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[InstCombine] Canonicalize switch(X^C) expressions to switch(X) #143677

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[InstCombine] Canonicalize switch(X^C) expressions to switch(X) #143677

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b928647
[InstCombine] Introduce tests for PR143677 (NFC)
antoniofrighetto Aug 26, 2025
28f403a
[InstCombine] Canonicalize `switch(X^C)` to `switch(X)`
antoniofrighetto Aug 26, 2025
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45 changes: 26 additions & 19 deletions llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4229,26 +4229,33 @@ static Value *simplifySwitchOnSelectUsingRanges(SwitchInst &SI,
Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
Value *Cond = SI.getCondition();
Value *Op0;
ConstantInt *AddRHS;
if (match(Cond, m_Add(m_Value(Op0), m_ConstantInt(AddRHS)))) {
// Change 'switch (X+4) case 1:' into 'switch (X) case -3'.
for (auto Case : SI.cases()) {
Constant *NewCase = ConstantExpr::getSub(Case.getCaseValue(), AddRHS);
assert(isa<ConstantInt>(NewCase) &&
"Result of expression should be constant");
Case.setValue(cast<ConstantInt>(NewCase));
}
return replaceOperand(SI, 0, Op0);
}
const APInt *CondOpC;
using InvertFn = std::function<APInt(const APInt &Case, const APInt &C)>;

ConstantInt *SubLHS;
if (match(Cond, m_Sub(m_ConstantInt(SubLHS), m_Value(Op0)))) {
// Change 'switch (1-X) case 1:' into 'switch (X) case 0'.
for (auto Case : SI.cases()) {
Constant *NewCase = ConstantExpr::getSub(SubLHS, Case.getCaseValue());
assert(isa<ConstantInt>(NewCase) &&
"Result of expression should be constant");
Case.setValue(cast<ConstantInt>(NewCase));
auto MaybeInvertible = [&](Value *Cond) -> std::optional<InvertFn> {
if (match(Cond, m_Add(m_Value(Op0), m_APInt(CondOpC))))
// Change 'switch (X+C) case Case:' into 'switch (X) case Case-C'.
return [](const APInt &Case, const APInt &C) { return Case - C; };

if (match(Cond, m_Sub(m_APInt(CondOpC), m_Value(Op0))))
// Change 'switch (C-X) case Case:' into 'switch (X) case C-Case'.
return [](const APInt &Case, const APInt &C) { return C - Case; };

if (match(Cond, m_Xor(m_Value(Op0), m_APInt(CondOpC))) &&
!CondOpC->isMinSignedValue() && !CondOpC->isMaxSignedValue())
// Change 'switch (X^C) case Case:' into 'switch (X) case Case^C'.
// Prevent creation of large case values by excluding extremes.
return [](const APInt &Case, const APInt &C) { return Case ^ C; };

return std::nullopt;
};

// Attempt to invert and simplify the switch condition, as long as the
// condition is not used further, as it may be unprofitable otherwise.
if (auto InvertFn = MaybeInvertible(Cond); InvertFn && Cond->hasOneUse()) {
for (auto &Case : SI.cases()) {
const APInt &New = (*InvertFn)(Case.getCaseValue()->getValue(), *CondOpC);
Case.setValue(ConstantInt::get(SI.getContext(), New));
}
return replaceOperand(SI, 0, Op0);
}
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9 changes: 4 additions & 5 deletions llvm/test/Transforms/InstCombine/narrow-switch.ll
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Expand Up @@ -235,11 +235,10 @@ define i32 @trunc32to16(i32 %a0) #0 {
; ALL-NEXT: [[RETVAL:%.*]] = alloca i32, align 4
; ALL-NEXT: [[XOR:%.*]] = lshr i32 [[A0]], 16
; ALL-NEXT: [[TMP0:%.*]] = trunc nuw i32 [[XOR]] to i16
; ALL-NEXT: [[TRUNC:%.*]] = xor i16 [[TMP0]], 15784
; ALL-NEXT: switch i16 [[TRUNC]], label %[[SW_EPILOG:.*]] [
; ALL-NEXT: i16 63, label %[[SW_BB:.*]]
; ALL-NEXT: i16 1, label %[[SW_BB1:.*]]
; ALL-NEXT: i16 100, label %[[SW_BB2:.*]]
; ALL-NEXT: switch i16 [[TMP0]], label %[[SW_EPILOG:.*]] [
; ALL-NEXT: i16 15767, label %[[SW_BB:.*]]
; ALL-NEXT: i16 15785, label %[[SW_BB1:.*]]
; ALL-NEXT: i16 15820, label %[[SW_BB2:.*]]
; ALL-NEXT: ]
; ALL: [[SW_BB]]:
; ALL-NEXT: store i32 90, ptr [[RETVAL]], align 4
Expand Down
154 changes: 154 additions & 0 deletions llvm/test/Transforms/InstCombine/switch-xor.ll
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@@ -0,0 +1,154 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt < %s -passes=instcombine -S | FileCheck %s

define i1 @test_switch_with_xor(i32 %x) {
; CHECK-LABEL: define i1 @test_switch_with_xor(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: switch i32 [[X]], label %[[SW_DEFAULT:.*]] [
; CHECK-NEXT: i32 3, label %[[SW_BB:.*]]
; CHECK-NEXT: i32 0, label %[[SW_BB]]
; CHECK-NEXT: i32 1, label %[[SW_BB]]
; CHECK-NEXT: ]
; CHECK: [[SW_BB]]:
; CHECK-NEXT: ret i1 true
; CHECK: [[SW_DEFAULT]]:
; CHECK-NEXT: ret i1 false
;
entry:
%xor = xor i32 %x, 2
switch i32 %xor, label %sw.default [
i32 1, label %sw.bb
i32 2, label %sw.bb
i32 3, label %sw.bb
]

sw.bb:
ret i1 true
sw.default:
ret i1 false
}

; Negative tests.

