[llvm] [SimplifyCFG] Use range check in simplifyBranchOnICmpChain if possible (PR #165105)
Yingwei Zheng via llvm-commits
llvm-commits at lists.llvm.org
Tue Oct 28 10:33:22 PDT 2025
https://github.com/dtcxzyw updated https://github.com/llvm/llvm-project/pull/165105
>From a9be6d4fc253167524c473ce4a677dd2b640883a Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Sat, 25 Oct 2025 23:43:30 +0800
Subject: [PATCH 1/2] [SimplifyCFG] Use range check in
simplifyBranchOnICmpChain if possible
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 73 +++++---
llvm/test/Transforms/SimplifyCFG/pr165088.ll | 186 +++++++++++++++++++
2 files changed, 233 insertions(+), 26 deletions(-)
create mode 100644 llvm/test/Transforms/SimplifyCFG/pr165088.ll
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index d831c2737e5f8..50272ddc46699 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5228,32 +5228,53 @@ bool SimplifyCFGOpt::simplifyBranchOnICmpChain(BranchInst *BI,
CompVal, DL.getIntPtrType(CompVal->getType()), "magicptr");
}
- // Create the new switch instruction now.
- SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size());
- if (HasProfile) {
- // We know the weight of the default case. We don't know the weight of the
- // other cases, but rather than completely lose profiling info, we split
- // the remaining probability equally over them.
- SmallVector<uint32_t> NewWeights(Values.size() + 1);
- NewWeights[0] = BranchWeights[1]; // this is the default, and we swapped if
- // TrueWhenEqual.
- for (auto &V : drop_begin(NewWeights))
- V = BranchWeights[0] / Values.size();
- setBranchWeights(*New, NewWeights, /*IsExpected=*/false);
- }
-
- // Add all of the 'cases' to the switch instruction.
- for (ConstantInt *Val : Values)
- New->addCase(Val, EdgeBB);
-
- // We added edges from PI to the EdgeBB. As such, if there were any
- // PHI nodes in EdgeBB, they need entries to be added corresponding to
- // the number of edges added.
- for (BasicBlock::iterator BBI = EdgeBB->begin(); isa<PHINode>(BBI); ++BBI) {
- PHINode *PN = cast<PHINode>(BBI);
- Value *InVal = PN->getIncomingValueForBlock(BB);
- for (unsigned i = 0, e = Values.size() - 1; i != e; ++i)
- PN->addIncoming(InVal, BB);
+ // Check if we can represent the values can be represented as a contiguous
+ // range. If so, we use a range check + conditional branch instead of a
+ // switch.
+ if (Values.front()->getValue() - Values.back()->getValue() ==
+ Values.size() - 1) {
+ ConstantRange RangeToCheck = ConstantRange::getNonEmpty(
+ Values.back()->getValue(), Values.front()->getValue() + 1);
+ APInt Offset, RHS;
+ ICmpInst::Predicate Pred;
+ RangeToCheck.getEquivalentICmp(Pred, RHS, Offset);
+ Value *X = CompVal;
+ if (!Offset.isZero())
+ X = Builder.CreateAdd(X, ConstantInt::get(CompVal->getType(), Offset));
+ Value *Cond =
+ Builder.CreateICmp(Pred, X, ConstantInt::get(CompVal->getType(), RHS));
+ BranchInst *NewBI = Builder.CreateCondBr(Cond, EdgeBB, DefaultBB);
+ if (HasProfile)
+ setBranchWeights(*NewBI, BranchWeights, /*IsExpected=*/false);
+ // We don't need to update PHI nodes since we don't add any new edges.
+ } else {
+ // Create the new switch instruction now.
+ SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size());
+ if (HasProfile) {
+ // We know the weight of the default case. We don't know the weight of the
+ // other cases, but rather than completely lose profiling info, we split
+ // the remaining probability equally over them.
+ SmallVector<uint32_t> NewWeights(Values.size() + 1);
+ NewWeights[0] = BranchWeights[1]; // this is the default, and we swapped
+ // if TrueWhenEqual.
+ for (auto &V : drop_begin(NewWeights))
+ V = BranchWeights[0] / Values.size();
+ setBranchWeights(*New, NewWeights, /*IsExpected=*/false);
+ }
+
+ // Add all of the 'cases' to the switch instruction.
+ for (ConstantInt *Val : Values)
+ New->addCase(Val, EdgeBB);
+
+ // We added edges from PI to the EdgeBB. As such, if there were any
+ // PHI nodes in EdgeBB, they need entries to be added corresponding to
+ // the number of edges added.
