[MachineBlockPlacement] Don't make blocks "uneditable"

[sanjoy]

Summary

NB: I'm new to this area, so please review with care and distrust. :)

This fixes an issue with MachineBlockPlacement due to a badly timed call
to analyzeBranch with AllowModify set to true. The timeline is as
follows:

1. MachineBlockPlacement::maybeTailDuplicateBlock calls TailDup.shouldTailDuplicate on its argument, which in turn calls analyzeBranch with AllowModify set to true.
2. This analyzeBranch call edits the terminator sequence of the block based on the physical layout of the machine function, turning an unanalyzable non-fallthrough block to a unanalyzable fallthrough block. Normally MBP bails out of rearranging such blocks, but this block was unanalyzable non-fallthrough (and thus rearrangeable) the first time MBP looked at it, and so it goes ahead and decides where it should be placed in the function.
3. When placing this block MBP fails to analyze and thus update the block in keeping with the new physical layout.

Concretely, before (1) we have something like:

LBL0:
  < unknown terminator op that may branch to LBL1 >
  jmp LBL1

LBL1:
  ... A

LBL2:
  ... B

In (2), analyze branch simplifies this to

LBL0:
  < unknown terminator op that may branch to LBL2 >
  ;; jmp LBL1 <- redundant jump removed

LBL1:
  ... A

LBL2:
  ... B

In (3), MachineBlockPlacement goes ahead with its plan of putting LBL2
after the first block since that is profitable.

LBL0:
  < unknown terminator op that may branch to LBL2 >
  ;; jmp LBL1 <- redundant jump

LBL2:
  ... B

LBL1:
  ... A

and the program now has incorrect behavior (we no longer fall-through
from LBL0 to LBL1) because MBP can no longer edit LBL0.

There are several possible solutions, but I went with removing the teeth
off of the analyzeBranch calls in TailDuplicator. That makes thinking
about the result of these calls easier, and nothing in the lit test
suite broke when I did it.

I've also added some bookkeeping to the MachineBlockPlacement pass and
used that to write an assert that would have caught this issue.

[iteratee]

I'm OK with the debugging, but I think it's a little over-engineered. Basically you're keeping a list of all the blocks that you think have unanalyzable fallthrough and then you want to assert if during updateTerminators you encounter a block with unanalyzable fallthrough that is not on that list. Is that correct? If so simply keeping that set and testing for membership is sufficient, and simpler.

I'm also OK removing the teeth in TailDuplicator. It bothers me that analyzeBranch has that flag at all.

My real surprise moment is that analyzeBranch would choose to modify a branch that it claims it can't analyze. Really! I'm not convinced that's correct, but I'm fine with some defensive programming in response to it.

lib/CodeGen/MachineBlockPlacement.cpp

179

I think this should just be a list global to the pass. Each unanalyzable fallthrough gets merged exactly once.

209

I don't really like this function. This is basically all blocks without unanalyzable fallthrough. I'd rather just test for membership at the point where we need it.

1599–1600

Rather than polluting the merge api, BB is available right here, just mark it.

[sanjoy]

In D27783#623014, @iteratee wrote:

I'm OK with the debugging, but I think it's a little over-engineered. Basically you're keeping a list of all the blocks that you think have unanalyzable fallthrough and then you want to assert if during updateTerminators you encounter a block with unanalyzable fallthrough that is not on that list. Is that correct? If so simply keeping that set and testing for membership is sufficient, and simpler.

Sounds good -- I'll move the set to live on the MachineFunctionPass object then.

I'm also OK removing the teeth in TailDuplicator.

It bothers me that analyzeBranch has that flag at all.

Me too. :)

My real surprise moment is that analyzeBranch would choose to modify a branch that it claims it can't analyze. Really! I'm not convinced that's correct, but I'm fine with some defensive programming in response to it.

I agree that's hard to defend, and I was initially considering fixing analyzeBranch to edit a branch only if it would return false. However, it is also reasonable to also say that the transform it did here is a pretty obvious peephole transform which it _should_ be able to do without fully understanding the branch.

