[WebAssembly][LiveDebugValues] Handle target index defs

[aheejin]

Summary

This adds the missing handling for defs for target index operands, as is
already done for registers.

There are two kinds of target indices: local indices and stack operands.

• Locals are something similar to registers in Wasm-land. For local indices, we can check for local-defining instructions (local.set or local.tee).

• Wasm is a stack machine, so we have values in certain Wasm value stack location, which change when Wasm instructions produce or consume values. So basically any value-producing instrucion, i.e., instruction with defs, can change values in the Wasm stack. But I think we don't need to worry about this here, because WebAssemblyDebugFixup, which runs right before this analysis, makes sure to insert terminating DBG_VALUE $noreg instructions whenever a stack value gets popped. After WebAssemblyDebugFixup, there shouldn't be any DBG_VALUEs for stack operands that don't have a terminating DBG_VALUE $noreg within the same BB.

So this CL only works on DBG_VALUEs for locals. When we encounter a
local.set or local.tee instructions, we delete DBG_VALUEs for
those target index locations from the open range set, so they will not
be availble in OutLocs. For example,

bb.0:
  successors: %bb.1
  DBG_VALUE target-index(wasm-local) + 2, $noreg, "var", ...
  ...
  local.set 2 ...

bb.1:
; predecessors: %bb.0
  ; We shouldn't add `DBG_VALUE target (wasm-local) + 2 here because
  ; it was killed by 'local.set' in bb.0

After disabling register coalescing at -O1, the average PC ranges covered
for Emscripten core benchmarks is currently 20.6% in the LLVM
tot. After applying D138943 and this CL, the coverage goes up to 57%.
This also enables LiveDebugValues analysis in the Wasm pipeline by
default.

[aheejin]

Remove debug printing

[aheejin]

@jmorse I have a separate question again, if you don't mind 😅

When there's a register def, this pass doesn't insert DBG_VALUE $noreg right after it to terminate its impact; this just deletes its info from OpenRanges so that it doesn't get propagated to its OutLocs. I did the same for our Wasm target indices, because it looks that's the way this pass works. But do you know why this pass was designed that way? I think adding DBG_VALUE $noreg right after the new def would make debug info more precise within the BB.

And another thing, I didn't add anything to copy handling because Wasm doesn't have copy instructions. (This COPY instruction is a pseudo instruction and should have been lowered into other instructions at this phase.)

[dschuff]

In D140373#4008491, @aheejin wrote:

And another thing, I didn't add anything to copy handling because Wasm doesn't have copy instructions. (This COPY instruction is a pseudo instruction and should have been lowered into other instructions at this phase.)

Regarding the latter bit, I think that's fine for wasm. My understanding is that COPY instructions are mostly the product of register allocation (and a few other random special cases in backends) but because wasm doesn't do register allocation the way other targets do (and in any case the set of locals is basically an infinite virtual register file we leave the part of regalloc that typically requires copies to the engine).
I think a similar explanation applies to the discussion on D138943 about memory because Wasm likewise has no spills (no callee-saved or caller-saved registers, etc)

[dschuff]

llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp

1657

llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.cpp

208

Technically this function is "wrong" because stack value defs and global.sets are also target index defs. Your commit message explains why we only care about local defs and I agree that's the behavior we want; but it is a bit confusing.
One option would be to implement the "correct" behavior here (i.e. return true for all of those), and then filter out the non-local ones in the caller. But that would be more code and slower for no good reason. The other option would be to change the name of the function to better reflect what we want it to do, but because it's technically target-independent I don't know of a good name. Another option would be to keep the name and behavior the same but put a big comment somewhere with an explanation. None of those sound particularly great, or particularly catastrophic. Thoughts?

[aheejin]

Fix a typo

[aheejin]

llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.cpp

208

Doing something in the caller doesn't work because VarLocBasedImpl.cpp is under lib/CodeGen and it cannot access WebAssembly:: namespace or any of its instruction names such as WebAssembly::LOCAL_SET there. We need lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCTargetDesc.h to access them, and this file is only accessible within lib/Target/WebAassembly. This the reason I added this utility function here in TargetInstrInfo in the first place.

Also, implementing the "correct" behavior means returning true for all instructions with a def, because they produce stack values, which I guess is more than half of Wasm instructions anyway so there's no point of doing that. Also what we want here is a target index and an operand, which only local.*** instructions have.

That leaves us only with the option of changing the function name or putting a big comment blob or both. Do you have any suggestions for a better name?