+ for (BasicBlock::iterator BBI = EdgeBB->begin(); isa<PHINode>(BBI); ++BBI) {
+ PHINode *PN = cast<PHINode>(BBI);
+ Value *InVal = PN->getIncomingValueForBlock(BB);
+ for (unsigned i = 0, e = Values.size() - 1; i != e; ++i)
+ PN->addIncoming(InVal, BB);
+ }
}
// Erase the old branch instruction.
diff --git a/llvm/test/Transforms/SimplifyCFG/pr165088.ll b/llvm/test/Transforms/SimplifyCFG/pr165088.ll
new file mode 100644
index 0000000000000..4514a1927b586
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/pr165088.ll
@@ -0,0 +1,186 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -S -passes="simplifycfg<switch-range-to-icmp>" < %s | FileCheck %s
+
+; Avoid getting stuck in the cycle pr165088_cycle_[1-4].
+
+define void @pr165088_cycle_1(i8 %x) {
+; CHECK-LABEL: define void @pr165088_cycle_1(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: [[TMP0:%.*]] = icmp ult i8 [[X]], 2
+; CHECK-NEXT: br i1 [[TMP0]], label %[[BLOCK2:.*]], label %[[BLOCK3:.*]]
+; CHECK: [[BLOCK1:.*]]:
+; CHECK-NEXT: [[COND2:%.*]] = icmp ugt i8 [[X]], 1
+; CHECK-NEXT: br i1 [[COND2]], label %[[BLOCK3]], label %[[BLOCK2]]
+; CHECK: [[BLOCK2]]:
+; CHECK-NEXT: br label %[[BLOCK3]]
+; CHECK: [[BLOCK3]]:
+; CHECK-NEXT: [[COND3:%.*]] = icmp eq i8 [[X]], 0
+; CHECK-NEXT: br i1 [[COND3]], label %[[EXIT:.*]], label %[[BLOCK1]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ %switch = icmp uge i8 %x, 2
+ %cond1 = icmp ugt i8 %x, 1
+ %or.cond = and i1 %switch, %cond1
+ br i1 %or.cond, label %block3, label %block2
+
+block1:
+ %cond2 = icmp ugt i8 %x, 1
+ br i1 %cond2, label %block3, label %block2
+
+block2:
+ br label %block3
+
+block3:
+ %cond3 = icmp eq i8 %x, 0
+ br i1 %cond3, label %exit, label %block1
+
+exit:
+ ret void
+}
+
+define void @pr165088_cycle_2(i8 %x) {
+; CHECK-LABEL: define void @pr165088_cycle_2(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: [[SWITCH:%.*]] = icmp ult i8 [[X]], 2
+; CHECK-NEXT: br i1 [[SWITCH]], label %[[BLOCK2:.*]], label %[[BLOCK3:.*]]
+; CHECK: [[BLOCK1:.*]]:
+; CHECK-NEXT: [[COND2:%.*]] = icmp ugt i8 [[X]], 1
+; CHECK-NEXT: br i1 [[COND2]], label %[[BLOCK3]], label %[[BLOCK2]]
+; CHECK: [[BLOCK2]]:
+; CHECK-NEXT: br label %[[BLOCK3]]
+; CHECK: [[BLOCK3]]:
+; CHECK-NEXT: [[COND3:%.*]] = icmp eq i8 [[X]], 0
+; CHECK-NEXT: br i1 [[COND3]], label %[[EXIT:.*]], label %[[BLOCK1]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ switch i8 %x, label %block3 [
+ i8 1, label %block2
+ i8 0, label %block2
+ ]
+
+block1: ; preds = %block3
+ %cond2 = icmp ugt i8 %x, 1
+ br i1 %cond2, label %block3, label %block2
+
+block2: ; preds = %entry, %entry, %block1
+ br label %block3
+
+block3: ; preds = %entry, %block2, %block1
+ %cond3 = icmp eq i8 %x, 0
+ br i1 %cond3, label %exit, label %block1
+
+exit: ; preds = %block3
+ ret void
+}
+
+define void @pr165088_cycle_3(i8 %x) {
+; CHECK-LABEL: define void @pr165088_cycle_3(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: br label %[[BLOCK3:.*]]
+; CHECK: [[BLOCK3]]:
+; CHECK-NEXT: [[COND3:%.*]] = icmp eq i8 [[X]], 0
+; CHECK-NEXT: br i1 [[COND3]], label %[[EXIT:.*]], label %[[BLOCK3]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ switch i8 %x, label %block1 [
+ i8 1, label %block2
+ i8 0, label %block2
+ ]
+
+block1: ; preds = %entry, %block3
+ %cond2 = icmp ugt i8 %x, 1
+ br i1 %cond2, label %block3, label %block2
+
