From 29d054bf126870706f909a821895770bd9af42b3 Mon Sep 17 00:00:00 2001 From: Max Kazantsev Date: Wed, 8 Sep 2021 13:17:23 +0700 Subject: [PATCH] [SimplifyCFG] Preserve knowledge about guarding condition by adding assume This improvement adds "assume" after removal of branch basing on UB in successor block. Consider the following example: ``` pred: x = ... cond = x > 10 br cond, bb, other.succ bb: phi [nullptr, pred], ... // other possible preds load(phi) // UB if we came from pred other.succ: // here we know that x <= 10, but this knowledge is lost // after the branch is turned to unconditional unless we // preserve it with assume. ``` If we remove the branch basing on knowledge about UB in a successor block, then the fact that x <= 10 is other.succ might be lost if this condition is not inferrable from any dominating condition. To preserve this knowledge, we can add assume intrinsic with (possibly inverted) branch condition. Patch by Dmitry Bakunevich! Differential Revision: https://reviews.llvm.org/D109054 Reviewed By: lebedev.ri --- llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 10 +++++++++- .../Transforms/SimplifyCFG/tautological-conditional-branch.ll | 6 +++++- 2 files changed, 14 insertions(+), 2 deletions(-) diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp index ba3129f..737b4f9 100644 --- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp +++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp @@ -6598,9 +6598,17 @@ static bool removeUndefIntroducingPredecessor(BasicBlock *BB, // destination from conditional branches. if (BI->isUnconditional()) Builder.CreateUnreachable(); - else + else { + // Preserve guarding condition in assume, because it might not be + // inferrable from any dominating condition. + Value *Cond = BI->getCondition(); + if (BI->getSuccessor(0) == BB) + Builder.CreateAssumption(Builder.CreateNot(Cond)); + else + Builder.CreateAssumption(Cond); Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1) : BI->getSuccessor(0)); + } BI->eraseFromParent(); if (DTU) DTU->applyUpdates({{DominatorTree::Delete, Predecessor, BB}}); diff --git a/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll b/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll index 9125981..91ab3e5 100644 --- a/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll +++ b/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll @@ -14,8 +14,12 @@ define void @eggs(i1 %arg, i1 %arg16, %0* %arg17, %0* %arg18, %0* %arg19) { ; CHECK-NEXT: unreachable ; CHECK: bb21: ; CHECK-NEXT: [[I:%.*]] = icmp eq %0* [[ARG17:%.*]], null +; CHECK-NEXT: [[TMP0:%.*]] = xor i1 [[I]], true +; CHECK-NEXT: call void @llvm.assume(i1 [[TMP0]]) ; CHECK-NEXT: call void @hoge() -; CHECK-NEXT: [[I27:%.*]] = getelementptr inbounds [[TMP0:%.*]], %0* [[ARG19:%.*]], i64 0, i32 0 +; CHECK-NEXT: [[TMP1:%.*]] = xor i1 [[ARG16:%.*]], true +; CHECK-NEXT: call void @llvm.assume(i1 [[TMP1]]) +; CHECK-NEXT: [[I27:%.*]] = getelementptr inbounds [[TMP0]], %0* [[ARG19:%.*]], i64 0, i32 0 ; CHECK-NEXT: [[I28:%.*]] = load %1*, %1** [[I27]], align 8 ; CHECK-NEXT: call void @pluto.1(%1* [[I28]]) ; CHECK-NEXT: call void @pluto() -- 2.7.4