[aheejin]

Rebase onto main

[dschuff]

llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.cpp

208

Yeah it's kind of weird because VarLocBasedImpl is technically target-neutral but has wasm-specific logic; I didn't think about not even being able to access the namespace and instruction names.
But you did just call out the fundamental thing that separates the local-defs we care about from stack defs is that they have explicit operands. Maybe it's enough to call it isExplicitTargetIndexDef and for the blob comment to say that the intention is for this to cover explicit TargetIndex defs that can be tracked by their offset. (It's probably useful for the comment to say a bit more than just that, i.e. maybe also briefly describe the actual use of the function).

[jmorse]

Thanks for assurance about the stack-popping being addressed elsewhere -- it might be worth putting a comment in D138943 next to the definition of struct WasmLoc about that being a deliberate non-objective, to enlighten future readers.

The LiveDebugValues bits look fine -- it's quite a small diff,

And another thing, I didn't add anything to copy handling because Wasm doesn't have copy instructions. (This COPY instruction is a pseudo instruction and should have been lowered into other instructions at this phase.)

If there's no movement of values with COPYs / spills / restores in webassembly then that's great, no extra logic is needed. Having to follow multiple machine locations around is an artefact of the limited space available on physical machines.

A note on the performance characteristics -- the set of "live" variable locations that are being tracked are kept in a sparse bitvector, one bit per {Variable,MachineLocation} pair. Some large projects can have hundreds (or thousands) of variables for each machine location, which creates a lot of overhead when updating the sparse bitvector. So, a couple of years ago the sparse bitvector became a coalescing bitvector that groups bits together, and the numerical space of the locations were grouped by machine location to make them coalesce better, improving compile-time performance. There's more explanation at D74986 and that stack.

As you're placing all target index locations in the kWasmLocation location, for large projects there's a risk that the tracking of variable locations won't compress very well, because the bit numbers given for them are randomly spread between variables and target indexes. If you gave each target index it's own u32_location_t, they would group together better and compress more.

This isn't actually anything to care about unless you're dealing with absolutely massive functions with lots of inlining, but I figured I'd explain in case you see poor compile-time performance in this area in the future.

When there's a register def, this pass doesn't insert DBG_VALUE $noreg right after it to terminate its impact; this just deletes its info from OpenRanges so that it doesn't get propagated to its OutLocs. I did the same for our Wasm target indices, because it looks that's the way this pass works. But do you know why this pass was designed that way? I think adding DBG_VALUE $noreg right after the new def would make debug info more precise within the BB.

That's correct -- as it stands, there's a certain amount of redundancy because the DbgEntityHistoryCalculatorclass has to re-compute that information, even though LiveDebugValues already had to do it. I don't really know how things ended up being this way, my best guess is that other parts of LLVM used to propagate variable locations between blocks and did so incorrectly, and DbgEntityHistoryCalculator was needed to find out where variable ranges ended. I think LiveDebugValues was added in 2015, to more accurately propagate variable locations, but DbgEntityHistoryCalculator wasn't removed or re-designed as it was still doing useful things (i.e., building location lists), and there was no reason to change it. So, I think the redundancy is just a case of technical debt.

A more ideal design would be one where LiveDebugValues was an analysis rather than a pass, and it directly produced variable location ranges instead of DBG_VALUE instructions. In functions with lots of inlining and lots of blocks, LiveDebugValues effectively produces the cross product of all locations for all variables in all blocks in DBG_VALUEs, which can be a massive number of instructions, and is really inefficient! I tried a while back to convert it to an analysis, but doing that changed the line tables for some reason that I couldn't pin down X_X...

llvm/include/llvm/CodeGen/TargetInstrInfo.h

2064–2065

Nit: could I suggest writing "target index" as "TargetIndex" to make it clear it's a noun, and an operand kind. That makes it very clear to readers that it's a specific data structure rather than some higher level concept applicable to all targets.

llvm/test/DebugInfo/WebAssembly/live-debug-values.mir

199

[aheejin]

Address comments + move code a bit

llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.cpp

208

• Changed the function name to isExplicitTargetIndexDef.
• Expanded the comment a little more in TargetInstrInfo.h, and also added more detailed comments in this file (WebAssemblyInstrInfo.cpp), where the WebAssemblyInstrInfo::isTargetIndexDef is defined.
• In theory we need to add global.set here too, but we don't have global indices at this point because they are relocatable and we address them by names until linking, so we don't have 'offsets' (which are used to store local/global indices) to deal with in LiveDebugValues. And AFAIK we don't really store values that have debug info in wasm globals anyway.

[aheejin]

Add a TODO on kWasmLocation

[aheejin]

@jmorse

In D140373#4016029, @jmorse wrote:

Thanks for assurance about the stack-popping being addressed elsewhere -- it might be worth putting a comment in D138943 next to the definition of struct WasmLoc about that being a deliberate non-objective, to enlighten future readers.

I tried that but I was not sure whether it was the best location. I wrote about it in https://reviews.llvm.org/D138943#4027424.