I think an overall better solution is to have two TII hooks, analyzeBranch and simplifyBranch. simplifyBranch can then be specified to not indicate anything about the understandability of the terminator sequence -- its job would be to do locally obvious peephole simplification without any global understanding.

[sanjoy]

• review

[iteratee]

One minor fix, but otherwise LGTM

lib/CodeGen/MachineBlockPlacement.cpp

1680

Can you make the assert comment here more descriptive, something like:
"Unexpected block with un-analyzable fallthrough detected."

lib/CodeGen/MachineBlockPlacement.cpp

class BlockChain {
   /// out in-order within the function.
   SmallVector<MachineBasicBlock *, 4> Blocks;
 
   /// \brief A handle to the function-wide basic block to block chain mapping.
   ///
   /// This is retained in each block chain to simplify the computation of child
   /// block chains for SCC-formation and iteration. We store the edges to child
   /// basic blocks, and map them back to their associated chains using this
   /// structure.
   BlockToChainMapType &BlockToChain;
 
 public:
   /// \brief Construct a new BlockChain.
   ///
   /// This builds a new block chain representing a single basic block in the
   /// function. It also registers itself as the chain that block participates
   /// in with the BlockToChain mapping.
public:
   iterator end() { return Blocks.end(); }
 
   bool remove(MachineBasicBlock* BB) {
     for(iterator i = begin(); i != end(); ++i) {
       if (*i == BB) {
         Blocks.erase(i);
         return true;
       }
     }
     return false;
   }
 
   /// \brief Merge a block chain into this one.
   ///
   /// This routine merges a block chain into this one. It takes care of forming
   /// a contiguous sequence of basic blocks, updating the edge list, and
   /// updating the block -> chain mapping. It does not free or tear down the
class MachineBlockPlacement : public MachineFunctionPass {
   /// \brief Function wide BasicBlock to BlockChain mapping.
   ///
   /// This mapping allows efficiently moving from any given basic block to the
   /// BlockChain it participates in, if any. We use it to, among other things,
   /// allow implicitly defining edges between chains as the existing edges
   /// between basic blocks.
   DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
 
+#ifndef NDEBUG
+  /// The set of basic blocks that have terminators that cannot be fully
+  /// analyzed.  These basic blocks cannot be re-ordered safely by
+  /// MachineBlockPlacement, and we must preserve physical layout of these
+  /// blocks and their successors through the pass.
+  SmallPtrSet<MachineBasicBlock *, 4> BlocksWithUnanalyzableExits;
+#endif
+
   /// Decrease the UnscheduledPredecessors count for all blocks in chain, and
   /// if the count goes to 0, add them to the appropriate work list.
   void markChainSuccessors(BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
                            const BlockFilterSet *BlockFilter = nullptr);
 
   /// Decrease the UnscheduledPredecessors count for a single block, and
   /// if the count goes to 0, add them to the appropriate work list.
   void markBlockSuccessors(
for (;;) {
       MachineFunction::iterator NextFI = std::next(FI);
       MachineBasicBlock *NextBB = &*NextFI;
       // Ensure that the layout successor is a viable block, as we know that
       // fallthrough is a possibility.
       assert(NextFI != FE && "Can't fallthrough past the last block.");
       DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
                    << getBlockName(BB) << " -> " << getBlockName(NextBB)
                    << "\n");
       Chain->merge(NextBB, nullptr);
+      BlocksWithUnanalyzableExits.insert(&*BB);
       FI = NextFI;
       BB = NextBB;
     }
   }
 
   // Turned on with OutlineOptionalBranches option
   collectMustExecuteBBs();
 
for (MachineBasicBlock *ChainBB : FunctionChain) {
     MachineBasicBlock *PrevBB = &*std::prev(MachineFunction::iterator(ChainBB));
 
     // FIXME: It would be awesome of updateTerminator would just return rather
     // than assert when the branch cannot be analyzed in order to remove this
     // boiler plate.
     Cond.clear();
     MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
 
