[llvm] [SimplifyCFG] Enable MergeBlockIntoPredecessor with two successors (PR #143766)
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llvm-commits at lists.llvm.org
Wed Jun 11 19:44:59 PDT 2025
https://github.com/HighW4y2H3ll updated https://github.com/llvm/llvm-project/pull/143766
>From 69813cf107a23b9fd70ef121769fe6c70488eec7 Mon Sep 17 00:00:00 2001
From: h2h <h2h at meta.com>
Date: Tue, 10 Jun 2025 15:41:26 -0700
Subject: [PATCH 1/2] [SimplifyCFG] Enable MergeBlockIntoPredecessor with two
successors
---
.../lib/Transforms/Scalar/SimplifyCFGPass.cpp | 2 +
llvm/lib/Transforms/Utils/BasicBlockUtils.cpp | 34 +++++++++++
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 2 +-
.../SimplifyCFG/two-succ-merge-block.ll | 58 +++++++++++++++++++
4 files changed, 95 insertions(+), 1 deletion(-)
create mode 100644 llvm/test/Transforms/SimplifyCFG/two-succ-merge-block.ll
diff --git a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
index 4e437e9abeb43..864a3c35ed79e 100644
--- a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
+++ b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
@@ -264,6 +264,8 @@ static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
if (simplifyCFG(&BB, TTI, DTU, Options, LoopHeaders)) {
LocalChange = true;
++NumSimpl;
+ // Function iterator might not be valid anymore
+ break;
}
}
Changed |= LocalChange;
diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 6608515e1cbbc..473aaa90962be 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -207,6 +207,10 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
if (PredecessorWithTwoSuccessors) {
if (!(PredBB_BI = dyn_cast<BranchInst>(PTI)))
return false;
+ // Can't merge if any of the instructions has side effects
+ for (Instruction &candid : *BB)
+ if (candid.mayHaveSideEffects())
+ return false;
BranchInst *BB_JmpI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BB_JmpI || !BB_JmpI->isUnconditional())
return false;
@@ -283,6 +287,36 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
if (MSSAU)
MSSAU->moveAllAfterMergeBlocks(BB, PredBB, Start);
+ // PHI nodes that references the Pred shall be replaced with a SelectInst
+ for (BasicBlock *Succ : successors(PTI)) {
+ if (Succ == BB)
+ continue;
+ for (PHINode &PhiSucc : Succ->phis()) {
+ if (llvm::is_contained(PhiSucc.blocks(), PredBB)) {
+ if (PredBB_BI->isConditional()) {
+ Value *Cond = PredBB_BI->getCondition();
+ Value *OrigPhi0 = PhiSucc.removeIncomingValue(PredBB, false);
+ Value *newPhi = OrigPhi0;
+
+ if (PhiSucc.getNumOperands() > 0) {
+ if (llvm::is_contained(PhiSucc.blocks(), BB)) {
+ Value *OrigPhi1 = PhiSucc.removeIncomingValue(BB, false);
+ // Swap branch given the conditions
+ if (PredBB_BI->getSuccessor(0) == Succ) {
+ newPhi = SelectInst::Create(Cond, OrigPhi0, OrigPhi1, "", PTI->getIterator());
+ } else {
+ newPhi = SelectInst::Create(Cond, OrigPhi1, OrigPhi0, "", PTI->getIterator());
+ }
+ PhiSucc.addIncoming(newPhi, PredBB);
+ }
+ }
+
+ PhiSucc.addIncoming(newPhi, PredBB);
+ }
+ }
+ }
+ }
+
// Make all PHI nodes that referred to BB now refer to Pred as their
// source...