A note on the performance characteristics -- the set of "live" variable locations that are being tracked are kept in a sparse bitvector, one bit per {Variable,MachineLocation} pair. Some large projects can have hundreds (or thousands) of variables for each machine location, which creates a lot of overhead when updating the sparse bitvector. So, a couple of years ago the sparse bitvector became a coalescing bitvector that groups bits together, and the numerical space of the locations were grouped by machine location to make them coalesce better, improving compile-time performance. There's more explanation at D74986 and that stack.

As you're placing all target index locations in the kWasmLocation location, for large projects there's a risk that the tracking of variable locations won't compress very well, because the bit numbers given for them are randomly spread between variables and target indexes. If you gave each target index it's own u32_location_t, they would group together better and compress more.

This isn't actually anything to care about unless you're dealing with absolutely massive functions with lots of inlining, but I figured I'd explain in case you see poor compile-time performance in this area in the future.

Thanks a lot for the info! I tried the current patch with various Emscripten benchmarks, which includes sizable programs, and I didn't notice pathological slowdown for them. But this is certainly possible with larger programs, and I'll probably do this as a followup if the need arises. I added a TODO describing this possibility above the definition of kWasmLocation.

When there's a register def, this pass doesn't insert DBG_VALUE $noreg right after it to terminate its impact; this just deletes its info from OpenRanges so that it doesn't get propagated to its OutLocs. I did the same for our Wasm target indices, because it looks that's the way this pass works. But do you know why this pass was designed that way? I think adding DBG_VALUE $noreg right after the new def would make debug info more precise within the BB.

That's correct -- as it stands, there's a certain amount of redundancy because the DbgEntityHistoryCalculatorclass has to re-compute that information, even though LiveDebugValues already had to do it. I don't really know how things ended up being this way, my best guess is that other parts of LLVM used to propagate variable locations between blocks and did so incorrectly, and DbgEntityHistoryCalculator was needed to find out where variable ranges ended. I think LiveDebugValues was added in 2015, to more accurately propagate variable locations, but DbgEntityHistoryCalculator wasn't removed or re-designed as it was still doing useful things (i.e., building location lists), and there was no reason to change it. So, I think the redundancy is just a case of technical debt.

A more ideal design would be one where LiveDebugValues was an analysis rather than a pass, and it directly produced variable location ranges instead of DBG_VALUE instructions. In functions with lots of inlining and lots of blocks, LiveDebugValues effectively produces the cross product of all locations for all variables in all blocks in DBG_VALUEs, which can be a massive number of instructions, and is really inefficient! I tried a while back to convert it to an analysis, but doing that changed the line tables for some reason that I couldn't pin down X_X...

Thank you for the info and the history! Yeah, I also think this would be better as an analysis. That way, subscribers to this analysis can choose the granularity with which they want to place DBG_VALUE or DBG_VALUE $noreg instructions themselves, depending on the optimization level or and function size.

[dschuff]

It looks like all the feedback has been address for now, so LGTM.
We can always update the comments if we have more discussion.

[aheejin]

@jmorse I would like to land this soon because we have waiting users. If you have remaining comments, I'll address them as follow-ups. Thank you!

[jmorse]

LGTM

llvm/include/llvm/CodeGen/TargetInstrInfo.h

public:
   }
 
   /// Return the uniformity behavior of the given instruction.
   virtual InstructionUniformity
   getInstructionUniformity(const MachineInstr &MI) const {
     return InstructionUniformity::Default;
   }
 
+  /// Returns true if the given \p MI defines a TargetIndex operand that can be
+  /// tracked by their offset, can have values, and can have debug info
+  /// associated with it. If so, sets \p Index and \p Offset of the target index
+  /// operand.
+  virtual bool isExplicitTargetIndexDef(const MachineInstr &MI, int &Index,
+                                        int64_t &Offset) const {
+    return false;
+  }
+
 private:
   mutable std::unique_ptr<MIRFormatter> Formatter;
   unsigned CallFrameSetupOpcode, CallFrameDestroyOpcode;
   unsigned CatchRetOpcode;
   unsigned ReturnOpcode;
 };
 
 /// Provide DenseMapInfo for TargetInstrInfo::RegSubRegPair.

llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp

 //===- VarLocBasedImpl.cpp - Tracking Debug Value MIs with VarLoc class----===//
 //
 // 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 VarLocBasedImpl.cpp
 ///
 /// LiveDebugValues is an optimistic "available expressions" dataflow
 /// algorithm. The set of expressions is the set of machine locations
-/// (registers, spill slots, constants) that a variable fragment might be
-/// located, qualified by a DIExpression and indirect-ness flag, while each
-/// variable is identified by a DebugVariable object. The availability of an
-/// expression begins when a DBG_VALUE instruction specifies the location of a
-/// DebugVariable, and continues until that location is clobbered or
-/// re-specified by a different DBG_VALUE for the same DebugVariable.
+/// (registers, spill slots, constants, and target indices) that a variable
+/// fragment might be located, qualified by a DIExpression and indirect-ness
+/// flag, while each variable is identified by a DebugVariable object. The
+/// availability of an expression begins when a DBG_VALUE instruction specifies
+/// the location of a DebugVariable, and continues until that location is
+/// clobbered or re-specified by a different DBG_VALUE for the same
+/// DebugVariable.
 ///
 /// The output of LiveDebugValues is additional DBG_VALUE instructions,
 /// placed to extend variable locations as far they're available. This file
 /// and the VarLocBasedLDV class is an implementation that explicitly tracks
 /// locations, using the VarLoc class.
 ///
 /// The canonical "available expressions" problem doesn't have expression
 /// clobbering, instead when a variable is re-assigned, any expressions using
struct LocIndex {
   /// SpillLocKind.
   static constexpr u32_location_t kSpillLocation = kFirstInvalidRegLocation;
 
   /// A special location reserved for VarLocs of kind EntryValueBackupKind and
   /// EntryValueCopyBackupKind.
   static constexpr u32_location_t kEntryValueBackupLocation =
       kFirstInvalidRegLocation + 1;
 
+  /// A special location reserved for VarLocs with locations of kind
+  /// WasmLocKind.
+  /// TODO Placing all Wasm target index locations in this single kWasmLocation
+  /// may cause slowdown in compilation time in very large functions. Consider
+  /// giving a each target index/offset pair its own u32_location_t if this
+  /// becomes a problem.
+  static constexpr u32_location_t kWasmLocation = kFirstInvalidRegLocation + 2;
+
   LocIndex(u32_location_t Location, u32_index_t Index)
       : Location(Location), Index(Index) {}
 
   uint64_t getAsRawInteger() const {
     return (static_cast<uint64_t>(Location) << 32) | Index;
   }
 
   template<typename IntT> static LocIndex fromRawInteger(IntT ID) {
struct SpillLoc {
       }
       bool operator!=(const SpillLoc &Other) const {
         return !(*this == Other);
       }
     };
 
     // Target indices used for wasm-specific locations.
     struct WasmLoc {
-      int Index; // One of TargetIndex values defined in WebAssembly.h
+      // One of TargetIndex values defined in WebAssembly.h. We deal with
+      // local-related TargetIndex in this analysis (TI_LOCAL and
+      // TI_LOCAL_INDIRECT). Stack operands (TI_OPERAND_STACK) will be handled
+      // separately WebAssemblyDebugFixup pass, and we don't associate debug
+      // info with values in global operands (TI_GLOBAL_RELOC) at the moment.
+      int Index;
       int64_t Offset;
       bool operator==(const WasmLoc &Other) const {
         return Index == Other.Index && Offset == Other.Offset;
       }
       bool operator!=(const WasmLoc &Other) const { return !(*this == Other); }
     };
 
     /// Identity of the variable at this location.
struct VarLoc {
     unsigned getSpillLocIdx(SpillLoc SpillLocation) const {
       for (unsigned Idx = 0; Idx < Locs.size(); ++Idx)
         if (Locs[Idx].Kind == MachineLocKind::SpillLocKind &&
             Locs[Idx].Value.SpillLocation == SpillLocation)
           return Idx;
       llvm_unreachable("Could not find given SpillLoc in Locs");
     }
 
+    bool containsWasmLocs() const {
+      return any_of(Locs, [](VarLoc::MachineLoc ML) {
+        return ML.Kind == VarLoc::MachineLocKind::WasmLocKind;
+      });
+    }
+
+    /// If this variable is described in whole or part by \p WasmLocation,
+    /// return true.
+    bool usesWasmLoc(WasmLoc WasmLocation) const {
+      MachineLoc WasmML;
+      WasmML.Kind = MachineLocKind::WasmLocKind;
+      WasmML.Value.WasmLocation = WasmLocation;
+      return is_contained(Locs, WasmML);
+    }
+
     /// Determine whether the lexical scope of this value's debug location
     /// dominates MBB.
     bool dominates(LexicalScopes &LS, MachineBasicBlock &MBB) const {
       return LS.dominates(MI.getDebugLoc().get(), &MBB);
     }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
     // TRI and TII can be null.
LocIndices insert(const VarLoc &VL) {
       // always have an index at the universal location index as the last index.
       if (VL.EVKind == VarLoc::EntryValueLocKind::NonEntryValueKind) {
         VL.getDescribingRegs(Locations);
         assert(all_of(Locations,
                       [](auto RegNo) {
                         return RegNo < LocIndex::kFirstInvalidRegLocation;
                       }) &&
                "Physreg out of range?");
-        if (VL.containsSpillLocs()) {
-          LocIndex::u32_location_t Loc = LocIndex::kSpillLocation;
-          Locations.push_back(Loc);
-        }
+        if (VL.containsSpillLocs())
+          Locations.push_back(LocIndex::kSpillLocation);
+        if (VL.containsWasmLocs())
+          Locations.push_back(LocIndex::kWasmLocation);
       } else if (VL.EVKind != VarLoc::EntryValueLocKind::EntryValueKind) {
         LocIndex::u32_location_t Loc = LocIndex::kEntryValueBackupLocation;
         Locations.push_back(Loc);
       }
       Locations.push_back(LocIndex::kUniversalLocation);
       for (LocIndex::u32_location_t Location : Locations) {
         auto &Vars = Loc2Vars[Location];
         Indices.push_back(
public:
     }
 