define i1 @test_switch_with_xor_nonconstant_ops(i32 %x, i32 %y) {
; CHECK-LABEL: define i1 @test_switch_with_xor_nonconstant_ops(
; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: [[XOR:%.*]] = xor i32 [[X]], [[Y]]
; CHECK-NEXT: switch i32 [[XOR]], label %[[SW_DEFAULT:.*]] [
; CHECK-NEXT: i32 1, label %[[SW_BB:.*]]
; CHECK-NEXT: i32 2, label %[[SW_BB]]
; CHECK-NEXT: i32 3, label %[[SW_BB]]
; CHECK-NEXT: ]
; CHECK: [[SW_BB]]:
; CHECK-NEXT: ret i1 true
; CHECK: [[SW_DEFAULT]]:
; CHECK-NEXT: ret i1 false
;
entry:
%xor = xor i32 %x, %y
switch i32 %xor, label %sw.default [
i32 1, label %sw.bb
i32 2, label %sw.bb
i32 3, label %sw.bb
]

sw.bb:
ret i1 true
sw.default:
ret i1 false
}

define i1 @test_switch_with_xor_condition_multiple_uses(i32 %x) {
; CHECK-LABEL: define i1 @test_switch_with_xor_condition_multiple_uses(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: [[XOR:%.*]] = xor i32 [[X]], 2
; CHECK-NEXT: call void @opaque(i32 [[XOR]])
; CHECK-NEXT: switch i32 [[XOR]], label %[[SW_DEFAULT:.*]] [
; CHECK-NEXT: i32 1, label %[[SW_BB:.*]]
; CHECK-NEXT: i32 2, label %[[SW_BB]]
; CHECK-NEXT: i32 3, label %[[SW_BB]]
; CHECK-NEXT: ]
; CHECK: [[SW_BB]]:
; CHECK-NEXT: ret i1 true
; CHECK: [[SW_DEFAULT]]:
; CHECK-NEXT: ret i1 false
;
entry:
%xor = xor i32 %x, 2
call void @opaque(i32 %xor)
switch i32 %xor, label %sw.default [
i32 1, label %sw.bb
i32 2, label %sw.bb
i32 3, label %sw.bb
]

sw.bb:
ret i1 true
sw.default:
ret i1 false
}

define i1 @test_switch_with_xor_by_int_min(i64 %x) {
; CHECK-LABEL: define i1 @test_switch_with_xor_by_int_min(
; CHECK-SAME: i64 [[X:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: [[XOR:%.*]] = xor i64 [[X]], -9223372036854775808
; CHECK-NEXT: switch i64 [[XOR]], label %[[SW_DEFAULT:.*]] [
; CHECK-NEXT: i64 1, label %[[SW_BB:.*]]
; CHECK-NEXT: i64 2, label %[[SW_BB]]
; CHECK-NEXT: i64 3, label %[[SW_BB]]
; CHECK-NEXT: ]
; CHECK: [[SW_BB]]:
; CHECK-NEXT: ret i1 true
; CHECK: [[SW_DEFAULT]]:
; CHECK-NEXT: ret i1 false
;
entry:
%xor = xor i64 %x, -9223372036854775808
switch i64 %xor, label %sw.default [
i64 1, label %sw.bb
i64 2, label %sw.bb
i64 3, label %sw.bb
]

sw.bb: ; preds = %entry, %entry, %entry
ret i1 true

sw.default: ; preds = %entry
ret i1 false
}

define i1 @test_switch_with_xor_by_int_max(i64 %x) {
; CHECK-LABEL: define i1 @test_switch_with_xor_by_int_max(
; CHECK-SAME: i64 [[X:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: [[XOR:%.*]] = xor i64 [[X]], 9223372036854775807
; CHECK-NEXT: switch i64 [[XOR]], label %[[SW_DEFAULT:.*]] [
; CHECK-NEXT: i64 1, label %[[SW_BB:.*]]
; CHECK-NEXT: i64 2, label %[[SW_BB]]
; CHECK-NEXT: i64 3, label %[[SW_BB]]
; CHECK-NEXT: ]
; CHECK: [[SW_BB]]:
; CHECK-NEXT: ret i1 true
; CHECK: [[SW_DEFAULT]]:
; CHECK-NEXT: ret i1 false
;
entry:
%xor = xor i64 %x, 9223372036854775807
switch i64 %xor, label %sw.default [
i64 1, label %sw.bb
i64 2, label %sw.bb
i64 3, label %sw.bb
]

sw.bb: ; preds = %entry, %entry, %entry
ret i1 true

sw.default: ; preds = %entry
ret i1 false
}

declare void @opaque(i32)
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