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[SimplifyCFG]: Switch on umin replaces default #164097

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fba6d47
[SimplifyCFG]: Remove unreachable when switch with umin
kper Oct 18, 2025
c131865
[SimplifyCFG]: Added test
kper Oct 18, 2025
4866e8f
[SimplifyCFG]: Addressed feedback
kper Oct 18, 2025
27cfa8b
[SimplifyCFG]: Updated weights
kper Oct 18, 2025
8a5bd8c
[SimplifyCFG]: Remove dead cases
kper Oct 19, 2025
64443b7
[SimplifyCFG]: Remove header
kper Oct 19, 2025
cf5a0c2
[SimplifyCFG]: Addressed feedback
kper Oct 20, 2025
7443e6e
[SimplifyCFG]: Update default case weight
kper Oct 20, 2025
8865c3f
[SimplifyCFG]: Addressed feedback
kper Oct 21, 2025
bf76fa4
[SimplifyCFG]: Remove dead cases when umin pattern matches
kper Oct 21, 2025
fb77844
[SimplifyCFG]: Fixed return when dead cases are eliminated
kper Oct 22, 2025
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5 changes: 5 additions & 0 deletions llvm/include/llvm/IR/Instructions.h
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Original file line number Diff line number Diff line change
Expand Up @@ -3556,6 +3556,11 @@ class SwitchInstProfUpdateWrapper {
/// correspondent branch weight.
LLVM_ABI SwitchInst::CaseIt removeCase(SwitchInst::CaseIt I);

/// Replace the default destination by given case. Delegate the call to
/// the underlying SwitchInst::setDefaultDest and remove correspondent branch
/// weight.
LLVM_ABI void replaceDefaultDest(SwitchInst::CaseIt I);

/// Delegate the call to the underlying SwitchInst::addCase() and set the
/// specified branch weight for the added case.
LLVM_ABI void addCase(ConstantInt *OnVal, BasicBlock *Dest, CaseWeightOpt W);
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10 changes: 10 additions & 0 deletions llvm/lib/IR/Instructions.cpp
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Expand Up @@ -4171,6 +4171,16 @@ SwitchInstProfUpdateWrapper::removeCase(SwitchInst::CaseIt I) {
return SI.removeCase(I);
}

void SwitchInstProfUpdateWrapper::replaceDefaultDest(SwitchInst::CaseIt I) {
auto *DestBlock = I->getCaseSuccessor();
if (Weights) {
auto Weight = getSuccessorWeight(I->getCaseIndex() + 1);
(*Weights)[0] = Weight.value();
}

SI.setDefaultDest(DestBlock);
}

void SwitchInstProfUpdateWrapper::addCase(
ConstantInt *OnVal, BasicBlock *Dest,
SwitchInstProfUpdateWrapper::CaseWeightOpt W) {
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79 changes: 79 additions & 0 deletions llvm/lib/Transforms/Utils/SimplifyCFG.cpp
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Expand Up @@ -7540,6 +7540,82 @@ static bool reduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
return true;
}

/// Tries to transform the switch when the condition is umin with a constant.
/// In that case, the default branch can be replaced by the constant's branch.
/// This method also removes dead cases when the simplification cannot replace
/// the default branch.
///
/// For example:
/// switch(umin(a, 3)) {
/// case 0:
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What about [4,1,2,3] or [1,2,3]?

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

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@dianqk dianqk Oct 19, 2025 •
edited
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Sorry, I mean we can remove dead edges: https://alive2.llvm.org/ce/z/hC3Dbm.

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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).

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

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

/// case 1:
/// case 2:
/// case 3:
/// case 4:
/// // ...
/// default:
/// unreachable
/// }
///
/// Transforms into:
///
/// switch(a) {
/// case 0:
/// case 1:
/// case 2:
/// default:
/// // This is case 3
/// }
static bool simplifySwitchWhenUMin(SwitchInst *SI, DomTreeUpdater *DTU) {
Value *A;
ConstantInt *Constant;

if (!match(SI->getCondition(), m_UMin(m_Value(A), m_ConstantInt(Constant))))
return false;

SmallVector<DominatorTree::UpdateType> Updates;
SwitchInstProfUpdateWrapper SIW(*SI);
BasicBlock *BB = SIW->getParent();