+#ifndef NDEBUG
+    if (!BlocksWithUnanalyzableExits.count(PrevBB)) {
+      // Given the exact block placement we chose, we may actually not _need_ to
+      // be able to edit PrevBB's terminator sequence, but not being _able_ to
+      // do that at this point is a bug.
+      assert((!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) ||
+              !PrevBB->canFallThrough()) &&
+             "Precondition violated!");
+      Cond.clear();
+      TBB = FBB = nullptr;
+    }
+#endif
+
     // The "PrevBB" is not yet updated to reflect current code layout, so,
     //   o. it may fall-through to a block without explicit "goto" instruction
     //      before layout, and no longer fall-through it after layout; or
     //   o. just opposite.
     //
     // analyzeBranch() may return erroneous value for FBB when these two
     // situations take place. For the first scenario FBB is mistakenly set NULL;
     // for the 2nd scenario, the FBB, which is expected to be NULL, is

lib/CodeGen/TailDuplicator.cpp

else
     MaxDuplicateCount = TailDupSize;
 
   // If the block to be duplicated ends in an unanalyzable fallthrough, don't
   // duplicate it.
   // A similar check is necessary in MachineBlockPlacement to make sure pairs of
   // blocks with unanalyzable fallthrough get layed out contiguously.
   MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
   SmallVector<MachineOperand, 4> PredCond;
-  if (TII->analyzeBranch(TailBB, PredTBB, PredFBB, PredCond, true)
-      && TailBB.canFallThrough())
+  if (TII->analyzeBranch(TailBB, PredTBB, PredFBB, PredCond) &&
+      TailBB.canFallThrough())
     return false;
 
   // If the target has hardware branch prediction that can handle indirect
   // branches, duplicating them can often make them predictable when there
   // are common paths through the code.  The limit needs to be high enough
   // to allow undoing the effects of tail merging and other optimizations
   // that rearrange the predecessors of the indirect branch.
 

 
 bool TailDuplicator::canCompletelyDuplicateBB(MachineBasicBlock &BB) {
   for (MachineBasicBlock *PredBB : BB.predecessors()) {
     if (PredBB->succ_size() > 1)
       return false;
 
     MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
     SmallVector<MachineOperand, 4> PredCond;
-    if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
+    if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
       return false;
 
     if (!PredCond.empty())
       return false;
   }
   return true;
 }
 
for (MachineBasicBlock *PredBB : Preds) {
     if (PredBB->hasEHPadSuccessor())
       continue;
 
     if (bothUsedInPHI(*PredBB, Succs))
       continue;
 
     MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
     SmallVector<MachineOperand, 4> PredCond;
-    if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
+    if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
       continue;
 
     Changed = true;
     DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
                  << "From simple Succ: " << *TailBB);
 
     MachineBasicBlock *NewTarget = *TailBB->succ_begin();
     MachineBasicBlock *NextBB = PredBB->getNextNode();

 bool TailDuplicator::canTailDuplicate(MachineBasicBlock *TailBB,
                                       MachineBasicBlock *PredBB) {
   // EH edges are ignored by analyzeBranch.
   if (PredBB->succ_size() > 1)
     return false;
 
   MachineBasicBlock *PredTBB, *PredFBB;
   SmallVector<MachineOperand, 4> PredCond;
-  if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
+  if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
     return false;
   if (!PredCond.empty())
     return false;
   return true;
 }
 
 /// If it is profitable, duplicate TailBB's contents in each
 /// of its predecessors.
while (I != TailBB->instr_end()) {
         duplicateInstruction(MI, TailBB, PredBB, LocalVRMap, UsedByPhi);
       }
     }
     appendCopies(PredBB, CopyInfos, Copies);
 
     // Simplify
     MachineBasicBlock *PredTBB, *PredFBB;
     SmallVector<MachineOperand, 4> PredCond;
-    TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true);
+    TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond);
 
