[SimplifyCFG]: Switch on umin replaces default #164097

kper · 2025-10-18T15:44:34Z

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: #162111

llvmbot · 2025-10-18T15:45:02Z

@llvm/pr-subscribers-llvm-ir

@llvm/pr-subscribers-llvm-transforms

Author: None (kper)

Changes

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: #162111

Full diff: https://github.com/llvm/llvm-project/pull/164097.diff

2 Files Affected:

(modified) llvm/lib/Transforms/Utils/SimplifyCFG.cpp (+59)
(added) llvm/test/Transforms/SimplifyCFG/switch-umin.ll (+105)

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;
 }
 
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
+}

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

dtcxzyw · 2025-10-18T16:46:07Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+///
+/// Transforms into:
+///
+/// switch(umin(a, 3)) {


Suggested change

/// switch(umin(a, 3)) {

/// switch(a) {

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

dianqk · 2025-10-19T14:45:05Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+/// In that case, the default branch can be replaced by the constant's branch.
+/// For example:
+/// switch(umin(a, 3)) {
+/// case 0:


What about [4,1,2,3] or [1,2,3]?

Dead edges should be removed. Otherwise it will cause miscompilation: https://alive2.llvm.org/ce/z/Faeck4

Sorry, I mean we can remove dead edges: https://alive2.llvm.org/ce/z/hC3Dbm.

I think this needs to be explicitly checked in the transform, we shouldn't rely on eliminateDeadSwitchCases having removed such cases (looking at the implementation, it uses known bits rather than ranges, so I think it may not eliminate all dead cases if the umin is not at a power of two boundary).

Thanks :)
I have added a new commit which deletes cases where the value is higher than the constant

Missing test cases? I think you need add both tests of [1,2,3] that has holes and [4,1,2,3]?

dtcxzyw

LGTM. Thanks.
It should be free to handle smax/smin/umax as well, though these patterns are rare.
Output of https://github.com/dtcxzyw/llvm-tools/blob/main/minmax-switch.cpp on llvm-opt-benchmark:

Count: 4
llvm.smax 2
llvm.smin 4
llvm.umax 9
llvm.umin 10395

XChy

LGTM.

llvm/test/Transforms/SimplifyCFG/switch-umin.ll

XChy

Test coverage is needed for the new change.

XChy · 2025-10-20T09:17:08Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+
+  // A case is dead when its value is higher than the Constant.
+  SmallVector<ConstantInt *, 4> DeadCases;
+  for (auto Case = SI->case_begin(), e = SI->case_end(); Case != e; Case++) {


Suggested change

for (auto Case = SI->case_begin(), e = SI->case_end(); Case != e; Case++) {

for (auto Case : SI->cases()) {

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

kper · 2025-10-20T13:10:51Z

I applied the suggestions
I have added two new test cases for [1,2,3] and [4,1,2,3]
I created a new overloaded method SwitchInstProfUpdateWrapper::setDefaultDest which takes the CaseIt instead of the BB. This method will update the weight accordingly.

XChy

I would expect the dead cases to be removed even if no optimization opportunities in the motivating example.

XChy · 2025-10-21T10:46:51Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+  for (auto Case : SI->cases()) {
+    if (Case.getCaseValue()->getValue().ugt(Constant->getValue())) {
+      DeadCases.push_back(Case.getCaseValue());
+    }
+  }


Suggested change

for (auto Case : SI->cases()) {

if (Case.getCaseValue()->getValue().ugt(Constant->getValue())) {

DeadCases.push_back(Case.getCaseValue());

}

}

for (auto Case : SI->cases())

if (Case.getCaseValue()->getValue().ugt(Constant->getValue()))

DeadCases.push_back(Case.getCaseValue());

XChy · 2025-10-21T10:50:39Z

llvm/include/llvm/IR/Instructions.h


+  /// Delegate the call to the underlying SwitchInst::setDefaultCase and
+  /// remove correspondent branch weight.
+  LLVM_ABI void setDefaultDest(SwitchInst::CaseIt I);


replaceDefaultDest?

