[llvm] [SimplifyCFG]: Switch on umin replaces default (PR #164097)
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Sat Oct 18 08:44:41 PDT 2025
https://github.com/kper created https://github.com/llvm/llvm-project/pull/164097
A switch on `umin` can eliminate the default case by making the `umin`'s constant the default case.
Proof: https://alive2.llvm.org/ce/z/_N6nfs
Fixes: https://github.com/llvm/llvm-project/issues/162111
>From fba6d47789707e2bfb92addcd605210ce698299c Mon Sep 17 00:00:00 2001
From: Kevin Per <kevin.per at protonmail.com>
Date: Sat, 18 Oct 2025 12:26:41 +0000
Subject: [PATCH 1/2] [SimplifyCFG]: Remove unreachable when switch with umin
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 59 +++++++++++++++++++++++
1 file changed, 59 insertions(+)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index d831c2737e5f8..8d3c91e69ad48 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -7540,6 +7540,62 @@ static bool reduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
return true;
}
+/// Tries to transform the switch when the condition is umin and a constant.
+/// In that case, the default branch can be replaced by the constant's branch.
+/// For example:
+/// switch(umin(a, 3)) {
+/// case 0:
+/// case 1:
+/// case 2:
+/// case 3:
+/// // ...
+/// default:
+/// unreachable
+/// }
+///
+/// Transforms into:
+///
+/// switch(umin(a, 3)) {
+/// case 0:
+/// case 1:
+/// case 2:
+/// default:
+/// // This is case 3
+/// }
+static bool simplifySwitchWhenUMin(SwitchInst *SI, IRBuilder<> &Builder) {
+ auto *Call = dyn_cast<IntrinsicInst>(SI->getCondition());
+
+ if (!Call)
+ return false;
+
+ if (Call->getIntrinsicID() != Intrinsic::umin)
+ return false;
+
+ if (!SI->defaultDestUnreachable())
+ return false;
+
+ // Extract the constant operand from the intrinsic.
+ auto *Constant = dyn_cast<ConstantInt>(Call->getArgOperand(1));
+
+ if (!Constant) {
+ return false;
+ }
+
+ for (auto Case = SI->case_begin(), e = SI->case_end(); Case != e; Case++) {
+ uint64_t CaseValue = Case->getCaseValue()->getValue().getZExtValue();
+
+ // We found the case which is equal to the case's umin argument.
+ // We can make the case the default case.
+ if (Constant->equalsInt(CaseValue)) {
+ SI->setDefaultDest(Case->getCaseSuccessor());
+ SI->removeCase(Case);
+ return true;
+ }
+ }
+
+ return false;
+}
+
/// Tries to transform switch of powers of two to reduce switch range.
/// For example, switch like:
/// switch (C) { case 1: case 2: case 64: case 128: }
@@ -7966,6 +8022,9 @@ bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
if (simplifyDuplicateSwitchArms(SI, DTU))
return requestResimplify();
+ if (simplifySwitchWhenUMin(SI, Builder))
+ return requestResimplify();
+
return false;
}
>From c131865a96a26987aab9cc0a6c5cd116b53a58c0 Mon Sep 17 00:00:00 2001
From: Kevin Per <kevin.per at protonmail.com>
Date: Sat, 18 Oct 2025 15:35:29 +0000
Subject: [PATCH 2/2] [SimplifyCFG]: Added test
---
.../Transforms/SimplifyCFG/switch-umin.ll | 105 ++++++++++++++++++
1 file changed, 105 insertions(+)
create mode 100644 llvm/test/Transforms/SimplifyCFG/switch-umin.ll
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-umin.ll b/llvm/test/Transforms/SimplifyCFG/switch-umin.ll
new file mode 100644
index 0000000000000..69f78bf377dae
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/switch-umin.ll
@@ -0,0 +1,105 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -S -passes=simplifycfg < %s | FileCheck %s
+
+declare void @a()
+declare void @b()
+declare void @c()
+
+define void @switch_replace_default(i32 %x) {
+; CHECK-LABEL: define void @switch_replace_default(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT: [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 3)
+; CHECK-NEXT: switch i32 [[MIN]], label %[[COMMON_RET:.*]] [
+; CHECK-NEXT: i32 0, label %[[CASE0:.*]]
+; CHECK-NEXT: i32 1, label %[[CASE1:.*]]
+; CHECK-NEXT: i32 2, label %[[CASE2:.*]]
+; CHECK-NEXT: ]
+; CHECK: [[COMMON_RET]]:
+; CHECK-NEXT: ret void
+; CHECK: [[CASE0]]:
+; CHECK-NEXT: call void @a()
+; CHECK-NEXT: br label %[[COMMON_RET]]
+; CHECK: [[CASE1]]:
+; CHECK-NEXT: call void @b()
+; CHECK-NEXT: br label %[[COMMON_RET]]
+; CHECK: [[CASE2]]:
+; CHECK-NEXT: call void @c()
+; CHECK-NEXT: br label %[[COMMON_RET]]
+;
+ %min = call i32 @llvm.umin.i32(i32 %x, i32 3)
+ switch i32 %min, label %unreachable [
+ i32 0, label %case0
+ i32 1, label %case1
+ i32 2, label %case2
+ i32 3, label %case3
+ ]
+
+case0:
+ call void @a()
+ ret void
+
+case1:
+ call void @b()
+ ret void
+
+case2:
+ call void @c()
+ ret void
+
+case3:
+ ret void
+
+unreachable:
+ unreachable
+}
+
+define void @do_not_switch_replace_default(i32 %x, i32 %y) {
+; CHECK-LABEL: define void @do_not_switch_replace_default(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT: [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 [[Y]])
+; CHECK-NEXT: switch i32 [[MIN]], label %[[UNREACHABLE:.*]] [
+; CHECK-NEXT: i32 0, label %[[CASE0:.*]]
+; CHECK-NEXT: i32 1, label %[[CASE1:.*]]
+; CHECK-NEXT: i32 2, label %[[CASE2:.*]]
+; CHECK-NEXT: i32 3, label %[[COMMON_RET:.*]]
+; CHECK-NEXT: ]
+; CHECK: [[COMMON_RET]]:
+; CHECK-NEXT: ret void
+; CHECK: [[CASE0]]:
+; CHECK-NEXT: call void @a()
+; CHECK-NEXT: br label %[[COMMON_RET]]
+; CHECK: [[CASE1]]:
+; CHECK-NEXT: call void @b()
+; CHECK-NEXT: br label %[[COMMON_RET]]
+; CHECK: [[CASE2]]:
+; CHECK-NEXT: call void @c()
+; CHECK-NEXT: br label %[[COMMON_RET]]
+; CHECK: [[UNREACHABLE]]:
+; CHECK-NEXT: unreachable
+;
+ %min = call i32 @llvm.umin.i32(i32 %x, i32 %y)
+ switch i32 %min, label %unreachable [
+ i32 0, label %case0
+ i32 1, label %case1
+ i32 2, label %case2
+ i32 3, label %case3
+ ]
+
+case0:
+ call void @a()
+ ret void
+
+case1:
+ call void @b()
+ ret void
+
+case2:
+ call void @c()
+ ret void
+
+case3:
+ ret void
+
+unreachable:
+ unreachable
+}
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