     NumTailDupAdded += TailBB->size() - 1; // subtract one for removed branch
 
     // Update the CFG.
     PredBB->removeSuccessor(PredBB->succ_begin());
     assert(PredBB->succ_empty() &&
            "TailDuplicate called on block with multiple successors!");
     for (MachineBasicBlock *Succ : TailBB->successors())
if (!PrevBB)
     PrevBB = &*std::prev(TailBB->getIterator());
   MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
   SmallVector<MachineOperand, 4> PriorCond;
   // This has to check PrevBB->succ_size() because EH edges are ignored by
   // analyzeBranch.
   if (PrevBB->succ_size() == 1 &&
       // Layout preds are not always CFG preds. Check.
       *PrevBB->succ_begin() == TailBB &&
-      !TII->analyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true) &&
+      !TII->analyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond) &&
       PriorCond.empty() &&
       (!PriorTBB || PriorTBB == TailBB) &&
       TailBB->pred_size() == 1 &&
       !TailBB->hasAddressTaken()) {
     DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
                  << "From MBB: " << *TailBB);
     // There may be a branch to the layout successor. This is unlikely but it
     // happens. The correct thing to do is to remove the branch before

test/CodeGen/X86/block-placement.mir

+# RUN: llc -O3 -run-pass=block-placement -o - %s | FileCheck %s
+
+--- |
+  ; ModuleID = 'test.ll'
+  source_filename = "test.ll"
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+  
+  declare void @stub(i32*)
+  
+  define i32 @f(i32* %ptr, i1 %cond) {
+  entry:
+    br i1 %cond, label %left, label %right
+  
+  left:                                             ; preds = %entry
+    %is_null = icmp eq i32* %ptr, null
+    br i1 %is_null, label %null, label %not_null, !prof !0, !make.implicit !1
+  
+  not_null:                                         ; preds = %left
+    %val = load i32, i32* %ptr
+    ret i32 %val
+  
+  null:                                             ; preds = %left
+    call void @stub(i32* %ptr)
+    unreachable
+  
+  right:                                            ; preds = %entry
+    call void @stub(i32* null)
+    unreachable
+  }
+  
+  ; Function Attrs: nounwind
+  declare void @llvm.stackprotector(i8*, i8**) #0
+  
+  attributes #0 = { nounwind }
+  
+  !0 = !{!"branch_weights", i32 1048575, i32 1}
+  !1 = !{}
+
+...
+---
+# CHECK: name:            f
+name:            f
+alignment:       4
+tracksRegLiveness: true
+liveins:         
+  - { reg: '%rdi' }
+  - { reg: '%esi' }
+
+# CHECK: %eax = FAULTING_LOAD_OP %bb.3.null, 1684, killed %rdi, 1, _, 0, _ :: (load 4 from %ir.ptr)
+# CHECK-NEXT: JMP_1 %bb.2.not_null
+# CHECK: bb.3.null:
+# CHECK:  bb.4.right:
+# CHECK:  bb.2.not_null:
+
+body:             |
+  bb.0.entry:
+    successors: %bb.1.left(0x7ffff800), %bb.3.right(0x00000800)
+    liveins: %esi, %rdi
+  
+    frame-setup PUSH64r undef %rax, implicit-def %rsp, implicit %rsp
+    CFI_INSTRUCTION def_cfa_offset 16
+    TEST8ri %sil, 1, implicit-def %eflags, implicit killed %esi
+    JE_1 %bb.3.right, implicit killed %eflags
+  
+  bb.1.left:
+    successors: %bb.2.null(0x7ffff800), %bb.4.not_null(0x00000800)
+    liveins: %rdi
+  
+    %eax = FAULTING_LOAD_OP %bb.2.null, 1684, killed %rdi, 1, _, 0, _ :: (load 4 from %ir.ptr)
+    JMP_1 %bb.4.not_null
+  
+  bb.4.not_null:
+    liveins: %rdi, %eax
+  
+    %rcx = POP64r implicit-def %rsp, implicit %rsp
+    RETQ %eax
+
+  bb.2.null:
+    liveins: %rdi
+  
+    CALL64pcrel32 @stub, csr_64, implicit %rsp, implicit %rdi, implicit-def %rsp
+  
+  bb.3.right:
+    dead %edi = XOR32rr undef %edi, undef %edi, implicit-def dead %eflags, implicit-def %rdi
+    CALL64pcrel32 @stub, csr_64, implicit %rsp, implicit %rdi, implicit-def %rsp
+  
+...