT_EQ(InLocs[4][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[3][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[4][0], LiveInRsp);
+
+  // A def in loop3 should cause PHIs in every loop block: they're all
+  // reachable from each other.
+  TransferFunc[3].insert({RspLoc, RspDefInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk3);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk3);
+  TransferFunc[3].clear();
+
+  // A def in loop1 should cause a PHI in loop1, but not the other blocks.
+  // loop2 and loop3 are dominated by the def in loop1, so they should have
+  // that value live-through.
+  TransferFunc[1].insert({RspLoc, RspDefInBlk1});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspDefInBlk1);
+  EXPECT_EQ(InLocs[3][0], RspDefInBlk1);
+  EXPECT_EQ(InLocs[4][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[3][0], RspDefInBlk1);
+  EXPECT_EQ(OutLocs[4][0], RspDefInBlk1);
+  TransferFunc[1].clear();
+
+  // As with earlier tricks: copy $rsp to $rax in the entry block, then $rax
+  // to $rsp in block 3. The only def of $rsp is simply copying the same value
+  // back into itself, and the value of $rsp is LiveInRsp all the way through.
+  // PHIs should be created, then value-propagated away...  however this
+  // doesn't work in practice.
+  // Consider the entry to loop3: we can determine that there's an incoming
+  // PHI value from loop2, and LiveInRsp from the self-loop. This would still
+  // justify having a PHI on entry to loop3. The only way to completely
+  // value-propagate these PHIs away would be to speculatively explore what
+  // PHIs could be eliminated and what that would lead to; which is
+  // combinatorially complex.
+  // Happily:
+  //  a) In this scenario, we always have a tracked location for LiveInRsp
+  //     anyway, so there's no loss in availability,
+  //  b) Only DBG_PHIs of a register would be vunlerable to this scenario, and
+  //     even then only if a true PHI became a DBG_PHI and was then optimised
+  //     through branch folding to no longer be at a CFG join,
+  //  c) The register allocator can spot this kind of redundant COPY easily,
+  //     and eliminate it.
+  //
+  // This unit test left in as a reference for the limitations of this
+  // approach. PHIs will be left in $rsp on entry to each block.
+  TransferFunc[0].insert({RaxLoc, LiveInRsp});
+  TransferFunc[3].insert({RspLoc, RaxPHIInBlk3});
+  initValueArray(InLocsPtr, 5, 2);
+  initValueArray(OutLocsPtr, 5, 2);
+  buildMLocValueMap(InLocsPtr, OutLocsPtr, TransferFunc);
+  EXPECT_EQ(InLocs[0][0], LiveInRsp);
+  EXPECT_EQ(InLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(InLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(InLocs[3][0], RspPHIInBlk3);
+  EXPECT_EQ(InLocs[4][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][0], RspPHIInBlk1);
+  EXPECT_EQ(OutLocs[2][0], RspPHIInBlk2);
+  EXPECT_EQ(OutLocs[3][0], LiveInRsp);
+  EXPECT_EQ(OutLocs[4][0], LiveInRsp);
+  // Check $rax's value. It should have $rsps value from the entry block
+  // onwards.
+  EXPECT_EQ(InLocs[0][1], LiveInRax);
+  EXPECT_EQ(InLocs[1][1], LiveInRsp);
+  EXPECT_EQ(InLocs[2][1], LiveInRsp);
+  EXPECT_EQ(InLocs[3][1], LiveInRsp);
+  EXPECT_EQ(InLocs[4][1], LiveInRsp);
+  EXPECT_EQ(OutLocs[0][1], LiveInRsp);
+  EXPECT_EQ(OutLocs[1][1], LiveInRsp);
+  EXPECT_EQ(OutLocs[2][1], LiveInRsp);
+  EXPECT_EQ(OutLocs[3][1], LiveInRsp);
+  EXPECT_EQ(OutLocs[4][1], LiveInRsp);
+}