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[InstCombine] Canonicalize switch(X << C) into switch(X) (#77068)
This patch canonicalizes `switch(X << C)` to `switch(X)`. If the shift may wrap, an and instruction will be created to mask out all of the shifted bits. Alive2: https://alive2.llvm.org/ce/z/wSsL5y NOTE: We can relax the one-use constraint. But I don't see any benefit in my benchmark. Compile-time impact: http://llvm-compile-time-tracker.com/compare.php?from=a776740d6296520b8bde156aa3f8d9ecb32cddd9&to=6dd783b9f90ae5f258102d732953567d7e317c02&stat=instructions%3Au |stage1-O3|stage1-ReleaseThinLTO|stage1-ReleaseLTO-g|stage1-O0-g|stage2-O3|stage2-O0-g|stage2-clang| |--|--|--|--|--|--|--| |-0.00%|+0.01%|-0.02%|-0.01%|+0.02%|-0.00%|+0.01%|
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‎llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

Lines changed: 27 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3220,6 +3220,33 @@ Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
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return replaceOperand(SI, 0, Op0);
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}
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uint64_t ShiftAmt;
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if (match(Cond, m_Shl(m_Value(Op0), m_ConstantInt(ShiftAmt))) &&
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ShiftAmt < Op0->getType()->getScalarSizeInBits() &&
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all_of(SI.cases(), [&](const auto &Case) {
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return Case.getCaseValue()->getValue().countr_zero() >= ShiftAmt;
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})) {
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// Change 'switch (X << 2) case 4:' into 'switch (X) case 1:'.
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OverflowingBinaryOperator *Shl = cast<OverflowingBinaryOperator>(Cond);
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if (Shl->hasNoUnsignedWrap() || Shl->hasNoSignedWrap() ||
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Shl->hasOneUse()) {
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Value *NewCond = Op0;
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if (!Shl->hasNoUnsignedWrap() && !Shl->hasNoSignedWrap()) {
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// If the shift may wrap, we need to mask off the shifted bits.
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unsigned BitWidth = Op0->getType()->getScalarSizeInBits();
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NewCond = Builder.CreateAnd(
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Op0, APInt::getLowBitsSet(BitWidth, BitWidth - ShiftAmt));
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}
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for (auto Case : SI.cases()) {
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const APInt &CaseVal = Case.getCaseValue()->getValue();
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APInt ShiftedCase = Shl->hasNoSignedWrap() ? CaseVal.ashr(ShiftAmt)
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: CaseVal.lshr(ShiftAmt);
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Case.setValue(ConstantInt::get(SI.getContext(), ShiftedCase));
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}
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return replaceOperand(SI, 0, NewCond);
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}
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}
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KnownBits Known = computeKnownBits(Cond, 0, &SI);
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unsigned LeadingKnownZeros = Known.countMinLeadingZeros();
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unsigned LeadingKnownOnes = Known.countMinLeadingOnes();