     /// Get all set IDs for VarLocs of EVKind EntryValueBackupKind or
     /// EntryValueCopyBackupKind.
     auto getEntryValueBackupVarLocs() const {
       return LocIndex::indexRangeForLocation(
           getVarLocs(), LocIndex::kEntryValueBackupLocation);
     }
+
+    /// Get all set IDs for VarLocs with MLs of kind WasmLocKind.
+    auto getWasmVarLocs() const {
+      return LocIndex::indexRangeForLocation(getVarLocs(),
+                                             LocIndex::kWasmLocation);
+    }
   };
 
   /// Collect all VarLoc IDs from \p CollectFrom for VarLocs with MLs of kind
   /// RegisterKind which are located in any reg in \p Regs. The IDs for each
   /// VarLoc correspond to entries in the universal location bucket, which every
   /// VarLoc has exactly 1 entry for. Insert collected IDs into \p Collected.
   static void collectIDsForRegs(VarLocsInRange &Collected,
                                 const DefinedRegsSet &Regs,
void recordEntryValue(const MachineInstr &MI,
                         const DefinedRegsSet &DefinedRegs,
                         OpenRangesSet &OpenRanges, VarLocMap &VarLocIDs);
   void transferRegisterCopy(MachineInstr &MI, OpenRangesSet &OpenRanges,
                             VarLocMap &VarLocIDs, TransferMap &Transfers);
   void transferRegisterDef(MachineInstr &MI, OpenRangesSet &OpenRanges,
                            VarLocMap &VarLocIDs,
                            InstToEntryLocMap &EntryValTransfers,
                            RegDefToInstMap &RegSetInstrs);
+  void transferWasmDef(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                       VarLocMap &VarLocIDs);
   bool transferTerminator(MachineBasicBlock *MBB, OpenRangesSet &OpenRanges,
                           VarLocInMBB &OutLocs, const VarLocMap &VarLocIDs);
 
   void process(MachineInstr &MI, OpenRangesSet &OpenRanges,
                VarLocMap &VarLocIDs, TransferMap &Transfers,
                InstToEntryLocMap &EntryValTransfers,
                RegDefToInstMap &RegSetInstrs);
 
void VarLocBasedLDV::transferRegisterDef(MachineInstr &MI,
 
   if (TPC) {
     auto &TM = TPC->getTM<TargetMachine>();
     if (TM.Options.ShouldEmitDebugEntryValues())
       emitEntryValues(MI, OpenRanges, VarLocIDs, EntryValTransfers, KillSet);
   }
 }
 
+void VarLocBasedLDV::transferWasmDef(MachineInstr &MI,
+                                     OpenRangesSet &OpenRanges,
+                                     VarLocMap &VarLocIDs) {
+  // If this is not a Wasm local.set or local.tee, which sets local values,
+  // return.
+  int Index;
+  int64_t Offset;
+  if (!TII->isExplicitTargetIndexDef(MI, Index, Offset))
+    return;
+
+  // Find the target indices killed by MI, and delete those variable locations
+  // from the open range.
+  VarLocsInRange KillSet;
+  VarLoc::WasmLoc Loc{Index, Offset};
+  for (uint64_t ID : OpenRanges.getWasmVarLocs()) {
+    LocIndex Idx = LocIndex::fromRawInteger(ID);
+    const VarLoc &VL = VarLocIDs[Idx];
+    assert(VL.containsWasmLocs() && "Broken VarLocSet?");
+    if (VL.usesWasmLoc(Loc))
+      KillSet.insert(ID);
+  }
+  OpenRanges.erase(KillSet, VarLocIDs, LocIndex::kWasmLocation);
+}
+
 bool VarLocBasedLDV::isSpillInstruction(const MachineInstr &MI,
                                          MachineFunction *MF) {
   // TODO: Handle multiple stores folded into one.
   if (!MI.hasOneMemOperand())
     return false;
 