XChy · 2025-10-21T10:56:12Z

llvm/lib/IR/Instructions.cpp

+  auto DestBlock = I->getCaseSuccessor();
+  if (Weights) {
+    auto Weight = getSuccessorWeight(I->getCaseIndex() + 1);
+    if (Weight.has_value()) {


Weight should be available if Weights is available. Remove the check or use an assertion here.

kper · 2025-10-21T11:54:21Z

I would expect the dead cases to be removed even if no optimization opportunities in the motivating example.

I have included the recent suggestions. Also, I refactored the code s.t the dead case elimination happens first. I added a test which does not trigger the simplification because the dest is reachable but the dead cases were eliminated.

llvm/test/Transforms/SimplifyCFG/switch-umin.ll

dianqk · 2025-10-22T12:36:40Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+    BasicBlock *DeadCaseBB = DeadCase->getCaseSuccessor();
+    DeadCaseBB->removePredecessor(SIW->getParent());
+    SIW.removeCase(DeadCase);
+    Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});


Suggested change

Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});

Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});

Changed = true;

dianqk · 2025-10-22T12:41:56Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+  if (!SI->defaultDestUnreachable() || Case == SI->case_default()) {
+    if (DTU)
+      DTU->applyUpdates(Updates);
+    return false;
+  }
+
+  BasicBlock *Unreachable = SI->getDefaultDest();
+  SIW.replaceDefaultDest(Case);
+  SIW.removeCase(Case);
+  SIW->setCondition(A);
+
+  Updates.push_back({DominatorTree::Delete, BB, Unreachable});
+
+  if (DTU)
+    DTU->applyUpdates(Updates);
+
+  return true;


Why does it return false after removing the dead cases?

Suggested change

if (!SI->defaultDestUnreachable() || Case == SI->case_default()) {

if (DTU)

DTU->applyUpdates(Updates);

return false;

}

BasicBlock *Unreachable = SI->getDefaultDest();

SIW.replaceDefaultDest(Case);

SIW.removeCase(Case);

SIW->setCondition(A);

Updates.push_back({DominatorTree::Delete, BB, Unreachable});

if (DTU)

DTU->applyUpdates(Updates);

return true;

if (SI->defaultDestUnreachable() && Case != SI->case_default()) {

BasicBlock *Unreachable = SI->getDefaultDest();

SIW.replaceDefaultDest(Case);

SIW.removeCase(Case);

SIW->setCondition(A);

Updates.push_back({DominatorTree::Delete, BB, Unreachable});

Changed = true;

}

if (DTU)

DTU->applyUpdates(Updates);

return Changed;

kper added 2 commits October 18, 2025 12:27

[SimplifyCFG]: Remove unreachable when switch with umin

fba6d47

[SimplifyCFG]: Added test

c131865

llvmbot added the llvm:transforms label Oct 18, 2025

kper commented Oct 18, 2025

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dtcxzyw requested review from XChy, dianqk and nikic October 18, 2025 16:56

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[SimplifyCFG]: Addressed feedback

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dtcxzyw mentioned this pull request Oct 18, 2025

Task submission dtcxzyw/llvm-opt-benchmark#1312

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pre-commit: PR164097 dtcxzyw/llvm-opt-benchmark#2947

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[SimplifyCFG]: Updated weights

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pre-commit: PR164097 dtcxzyw/llvm-opt-benchmark#2950

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kper added 2 commits October 19, 2025 17:32

[SimplifyCFG]: Remove dead cases

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[SimplifyCFG]: Addressed feedback

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[SimplifyCFG]: Remove dead cases when umin pattern matches

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	for (auto Case = SI->case_begin(), e = SI->case_end(); Case != e; Case++) {
	for (auto Case : SI->cases()) {

	Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});
	Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});
	Changed = true;

[SimplifyCFG]: Switch on umin replaces default #164097

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[SimplifyCFG]: Switch on umin replaces default #164097

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kper commented Oct 18, 2025

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