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Make FoldBranchToCommonDest poison-safe by default

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This is a small patch to make FoldBranchToCommonDest poison-safe by default.
After fc3f0c9c, only two syntactic changes are needed to fix unit tests.
This does not cause any assembly difference in testsuite as well (-O3, X86-64 Manjaro).

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D99452
This commit is contained in:
Juneyoung Lee 2021-03-27 17:12:20 +09:00
parent 64dc4c67c5
commit c9eefe2dea
4 changed files with 8 additions and 17 deletions
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7
include/llvm/Transforms/Utils/Local.h
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@ -186,15 +186,10 @@ bool FlattenCFG(BasicBlock *BB, AAResults *AA = nullptr);
/// If this basic block is ONLY a setcc and a branch, and if a predecessor
/// branches to us and one of our successors, fold the setcc into the
/// predecessor and use logical operations to pick the right destination.
/// If PoisonSafe is true, use select i1 rather than and/or i1 to successfully
/// block unexpected propagation of poison when merging the branches. This is
/// set to false by default when used by LoopSimplify for performance, but this
/// should be turned on by default.
bool FoldBranchToCommonDest(BranchInst *BI, llvm::DomTreeUpdater *DTU = nullptr,
MemorySSAUpdater *MSSAU = nullptr,
const TargetTransformInfo *TTI = nullptr,
unsigned BonusInstThreshold = 1,
bool PoisonSafe = false);
unsigned BonusInstThreshold = 1);
/// This function takes a virtual register computed by an Instruction and
/// replaces it with a slot in the stack frame, allocated via alloca.

14
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -2902,7 +2902,6 @@ shouldFoldCondBranchesToCommonDestination(BranchInst *BI, BranchInst *PBI,
static bool performBranchToCommonDestFolding(BranchInst *BI, BranchInst *PBI,
DomTreeUpdater *DTU,
MemorySSAUpdater *MSSAU,
bool PoisonSafe,
const TargetTransformInfo *TTI) {
BasicBlock *BB = BI->getParent();
BasicBlock *PredBlock = PBI->getParent();
@ -3004,9 +3003,8 @@ static bool performBranchToCommonDestFolding(BranchInst *BI, BranchInst *PBI,
// or/and the two conditions together.
Value *NewCond = nullptr;
Value *BICond = VMap[BI->getCondition()];
bool UseBinOp = !PoisonSafe || impliesPoison(BICond, PBI->getCondition());
if (UseBinOp)
if (impliesPoison(BICond, PBI->getCondition()))
NewCond = Builder.CreateBinOp(Opc, PBI->getCondition(), BICond, "or.cond");
else
NewCond =
@ -3035,8 +3033,7 @@ static bool performBranchToCommonDestFolding(BranchInst *BI, BranchInst *PBI,
bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
MemorySSAUpdater *MSSAU,
const TargetTransformInfo *TTI,
unsigned BonusInstThreshold,
bool PoisonSafe) {
unsigned BonusInstThreshold) {
// If this block ends with an unconditional branch,
// let SpeculativelyExecuteBB() deal with it.
if (!BI->isConditional())
@ -3140,8 +3137,7 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
// Ok, we have the budget. Perform the transformation.
for (BasicBlock *PredBlock : Preds) {
auto *PBI = cast<BranchInst>(PredBlock->getTerminator());
return performBranchToCommonDestFolding(BI, PBI, DTU, MSSAU, PoisonSafe,
TTI);
return performBranchToCommonDestFolding(BI, PBI, DTU, MSSAU, TTI);
}
return Changed;
}
@ -6360,7 +6356,7 @@ bool SimplifyCFGOpt::simplifyUncondBranch(BranchInst *BI,
// predecessor and use logical operations to update the incoming value
// for PHI nodes in common successor.
if (FoldBranchToCommonDest(BI, DTU, /*MSSAU=*/nullptr, &TTI,
Options.BonusInstThreshold, true))
Options.BonusInstThreshold))
return requestResimplify();
return false;
}
@ -6424,7 +6420,7 @@ bool SimplifyCFGOpt::simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
// branches to us and one of our successors, fold the comparison into the
// predecessor and use logical operations to pick the right destination.
if (FoldBranchToCommonDest(BI, DTU, /*MSSAU=*/nullptr, &TTI,
Options.BonusInstThreshold, true))
Options.BonusInstThreshold))
return requestResimplify();
// We have a conditional branch to two blocks that are only reachable

2
test/Transforms/IRCE/bad_expander.ll
View File

@ -98,7 +98,7 @@ define void @test_03(i64* %p1, i64* %p2, i1 %maybe_exit) {
; CHECK-NEXT: %iv = phi i64 [ %iv.next, %guarded ], [ 0, %loop.preheader ]
; CHECK-NEXT: %iv.next = add nuw nsw i64 %iv, 1
; CHECK-NEXT: %rc = icmp slt i64 %iv.next, %div_result
; CHECK-NEXT: %or.cond = and i1 %maybe_exit, true
; CHECK-NEXT: %or.cond = select i1 %maybe_exit, i1 true, i1 false
; CHECK-NEXT: br i1 %or.cond, label %guarded, label %exit.loopexit1
; CHECK: guarded:
; CHECK-NEXT: %gep = getelementptr i64, i64* %p1, i64 %iv.next

2
test/Transforms/LoopSimplify/pr26682.ll
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@ -7,7 +7,7 @@ target triple = "x86_64-unknown-unknown"
; Check that loop-simplify merges two loop exits, but preserves LCSSA form.
; CHECK-LABEL: @foo
; CHECK: for:
; CHECK: %or.cond = and i1 %cmp1, %cmp2
; CHECK: %or.cond = select i1 %cmp1, i1 %cmp2, i1 false
; CHECK-NOT: for.cond:
; CHECK: for.end:
; CHECK: %a.lcssa = phi i32 [ %a, %for ]