   if (!MI.getSpillSize(TII) && !MI.getFoldedSpillSize(TII))
     return false; // This is not a spill instruction, since no valid size was
void VarLocBasedLDV::process(MachineInstr &MI, OpenRangesSet &OpenRanges,
                              InstToEntryLocMap &EntryValTransfers,
                              RegDefToInstMap &RegSetInstrs) {
   if (!MI.isDebugInstr())
     LastNonDbgMI = &MI;
   transferDebugValue(MI, OpenRanges, VarLocIDs, EntryValTransfers,
                      RegSetInstrs);
   transferRegisterDef(MI, OpenRanges, VarLocIDs, EntryValTransfers,
                       RegSetInstrs);
+  transferWasmDef(MI, OpenRanges, VarLocIDs);
   transferRegisterCopy(MI, OpenRanges, VarLocIDs, Transfers);
   transferSpillOrRestoreInst(MI, OpenRanges, VarLocIDs, Transfers);
 }
 
 /// This routine joins the analysis results of all incoming edges in @MBB by
 /// inserting a new DBG_VALUE instruction at the start of the @MBB - if the same
 /// source variable in all the predecessors of @MBB reside in the same location.
 bool VarLocBasedLDV::join(

llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCTargetDesc.h

inline bool isCatch(unsigned Opc) {
   case WebAssembly::CATCH_ALL:
   case WebAssembly::CATCH_ALL_S:
     return true;
   default:
     return false;
   }
 }
 
+inline bool isLocalGet(unsigned Opc) {
+  switch (Opc) {
+  case WebAssembly::LOCAL_GET_I32:
+  case WebAssembly::LOCAL_GET_I32_S:
+  case WebAssembly::LOCAL_GET_I64:
+  case WebAssembly::LOCAL_GET_I64_S:
+  case WebAssembly::LOCAL_GET_F32:
+  case WebAssembly::LOCAL_GET_F32_S:
+  case WebAssembly::LOCAL_GET_F64:
+  case WebAssembly::LOCAL_GET_F64_S:
+  case WebAssembly::LOCAL_GET_V128:
+  case WebAssembly::LOCAL_GET_V128_S:
+  case WebAssembly::LOCAL_GET_FUNCREF:
+  case WebAssembly::LOCAL_GET_FUNCREF_S:
+  case WebAssembly::LOCAL_GET_EXTERNREF:
+  case WebAssembly::LOCAL_GET_EXTERNREF_S:
+    return true;
+  default:
+    return false;
+  }
+}
+
+inline bool isLocalSet(unsigned Opc) {
+  switch (Opc) {
+  case WebAssembly::LOCAL_SET_I32:
+  case WebAssembly::LOCAL_SET_I32_S:
+  case WebAssembly::LOCAL_SET_I64:
+  case WebAssembly::LOCAL_SET_I64_S:
+  case WebAssembly::LOCAL_SET_F32:
+  case WebAssembly::LOCAL_SET_F32_S:
+  case WebAssembly::LOCAL_SET_F64:
+  case WebAssembly::LOCAL_SET_F64_S:
+  case WebAssembly::LOCAL_SET_V128:
+  case WebAssembly::LOCAL_SET_V128_S:
+  case WebAssembly::LOCAL_SET_FUNCREF:
+  case WebAssembly::LOCAL_SET_FUNCREF_S:
+  case WebAssembly::LOCAL_SET_EXTERNREF:
+  case WebAssembly::LOCAL_SET_EXTERNREF_S:
+    return true;
+  default:
+    return false;
+  }
+}
+
+inline bool isLocalTee(unsigned Opc) {
+  switch (Opc) {
+  case WebAssembly::LOCAL_TEE_I32:
+  case WebAssembly::LOCAL_TEE_I32_S:
+  case WebAssembly::LOCAL_TEE_I64:
+  case WebAssembly::LOCAL_TEE_I64_S:
+  case WebAssembly::LOCAL_TEE_F32:
+  case WebAssembly::LOCAL_TEE_F32_S:
+  case WebAssembly::LOCAL_TEE_F64:
+  case WebAssembly::LOCAL_TEE_F64_S:
+  case WebAssembly::LOCAL_TEE_V128:
+  case WebAssembly::LOCAL_TEE_V128_S:
+  case WebAssembly::LOCAL_TEE_FUNCREF:
+  case WebAssembly::LOCAL_TEE_FUNCREF_S:
+  case WebAssembly::LOCAL_TEE_EXTERNREF:
+  case WebAssembly::LOCAL_TEE_EXTERNREF_S:
+    return true;
+  default:
+    return false;
+  }
+}
+
 } // end namespace WebAssembly
 } // end namespace llvm
 