‎llvm/test/Transforms/InstCombine/switch-shl.ll

Lines changed: 185 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,185 @@
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; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
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; RUN: opt < %s -passes=instcombine -S | FileCheck %s
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define i1 @test_switch_with_shl_mask(i32 %a) {
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; CHECK-LABEL: define i1 @test_switch_with_shl_mask(
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; CHECK-SAME: i32 [[A:%.*]]) {
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[TRUNC:%.*]] = trunc i32 [[A]] to i8
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; CHECK-NEXT: switch i8 [[TRUNC]], label [[SW_DEFAULT:%.*]] [
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; CHECK-NEXT: i8 0, label [[SW_BB:%.*]]
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; CHECK-NEXT: i8 1, label [[SW_BB]]
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; CHECK-NEXT: i8 -128, label [[SW_BB]]
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; CHECK-NEXT: ]
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; CHECK: sw.bb:
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; CHECK-NEXT: ret i1 true
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; CHECK: sw.default:
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; CHECK-NEXT: ret i1 false
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;
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entry:
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%b = shl i32 %a, 24
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switch i32 %b, label %sw.default [
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i32 0, label %sw.bb
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i32 16777216, label %sw.bb
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i32 2147483648, label %sw.bb
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]
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sw.bb:
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ret i1 true
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sw.default:
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ret i1 false
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}
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define i1 @test_switch_with_shl_nuw_multiuse(i32 %a) {
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; CHECK-LABEL: define i1 @test_switch_with_shl_nuw_multiuse(
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; CHECK-SAME: i32 [[A:%.*]]) {
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[B:%.*]] = shl nuw i32 [[A]], 24
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; CHECK-NEXT: call void @use(i32 [[B]])
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; CHECK-NEXT: switch i32 [[A]], label [[SW_DEFAULT:%.*]] [
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; CHECK-NEXT: i32 0, label [[SW_BB:%.*]]
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; CHECK-NEXT: i32 1, label [[SW_BB]]
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; CHECK-NEXT: i32 128, label [[SW_BB]]
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; CHECK-NEXT: ]
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; CHECK: sw.bb:
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; CHECK-NEXT: ret i1 true
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; CHECK: sw.default:
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; CHECK-NEXT: ret i1 false
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;
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entry:
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%b = shl nuw i32 %a, 24
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call void @use(i32 %b)
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switch i32 %b, label %sw.default [
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i32 0, label %sw.bb
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i32 16777216, label %sw.bb
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i32 2147483648, label %sw.bb
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]
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sw.bb:
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ret i1 true
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sw.default:
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ret i1 false
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}
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define i1 @test_switch_with_shl_nsw_multiuse(i32 %a) {
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; CHECK-LABEL: define i1 @test_switch_with_shl_nsw_multiuse(
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; CHECK-SAME: i32 [[A:%.*]]) {
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[B:%.*]] = shl nsw i32 [[A]], 24
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; CHECK-NEXT: call void @use(i32 [[B]])
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; CHECK-NEXT: switch i32 [[A]], label [[SW_DEFAULT:%.*]] [
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; CHECK-NEXT: i32 0, label [[SW_BB:%.*]]
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; CHECK-NEXT: i32 1, label [[SW_BB]]
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; CHECK-NEXT: i32 -128, label [[SW_BB]]
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; CHECK-NEXT: ]
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; CHECK: sw.bb:
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; CHECK-NEXT: ret i1 true
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; CHECK: sw.default:
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; CHECK-NEXT: ret i1 false
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;
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entry:
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%b = shl nsw i32 %a, 24
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call void @use(i32 %b)
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switch i32 %b, label %sw.default [
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i32 0, label %sw.bb
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i32 16777216, label %sw.bb
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i32 2147483648, label %sw.bb
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]
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sw.bb:
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ret i1 true
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sw.default:
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ret i1 false
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}
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; Negative tests
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define i1 @test_switch_with_shl_mask_multiuse(i32 %a) {
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; CHECK-LABEL: define i1 @test_switch_with_shl_mask_multiuse(
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; CHECK-SAME: i32 [[A:%.*]]) {
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[B:%.*]] = shl i32 [[A]], 24
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; CHECK-NEXT: call void @use(i32 [[B]])
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; CHECK-NEXT: switch i32 [[B]], label [[SW_DEFAULT:%.*]] [
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; CHECK-NEXT: i32 0, label [[SW_BB:%.*]]
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; CHECK-NEXT: i32 16777216, label [[SW_BB]]
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; CHECK-NEXT: i32 -2147483648, label [[SW_BB]]
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; CHECK-NEXT: ]
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; CHECK: sw.bb:
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; CHECK-NEXT: ret i1 true
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; CHECK: sw.default:
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; CHECK-NEXT: ret i1 false
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;
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entry:
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%b = shl i32 %a, 24
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call void @use(i32 %b)
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switch i32 %b, label %sw.default [
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i32 0, label %sw.bb
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i32 16777216, label %sw.bb
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i32 2147483648, label %sw.bb
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]
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sw.bb:
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ret i1 true
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sw.default:
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ret i1 false
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}
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define i1 @test_switch_with_shl_mask_unknown_shamt(i32 %a, i32 %shamt) {
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; CHECK-LABEL: define i1 @test_switch_with_shl_mask_unknown_shamt(
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; CHECK-SAME: i32 [[A:%.*]], i32 [[SHAMT:%.*]]) {
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[B:%.*]] = shl i32 [[A]], [[SHAMT]]
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; CHECK-NEXT: switch i32 [[B]], label [[SW_DEFAULT:%.*]] [
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; CHECK-NEXT: i32 0, label [[SW_BB:%.*]]
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; CHECK-NEXT: i32 16777216, label [[SW_BB]]
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; CHECK-NEXT: i32 -2147483648, label [[SW_BB]]
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; CHECK-NEXT: ]
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; CHECK: sw.bb:
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; CHECK-NEXT: ret i1 true
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; CHECK: sw.default:
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; CHECK-NEXT: ret i1 false
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;
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entry:
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%b = shl i32 %a, %shamt
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switch i32 %b, label %sw.default [
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i32 0, label %sw.bb
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i32 16777216, label %sw.bb
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i32 2147483648, label %sw.bb
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]
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sw.bb:
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ret i1 true
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sw.default:
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ret i1 false
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}
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define i1 @test_switch_with_shl_mask_poison(i32 %a) {
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; CHECK-LABEL: define i1 @test_switch_with_shl_mask_poison(
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; CHECK-SAME: i32 [[A:%.*]]) {
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; CHECK-NEXT: entry:
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; CHECK-NEXT: switch i32 poison, label [[SW_DEFAULT:%.*]] [
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; CHECK-NEXT: i32 0, label [[SW_BB:%.*]]
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; CHECK-NEXT: i32 16777216, label [[SW_BB]]
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; CHECK-NEXT: i32 -2147483648, label [[SW_BB]]
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; CHECK-NEXT: ]
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; CHECK: sw.bb:
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; CHECK-NEXT: ret i1 true
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; CHECK: sw.default:
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; CHECK-NEXT: ret i1 false
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;
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entry:
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%b = shl i32 %a, 32
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switch i32 %b, label %sw.default [
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i32 0, label %sw.bb
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i32 16777216, label %sw.bb
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i32 2147483648, label %sw.bb
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]
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sw.bb:
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ret i1 true
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sw.default:
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ret i1 false
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}
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declare void @use(i32)

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