// Dead cases are removed even when the simplification fails.
// A case is dead when its value is higher than the Constant.
SmallVector<ConstantInt *, 4> DeadCases;
for (auto Case : SI->cases())
if (Case.getCaseValue()->getValue().ugt(Constant->getValue()))
DeadCases.push_back(Case.getCaseValue());

for (ConstantInt *DeadCaseVal : DeadCases) {
SwitchInst::CaseIt DeadCase = SIW->findCaseValue(DeadCaseVal);
BasicBlock *DeadCaseBB = DeadCase->getCaseSuccessor();
DeadCaseBB->removePredecessor(SIW->getParent());
SIW.removeCase(DeadCase);
Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});
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Suggested change
Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});
Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});
Changed = true;

}

auto Case = SI->findCaseValue(Constant);
// If the case value is not found, `findCaseValue` returns the default case.
// In this scenario, since there is no explicit `case 3:`, the simplification
// fails. The simplification also fails when the switch’s default destination
// is reachable.
if (!SI->defaultDestUnreachable() || Case == SI->case_default()) {
if (DTU)
DTU->applyUpdates(Updates);
return !Updates.empty();
}

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;
}

/// 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: }
Expand Down Expand Up @@ -7966,6 +8042,9 @@ bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
if (simplifyDuplicateSwitchArms(SI, DTU))
return requestResimplify();

if (simplifySwitchWhenUMin(SI, DTU))
return requestResimplify();

return false;
}

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246 changes: 246 additions & 0 deletions llvm/test/Transforms/SimplifyCFG/switch-umin.ll
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@@ -0,0 +1,246 @@
; 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()
declare void @d()

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 [[X]], label %[[COMMON_RET:.*]] [
; CHECK-NEXT: i32 0, label %[[CASE0:.*]]
; CHECK-NEXT: i32 1, label %[[CASE1:.*]]
; CHECK-NEXT: i32 2, label %[[CASE2:.*]]
; CHECK-NEXT: ], !prof [[PROF0:![0-9]+]]
; 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
], !prof !0

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 @switch_replace_default_and_remove_dead_cases(i32 %x) {
; CHECK-LABEL: define void @switch_replace_default_and_remove_dead_cases(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 3)
; CHECK-NEXT: switch i32 [[X]], label %[[COMMON_RET:.*]] [
; CHECK-NEXT: i32 2, label %[[CASE2:.*]]
; CHECK-NEXT: i32 1, label %[[CASE1:.*]]
; CHECK-NEXT: ]
; CHECK: [[COMMON_RET]]:
; CHECK-NEXT: ret void
; 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 4, label %case4
i32 1, label %case1
i32 2, label %case2
i32 3, label %case3
]

case4:
call void @a()
ret void

case1:
call void @b()
ret void

case2:
call void @c()
ret void

case3:
ret void

unreachable:
unreachable
}

define void @switch_replace_default_when_holes(i32 %x) {
; CHECK-LABEL: define void @switch_replace_default_when_holes(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 3)
; CHECK-NEXT: switch i32 [[X]], label %[[COMMON_RET:.*]] [
; CHECK-NEXT: i32 1, label %[[CASE1:.*]]
; CHECK-NEXT: i32 2, label %[[CASE2:.*]]
; CHECK-NEXT: ]
; CHECK: [[COMMON_RET]]:
; CHECK-NEXT: ret void
; 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 1, label %case1
i32 2, label %case2
i32 3, label %case3
]

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
}

define void @do_not_replace_switch_default_but_remove_dead_cases(i32 %x) {
; CHECK-LABEL: define void @do_not_replace_switch_default_but_remove_dead_cases(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 3)
; CHECK-NEXT: switch i32 [[MIN]], label %[[CASE0:.*]] [
; CHECK-NEXT: i32 3, label %[[COMMON_RET:.*]]
; 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 %case0 [ ; default is reachable, therefore simplification not triggered
i32 0, label %case0
i32 1, label %case1
i32 2, label %case2
i32 3, label %case3
i32 4, label %case4
]

case0:
call void @a()
ret void

case1:
call void @b()
ret void

case2:
call void @c()
ret void

case3:
ret void

case4:
call void @d()
ret void

}


!0 = !{!"branch_weights", i32 1, i32 2, i32 3, i32 99, i32 5}
;.
; CHECK: [[PROF0]] = !{!"branch_weights", i32 5, i32 2, i32 3, i32 99}
;.