+block2: ; preds = %entry, %entry, %block1
+ br label %block3
+
+block3: ; preds = %block2, %block1
+ %cond3 = icmp eq i8 %x, 0
+ br i1 %cond3, label %exit, label %block1
+
+exit: ; preds = %block3
+ ret void
+}
+
+define void @pr165088_cycle_4(i8 %x) {
+; CHECK-LABEL: define void @pr165088_cycle_4(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: [[TMP0:%.*]] = icmp ult i8 [[X]], 2
+; CHECK-NEXT: br i1 [[TMP0]], label %[[BLOCK2:.*]], label %[[BLOCK3:.*]]
+; CHECK: [[BLOCK1:.*]]:
+; CHECK-NEXT: [[COND2_OLD:%.*]] = icmp ugt i8 [[X]], 1
+; CHECK-NEXT: br i1 [[COND2_OLD]], label %[[BLOCK3]], label %[[BLOCK2]]
+; CHECK: [[BLOCK2]]:
+; CHECK-NEXT: br label %[[BLOCK3]]
+; CHECK: [[BLOCK3]]:
+; CHECK-NEXT: [[COND3:%.*]] = icmp eq i8 [[X]], 0
+; CHECK-NEXT: br i1 [[COND3]], label %[[EXIT:.*]], label %[[BLOCK1]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ %switch = icmp ult i8 %x, 2
+ br i1 %switch, label %block2, label %block1
+
+block1: ; preds = %entry, %block3
+ %cond2 = icmp ugt i8 %x, 1
+ br i1 %cond2, label %block3, label %block2
+
+block2: ; preds = %entry, %block1
+ br label %block3
+
+block3: ; preds = %block2, %block1
+ %cond3 = icmp eq i8 %x, 0
+ br i1 %cond3, label %exit, label %block1
+
+exit: ; preds = %block3
+ ret void
+}
+
+define void @pr165088_original(i8 %x) {
+; CHECK-LABEL: define void @pr165088_original(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: [[TMP0:%.*]] = icmp ult i8 [[X]], 2
+; CHECK-NEXT: br i1 [[TMP0]], label %[[BLOCK2:.*]], label %[[BLOCK3:.*]]
+; CHECK: [[BLOCK1:.*]]:
+; CHECK-NEXT: [[COND3_OLD_OLD:%.*]] = icmp ugt i8 [[X]], 1
+; CHECK-NEXT: br i1 [[COND3_OLD_OLD]], label %[[BLOCK3]], label %[[BLOCK2]]
+; CHECK: [[BLOCK2]]:
+; CHECK-NEXT: br label %[[BLOCK3]]
+; CHECK: [[BLOCK3]]:
+; CHECK-NEXT: [[COND4:%.*]] = icmp eq i8 [[X]], 0
+; CHECK-NEXT: br i1 [[COND4]], label %[[EXIT:.*]], label %[[BLOCK1]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ %cond = icmp ne i8 %x, 0
+ %cond3 = icmp ne i8 %x, 0
+ %or.cond = and i1 %cond, %cond3
+ br i1 %or.cond, label %block3, label %block2
+
+block1: ; preds = %block3
+ %cond3.old = icmp ugt i8 %x, 1
+ br i1 %cond3.old, label %block3, label %block2
+
+block2: ; preds = %block1, %entry
+ br label %block3
+
+block3: ; preds = %block2, %block1, %entry
+ %cond4 = icmp eq i8 %x, 0
+ br i1 %cond4, label %exit, label %block1
+
+exit: ; preds = %block3
+ ret void
+}
>From 10254d4f63bf3b1cc0ad94349f7414c2fba35bb3 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 29 Oct 2025 01:33:06 +0800
Subject: [PATCH 2/2] [SimplifyCFG] Fix comments. NFC.
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 50272ddc46699..3cf7750b25eb8 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5228,9 +5228,8 @@ bool SimplifyCFGOpt::simplifyBranchOnICmpChain(BranchInst *BI,
CompVal, DL.getIntPtrType(CompVal->getType()), "magicptr");
}
- // Check if we can represent the values can be represented as a contiguous
- // range. If so, we use a range check + conditional branch instead of a
- // switch.
+ // Check if we can represent the values as a contiguous range. If so, we use a
+ // range check + conditional branch instead of a switch.
if (Values.front()->getValue() - Values.back()->getValue() ==
Values.size() - 1) {
ConstantRange RangeToCheck = ConstantRange::getNonEmpty(
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