 #endif

llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.h

unsigned insertBranch(MachineBasicBlock &MBB, MachineBasicBlock *TBB,
                         int *BytesAdded = nullptr) const override;
   bool
   reverseBranchCondition(SmallVectorImpl<MachineOperand> &Cond) const override;
 
   ArrayRef<std::pair<int, const char *>>
   getSerializableTargetIndices() const override;
 
   const MachineOperand &getCalleeOperand(const MachineInstr &MI) const override;
+
+  bool isExplicitTargetIndexDef(const MachineInstr &MI, int &Index,
+                                int64_t &Offset) const override;
 };
 
 } // end namespace llvm
 
 #endif

llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.cpp

static const std::pair<int, const char *> TargetIndices[] = {
       {WebAssembly::TI_LOCAL_INDIRECT, "wasm-local-indirect"}};
   return ArrayRef(TargetIndices);
 }
 
 const MachineOperand &
 WebAssemblyInstrInfo::getCalleeOperand(const MachineInstr &MI) const {
   return WebAssembly::getCalleeOp(MI);
 }
+
+// This returns true when the instruction defines a value of a TargetIndex
+// operand that can be tracked by offsets. For Wasm, this returns true for only
+// local.set/local.tees. This is currently used by LiveDebugValues analysis.
+//
+// These are not included:
+// - In theory we need to add global.set here too, but we don't have global
+//   indices at this point because they are relocatable and we address them by
+//   names until linking, so we don't have 'offsets' (which are used to store
+//   local/global indices) to deal with in LiveDebugValues. And we don't
+//   associate debug info in values in globals anyway.
+// - All other value-producing instructions, i.e. instructions with defs, can
+//   define values in the Wasm stack, which is represented by TI_OPERAND_STACK
+//   TargetIndex. But they don't have offset info within the instruction itself,
+//   and debug info analysis for them is handled separately in
+//   WebAssemblyDebugFixup pass, so we don't worry about them here.
+bool WebAssemblyInstrInfo::isExplicitTargetIndexDef(const MachineInstr &MI,
+                                                    int &Index,
+                                                    int64_t &Offset) const {
+  unsigned Opc = MI.getOpcode();
+  if (WebAssembly::isLocalSet(Opc) || WebAssembly::isLocalTee(Opc)) {
+    Index = WebAssembly::TI_LOCAL;
+    Offset = MI.explicit_uses().begin()->getImm();
+    return true;
+  }
+  return false;
+}

llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

void WebAssemblyPassConfig::addPostRegAlloc() {
 
   // These functions all require the NoVRegs property.
   disablePass(&MachineLateInstrsCleanupID);
   disablePass(&MachineCopyPropagationID);
   disablePass(&PostRAMachineSinkingID);
   disablePass(&PostRASchedulerID);
   disablePass(&FuncletLayoutID);
   disablePass(&StackMapLivenessID);
-  disablePass(&LiveDebugValuesID);
   disablePass(&PatchableFunctionID);
   disablePass(&ShrinkWrapID);
 
   // This pass hurts code size for wasm because it can generate irreducible
   // control flow.
   disablePass(&MachineBlockPlacementID);
 
   TargetPassConfig::addPostRegAlloc();

llvm/test/DebugInfo/WebAssembly/live-debug-values.mir

define void @test_target_indices() !dbg !14 {
     call void @llvm.dbg.value(metadata i32 0, metadata !15, metadata !DIExpression()), !dbg !16
     call void @llvm.dbg.value(metadata i32 0, metadata !17, metadata !DIExpression()), !dbg !16
     call void @llvm.dbg.value(metadata i32 0, metadata !18, metadata !DIExpression()), !dbg !16
     call void @llvm.dbg.value(metadata i32 0, metadata !19, metadata !DIExpression()), !dbg !16
     call void @llvm.dbg.value(metadata i32 0, metadata !20, metadata !DIExpression()), !dbg !16
     ret void
   }
 
+  define void @test_target_index_defs() !dbg !21 {
+    call void @llvm.dbg.value(metadata i32 0, metadata !22, metadata !DIExpression()), !dbg !23
+    call void @llvm.dbg.value(metadata i32 0, metadata !24, metadata !DIExpression()), !dbg !23
+    call void @llvm.dbg.value(metadata i32 0, metadata !25, metadata !DIExpression()), !dbg !23
+    ret void
+  }
+
   declare void @llvm.dbg.value(metadata, metadata, metadata)
 
