[llvm] [SimplifyCFG] Enable MergeBlockIntoPredecessor with two successors (PR #143766)
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Wed Jun 11 12:00:30 PDT 2025
https://github.com/HighW4y2H3ll created https://github.com/llvm/llvm-project/pull/143766
SimplifyCFG currently only merges basic block into its predecessor with `PredecessorWithTwoSuccessors` turned off, even though it's plausible to merge when the predecessor basic block has two successors and the instructions in the current basic block have no side effects. This patch re-enables this CFG transformation in the SimplifyCFG pass so that we can make more aggressive optimizations.
>From 1a8bf033379f1967e4eefa89e718ffb79f51b98c Mon Sep 17 00:00:00 2001
From: h2h <h2h at meta.com>
Date: Tue, 10 Jun 2025 15:41:26 -0700
Subject: [PATCH] [SimplifyCFG] Enable MergeBlockIntoPredecessor with two
successors
---
.../lib/Transforms/Scalar/SimplifyCFGPass.cpp | 2 +
llvm/lib/Transforms/Utils/BasicBlockUtils.cpp | 33 +++++++++++
llvm/lib/Transforms/Utils/Local.cpp | 4 +-
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 2 +-
.../SimplifyCFG/two-succ-merge-block.ll | 58 +++++++++++++++++++
5 files changed, 97 insertions(+), 2 deletions(-)
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..f9fda9625ea36 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 +287,35 @@ 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)) {
+ for (PHINode &PhiSucc : Succ->phis()) {
+ if (llvm::is_contained(PhiSucc.blocks(), PredBB) && llvm::is_contained(PhiSucc.blocks(), BB)) {
+ if (PredBB_BI->isConditional()) {
+ Value *Cond = PredBB_BI->getCondition();
+ Value *OrigPhi0 = PhiSucc.removeIncomingValue(PredBB, false);
+ Value *OrigPhi1 = PhiSucc.removeIncomingValue(BB, false);
+
+ SelectInst *newPhi = nullptr;
+ // 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());
+ }
+
+ // Erase PhiNode if its empty
+ if (PhiSucc.getNumOperands() == 0) {
+ PhiSucc.replaceAllUsesWith(newPhi);
+ PhiSucc.eraseFromParent();
+ break;
+ }
+ 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/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index be71cb69ad8cc..c382c1e385134 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -153,7 +153,9 @@ bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
// Let the basic block know that we are letting go of one copy of it.
assert(BI->getParent() && "Terminator not inserted in block!");
- Dest1->removePredecessor(BI->getParent());
+ // Keep the predecesor if Dest has multiple incoming edges
+ if (Dest1->getUniquePredecessor())
+ Dest1->removePredecessor(BI->getParent());
// Replace the conditional branch with an unconditional one.
BranchInst *NewBI = Builder.CreateBr(Dest1);
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}
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