[llvm] [SimplifyCFG]: Switch on umin replaces default (PR #164097)
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Sat Oct 18 10:46:32 PDT 2025
https://github.com/kper updated https://github.com/llvm/llvm-project/pull/164097
>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 1/3] [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 2/3] [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
+}
>From 4866e8f6666df8d88633f8363fafd30722420160 Mon Sep 17 00:00:00 2001
From: Kevin Per <kevin.per at protonmail.com>
Date: Sat, 18 Oct 2025 17:45:38 +0000
Subject: [PATCH 3/3] [SimplifyCFG]: Addressed feedback
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 42 +++++++++----------
.../Transforms/SimplifyCFG/switch-umin.ll | 2 +-
2 files changed, 20 insertions(+), 24 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 8d3c91e69ad48..ae2d216ad71ac 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -7555,45 +7555,41 @@ static bool reduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
///
/// Transforms into:
///
-/// switch(umin(a, 3)) {
+/// switch(a) {
/// 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;
+static bool simplifySwitchWhenUMin(SwitchInst *SI, DomTreeUpdater *DTU) {
+ Value *A;
+ ConstantInt *Constant;
- if (Call->getIntrinsicID() != Intrinsic::umin)
+ if (!match(SI->getCondition(), m_UMin(m_Value(A), m_ConstantInt(Constant))))
return false;
if (!SI->defaultDestUnreachable())
return false;
- // Extract the constant operand from the intrinsic.
- auto *Constant = dyn_cast<ConstantInt>(Call->getArgOperand(1));
+ auto Case = SI->findCaseValue(Constant);
- if (!Constant) {
+ // When the case value cannot be found then `findCaseValue` returns the
+ // default cause. This means that there is no `case 3:` and this
+ // simplification fails.
+ if (Case == SI->case_default())
return false;
- }
- for (auto Case = SI->case_begin(), e = SI->case_end(); Case != e; Case++) {
- uint64_t CaseValue = Case->getCaseValue()->getValue().getZExtValue();
+ BasicBlock *Unreachable = SI->getDefaultDest();
+ SI->setDefaultDest(Case->getCaseSuccessor());
+ SI->removeCase(Case);
+ SI->setCondition(A);
- // 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;
- }
- }
+ BasicBlock *BB = SI->getParent();
+ if (DTU)
+ DTU->applyUpdates({{DominatorTree::Delete, BB, Unreachable}});
- return false;
+ return true;
}
/// Tries to transform switch of powers of two to reduce switch range.
@@ -8022,7 +8018,7 @@ bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
if (simplifyDuplicateSwitchArms(SI, DTU))
return requestResimplify();
- if (simplifySwitchWhenUMin(SI, Builder))
+ if (simplifySwitchWhenUMin(SI, DTU))
return requestResimplify();
return false;
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-umin.ll b/llvm/test/Transforms/SimplifyCFG/switch-umin.ll
index 69f78bf377dae..fbc4d475b0264 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-umin.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-umin.ll
@@ -9,7 +9,7 @@ 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: switch i32 [[X]], label %[[COMMON_RET:.*]] [
; CHECK-NEXT: i32 0, label %[[CASE0:.*]]
; CHECK-NEXT: i32 1, label %[[CASE1:.*]]
; CHECK-NEXT: i32 2, label %[[CASE2:.*]]
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