   !llvm.dbg.cu = !{!0}
   !llvm.module.flags = !{!2, !3, !4}
 
   !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, emissionKind: FullDebug)
   !1 = !DIFile(filename: "test.c", directory: "")
   !2 = !{i32 7, !"Dwarf Version", i32 5}
--- |
   !17 = !DILocalVariable(name: "var1", scope: !14, file: !1, line: 11, type: !8)
   ; CHECK: ![[T1_VAR1:[0-9]+]] = !DILocalVariable(name: "var1", scope: ![[T1_SP]]
   !18 = !DILocalVariable(name: "var2", scope: !14, file: !1, line: 11, type: !8)
   ; CHECK: ![[T1_VAR2:[0-9]+]] = !DILocalVariable(name: "var2", scope: ![[T1_SP]]
   !19 = !DILocalVariable(name: "var3", scope: !14, file: !1, line: 11, type: !8)
   ; CHECK: ![[T1_VAR3:[0-9]+]] = !DILocalVariable(name: "var3", scope: ![[T1_SP]]
   !20 = !DILocalVariable(name: "var4", scope: !14, file: !1, line: 11, type: !8)
   ; CHECK: ![[T1_VAR4:[0-9]+]] = !DILocalVariable(name: "var4", scope: ![[T1_SP]]
+  !21 = distinct !DISubprogram(name: "test_target_index_defs", scope: !1, file: !1, line: 10, type: !6, scopeLine: 1, unit: !0)
+  !22 = !DILocalVariable(name: "var0", scope: !21, file: !1, line: 20, type: !8)
+  !23 = !DILocation(line: 10, scope: !21)
+  !24 = !DILocalVariable(name: "var1", scope: !21, file: !1, line: 20, type: !8)
+  !25 = !DILocalVariable(name: "var2", scope: !21, file: !1, line: 20, type: !8)
 ...
 
 ---
 # var0: Its debug value is set once in bb.0 and valid throughout the function.
 #       So a new DBG_VALUE will be added to the beginnings of all other BBs
 #       (bb.1, bb.2, and bb.3).
 # var1: Its debug value is set in bb.0, but is reset to no info in bb.1. So a
 #       new DBG_VALUE will be added in the beginning of bb.1 and bb.2, but bb.3
body: |
   ; CHECK-DAG: DBG_VALUE target-index(wasm-local), $noreg, ![[T1_VAR0]]
   ; CHECK-DAG: DBG_VALUE target-index(wasm-local) + 2, $noreg, ![[T1_VAR2]]
   ; CHECK-NOT: DBG_VALUE target-index(wasm-local){{.*}}, $noreg, ![[T1_VAR3]]
   ; CHECK-NOT: DBG_VALUE target-index(wasm-local){{.*}}, $noreg, ![[T1_VAR4]]
   bb.3:
   ; predecessors: %bb.1, %bb.2
     RETURN implicit-def dead $arguments
 ...
+
+---
+# bb.0 defines DBG_VALUEs for local index 2 and 3. The DBG_VALUE for local index
+# 2 is killed in bb.1 due to 'local.tee 2' there, and DBG_VALUE for local
+# index 3 is killed in bb.2 because there is 'local.set 3' there. As a result,
+# bb.3 shoudln't have any additional DBG_VALUEs added for those locals.
+# CHECK-LABEL: name: test_target_index_defs
+name: test_target_index_defs
+liveins:
+  - { reg: '$arguments' }
+body: |
+  bb.0:
+    successors: %bb.1, %bb.2
+    liveins: $arguments
+    DBG_VALUE target-index(wasm-local) + 2, $noreg, !22, !DIExpression(), debug-location !23
+    DBG_VALUE target-index(wasm-local) + 3, $noreg, !24, !DIExpression(), debug-location !23
+    %0:i32 = CONST_I32 1, implicit-def $arguments
+    BR_IF %bb.2, %0:i32, implicit-def $arguments
+
+  bb.1:
+  ; predecessors: %bb.0
+    successors: %bb.3
+    %0:i32 = CONST_I32 1, implicit-def $arguments
+    %1:i32 = LOCAL_TEE_I32 2, %0:i32, implicit-def $arguments
+    DROP_I32 killed %1:i32, implicit-def $arguments
+    BR %bb.3, implicit-def $arguments
+
+  bb.2:
+  ; predecessors: %bb.0
+    successors: %bb.3
+    %0:i32 = CONST_I32 1, implicit-def $arguments
+    LOCAL_SET_I32 3, %0:i32, implicit-def $arguments
+    BR %bb.3, implicit-def $arguments
+
+  ; CHECK: bb.3:
+  ; CHECK-NOT: DBG_VALUE target-index(wasm-local){{.*}}
+  bb.3:
+  ; predecessors: %bb.1, %bb.2
+    RETURN implicit-def dead $arguments
+...