BB->replaceAllUsesWith(PredBB);
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index e221022bb8361..27b0692ca7a4f 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -8337,7 +8337,7 @@ bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
- if (MergeBlockIntoPredecessor(BB, DTU))
+ if (MergeBlockIntoPredecessor(BB, DTU, nullptr, nullptr, nullptr, true))
return true;
if (SinkCommon && Options.SinkCommonInsts)
diff --git a/llvm/test/Transforms/SimplifyCFG/two-succ-merge-block.ll b/llvm/test/Transforms/SimplifyCFG/two-succ-merge-block.ll
new file mode 100644
index 0000000000000..eb8ac774b6e5b
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/two-succ-merge-block.ll
@@ -0,0 +1,58 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --tool ./opt --version 5
+; RUN: opt < %s -passes=simplifycfg -S | FileCheck %s
+
+define i1 @_Z7compareRK1SS1_(ptr %a, ptr %b) {
+; CHECK-LABEL: @_Z7compareRK1SS1_(
+; CHECK-SAME: ptr [[A:%.*]], ptr [[B:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: %0 = load i32, ptr %a, align 4, !tbaa !3
+; CHECK-NEXT: %1 = load i32, ptr %b, align 4, !tbaa !3
+; CHECK-NEXT: %cmp.i = icmp slt i32 %0, %1
+; CHECK-NEXT: %cmp.i19 = icmp eq i32 %0, %1
+; CHECK-NEXT: %y = getelementptr inbounds nuw i8, ptr %a, i64 4
+; CHECK-NEXT: %2 = load i32, ptr %y, align 4, !tbaa !8
+; CHECK-NEXT: %y14 = getelementptr inbounds nuw i8, ptr %b, i64 4
+; CHECK-NEXT: %3 = load i32, ptr %y14, align 4, !tbaa !8
+; CHECK-NEXT: %cmp = icmp slt i32 %2, %3
+; CHECK-NEXT: %4 = select i1 %cmp.i19, i1 %cmp, i1 false
+; CHECK-NEXT: %5 = select i1 %cmp.i, i1 true, i1 %4
+; CHECK-NEXT: br label %lor.end
+; CHECK-EMPTY:
+; CHECK-NEXT: lor.end: ; preds = %entry
+; CHECK-NEXT: ret i1 %5
+entry:
+ %0 = load i32, ptr %a, align 4, !tbaa !3
+ %1 = load i32, ptr %b, align 4, !tbaa !3
+ %cmp.i = icmp slt i32 %0, %1
+ br i1 %cmp.i, label %lor.end, label %lor.rhs
+
+lor.rhs: ; preds = %entry
+ %cmp.i19 = icmp eq i32 %0, %1
+ br i1 %cmp.i19, label %land.rhs, label %lor.end
+
+land.rhs: ; preds = %lor.rhs
+ %y = getelementptr inbounds nuw i8, ptr %a, i64 4
+ %2 = load i32, ptr %y, align 4, !tbaa !8
+ %y14 = getelementptr inbounds nuw i8, ptr %b, i64 4
+ %3 = load i32, ptr %y14, align 4, !tbaa !8
+ %cmp = icmp slt i32 %2, %3
+ br label %lor.end
+
+lor.end: ; preds = %lor.rhs, %land.rhs, %entry
+ %4 = phi i1 [ true, %entry ], [ false, %lor.rhs ], [ %cmp, %land.rhs ]
+ ret i1 %4
+}
+
+!llvm.module.flags = !{!0, !1}
+!llvm.ident = !{!2}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{i32 7, !"uwtable", i32 2}
+!2 = !{!"clang"}
+!3 = !{!4, !5, i64 0}
+!4 = !{!"_ZTS1S", !5, i64 0, !5, i64 4}
+!5 = !{!"int", !6, i64 0}
+!6 = !{!"omnipotent char", !7, i64 0}
+!7 = !{!"Simple C++ TBAA"}
+!8 = !{!4, !5, i64 4}
+!9 = !{!5, !5, i64 0}
>From e315f8942700310859f51d542874a927ee926af7 Mon Sep 17 00:00:00 2001
From: h2h <h2h at meta.com>
Date: Wed, 11 Jun 2025 19:28:32 -0700
Subject: [PATCH 2/2] Patch simplifyCFG back to merge one successor and check
for compatibility
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 27b0692ca7a4f..e221022bb8361 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -8337,7 +8337,7 @@ bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
- if (MergeBlockIntoPredecessor(BB, DTU, nullptr, nullptr, nullptr, true))
+ if (MergeBlockIntoPredecessor(BB, DTU))
return true;
if (SinkCommon && Options.SinkCommonInsts)
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