I'd like to nominate this fix for the 3.9.1 release.
- Matthias
> On Sep 19, 2016, at 6:14 PM, Matthias Braun via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>
> Author: matze
> Date: Mon Sep 19 20:14:42 2016
> New Revision: 281957
>
> URL: http://llvm.org/viewvc/llvm-project?rev=281957&view=rev
> Log:
> BranchFolder: Fix invalid undef flags after merge.
>
> It is legal to merge instructions with different undef flags; However we
> must drop the undef flag from the merged instruction if it isn't present
> everywhere.
>
> This fixes http://llvm.org/PR30199
>
> Added:
> llvm/trunk/test/CodeGen/X86/branchfolding-undef.mir
> Modified:
> llvm/trunk/lib/CodeGen/BranchFolding.cpp
>
> Modified: llvm/trunk/lib/CodeGen/BranchFolding.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/BranchFolding.cpp?rev=281957&r1=281956&r2=281957&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/BranchFolding.cpp (original)
> +++ llvm/trunk/lib/CodeGen/BranchFolding.cpp Mon Sep 19 20:14:42 2016
> @@ -801,9 +801,8 @@ bool BranchFolder::CreateCommonTailOnlyB
> }
>
> static void
> -mergeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
> - MachineBasicBlock &MBBCommon) {
> - // Merge MMOs from memory operations in the common block.
> +mergeOperations(MachineBasicBlock::iterator MBBIStartPos,
> + MachineBasicBlock &MBBCommon) {
> MachineBasicBlock *MBB = MBBIStartPos->getParent();
> // Note CommonTailLen does not necessarily matches the size of
> // the common BB nor all its instructions because of debug
> @@ -833,8 +832,18 @@ mergeMMOsFromMemoryOperations(MachineBas
> "Reached BB end within common tail length!");
> assert(MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!");
>
> + // Merge MMOs from memory operations in the common block.
> if (MBBICommon->mayLoad() || MBBICommon->mayStore())
> MBBICommon->setMemRefs(MBBICommon->mergeMemRefsWith(*MBBI));
> + // Drop undef flags if they aren't present in all merged instructions.
> + for (unsigned I = 0, E = MBBICommon->getNumOperands(); I != E; ++I) {
> + MachineOperand &MO = MBBICommon->getOperand(I);
> + if (MO.isReg() && MO.isUndef()) {
> + const MachineOperand &OtherMO = MBBI->getOperand(I);
> + if (!OtherMO.isUndef())
> + MO.setIsUndef(false);
> + }
> + }
>
> ++MBBI;
> ++MBBICommon;
> @@ -952,8 +961,8 @@ bool BranchFolder::TryTailMergeBlocks(Ma
> continue;
> DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
> << (i == e-1 ? "" : ", "));
> - // Merge MMOs from memory operations as needed.
> - mergeMMOsFromMemoryOperations(SameTails[i].getTailStartPos(), *MBB);
> + // Merge operations (MMOs, undef flags)
> + mergeOperations(SameTails[i].getTailStartPos(), *MBB);
> // Hack the end off BB i, making it jump to BB commonTailIndex instead.
> ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
> // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
>
> Added: llvm/trunk/test/CodeGen/X86/branchfolding-undef.mir
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/branchfolding-undef.mir?rev=281957&view=auto
> ==============================================================================
> --- llvm/trunk/test/CodeGen/X86/branchfolding-undef.mir (added)
> +++ llvm/trunk/test/CodeGen/X86/branchfolding-undef.mir Mon Sep 19 20:14:42 2016
> @@ -0,0 +1,29 @@
> +# RUN: llc -o - %s -march=x86 -run-pass branch-folder | FileCheck %s
> +# Test that tail merging drops undef flags that aren't present on all
> +# instructions to be merged.
> +--- |
> + define void @func() { ret void }
> +...
> +---
> +# CHECK-LABEL: name: func
> +# CHECK: bb.1:
> +# CHECK: %eax = MOV32ri 2
> +# CHECK-NOT: RET
> +# CHECK: bb.2:
> +# CHECK-NOT: RET 0, undef %eax
> +# CHECK: RET 0, %eax
> +name: func
> +tracksRegLiveness: true
> +body: |
> + bb.0:
> + successors: %bb.1, %bb.2
> + JE_1 %bb.1, implicit undef %eflags
> + JMP_1 %bb.2
> +
> + bb.1:
> + %eax = MOV32ri 2
> + RET 0, %eax
> +
> + bb.2:
> + RET 0, undef %eax
> +...
>
>
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