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[InstCombine] Simplify switch with selects (#84143)
An example from https://github.com/image-rs/image: ``` define void @test_ult_rhsc(i8 %x) { %val = add nsw i8 %x, -2 %cmp = icmp ult i8 %val, 11 %cond = select i1 %cmp, i8 %val, i8 6 switch i8 %cond, label %bb1 [ i8 0, label %bb2 i8 10, label %bb3 ] bb1: call void @func1() unreachable bb2: call void @func2() unreachable bb3: call void @func3() unreachable } ``` When `%cmp` evaluates to false, we can prove that the range of `%val` is [11, umax]. Thus we can safely replace `%cond` with `%val` since both `switch 6` and `switch %val` go to the default dest `%bb1`. Alive2: https://alive2.llvm.org/ce/z/uSTj6w Godbolt: https://godbolt.org/z/MGrG84bzr This patch will benefit many rust applications and some C/C++ applications (e.g., cvc5).
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‎llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

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@@ -3572,6 +3572,38 @@ Instruction *InstCombinerImpl::visitBranchInst(BranchInst &BI) {
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return nullptr;
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}
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// Replaces (switch (select cond, X, C)/(select cond, C, X)) with (switch X) if
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// we can prove that both (switch C) and (switch X) go to the default when cond
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// is false/true.
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static Value *simplifySwitchOnSelectUsingRanges(SwitchInst &SI,
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SelectInst *Select,
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bool IsTrueArm) {
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unsigned CstOpIdx = IsTrueArm ? 1 : 2;
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auto *C = dyn_cast<ConstantInt>(Select->getOperand(CstOpIdx));
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if (!C)
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return nullptr;
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BasicBlock *CstBB = SI.findCaseValue(C)->getCaseSuccessor();
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if (CstBB != SI.getDefaultDest())
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return nullptr;
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Value *X = Select->getOperand(3 - CstOpIdx);
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ICmpInst::Predicate Pred;
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const APInt *RHSC;
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if (!match(Select->getCondition(),
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m_ICmp(Pred, m_Specific(X), m_APInt(RHSC))))
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return nullptr;
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if (IsTrueArm)
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Pred = ICmpInst::getInversePredicate(Pred);
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// See whether we can replace the select with X
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ConstantRange CR = ConstantRange::makeExactICmpRegion(Pred, *RHSC);
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for (auto Case : SI.cases())
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if (!CR.contains(Case.getCaseValue()->getValue()))
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return nullptr;
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return X;
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}
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Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
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Value *Cond = SI.getCondition();
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Value *Op0;
@@ -3645,6 +3677,16 @@ Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
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}
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}
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// Fold switch(select cond, X, Y) into switch(X/Y) if possible
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if (auto *Select = dyn_cast<SelectInst>(Cond)) {
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if (Value *V =
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simplifySwitchOnSelectUsingRanges(SI, Select, /*IsTrueArm=*/true))
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return replaceOperand(SI, 0, V);
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if (Value *V =
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simplifySwitchOnSelectUsingRanges(SI, Select, /*IsTrueArm=*/false))
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return replaceOperand(SI, 0, V);
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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-select.ll

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@@ -0,0 +1,159 @@
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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 void @test_ult_rhsc(i8 %x) {
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; CHECK-LABEL: define void @test_ult_rhsc(
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; CHECK-SAME: i8 [[X:%.*]]) {
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; CHECK-NEXT: switch i8 [[X]], label [[BB1:%.*]] [
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; CHECK-NEXT: i8 2, label [[BB2:%.*]]
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; CHECK-NEXT: i8 12, label [[BB3:%.*]]
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; CHECK-NEXT: ]
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; CHECK: bb1:
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; CHECK-NEXT: call void @func1()
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; CHECK-NEXT: unreachable
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; CHECK: bb2:
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; CHECK-NEXT: call void @func2()
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; CHECK-NEXT: unreachable
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; CHECK: bb3:
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; CHECK-NEXT: call void @func3()
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; CHECK-NEXT: unreachable
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;
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%val = add nsw i8 %x, -2
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%cmp = icmp ult i8 %val, 11
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%cond = select i1 %cmp, i8 %val, i8 6
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switch i8 %cond, label %bb1 [
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i8 0, label %bb2
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i8 10, label %bb3
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]
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bb1:
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call void @func1()
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unreachable
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bb2:
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call void @func2()
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unreachable
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bb3:
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call void @func3()
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unreachable
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}
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define void @test_eq_lhsc(i8 %x) {
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; CHECK-LABEL: define void @test_eq_lhsc(
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; CHECK-SAME: i8 [[X:%.*]]) {
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; CHECK-NEXT: switch i8 [[X]], label [[BB1:%.*]] [
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; CHECK-NEXT: i8 0, label [[BB2:%.*]]
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; CHECK-NEXT: i8 10, label [[BB3:%.*]]
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; CHECK-NEXT: ]
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; CHECK: bb1:
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; CHECK-NEXT: call void @func1()
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; CHECK-NEXT: unreachable
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; CHECK: bb2:
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; CHECK-NEXT: call void @func2()
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; CHECK-NEXT: unreachable
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; CHECK: bb3:
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; CHECK-NEXT: call void @func3()
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; CHECK-NEXT: unreachable
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;
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%cmp = icmp eq i8 %x, 4
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%cond = select i1 %cmp, i8 6, i8 %x
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switch i8 %cond, label %bb1 [
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i8 0, label %bb2
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i8 10, label %bb3
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]
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bb1:
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call void @func1()
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unreachable
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bb2:
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call void @func2()
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unreachable
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bb3:
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call void @func3()
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unreachable
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}
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define void @test_ult_rhsc_invalid_cond(i8 %x, i8 %y) {
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; CHECK-LABEL: define void @test_ult_rhsc_invalid_cond(
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; CHECK-SAME: i8 [[X:%.*]], i8 [[Y:%.*]]) {
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; CHECK-NEXT: [[VAL:%.*]] = add nsw i8 [[X]], -2
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; CHECK-NEXT: [[CMP:%.*]] = icmp ult i8 [[Y]], 11
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; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i8 [[VAL]], i8 6
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; CHECK-NEXT: switch i8 [[COND]], label [[BB1:%.*]] [
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; CHECK-NEXT: i8 0, label [[BB2:%.*]]
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; CHECK-NEXT: i8 10, label [[BB3:%.*]]
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; CHECK-NEXT: i8 13, label [[BB3]]
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; CHECK-NEXT: ]
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; CHECK: bb1:
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; CHECK-NEXT: call void @func1()
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; CHECK-NEXT: unreachable
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; CHECK: bb2:
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; CHECK-NEXT: call void @func2()
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; CHECK-NEXT: unreachable
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; CHECK: bb3:
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; CHECK-NEXT: call void @func3()
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; CHECK-NEXT: unreachable
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;
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%val = add nsw i8 %x, -2
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%cmp = icmp ult i8 %y, 11
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%cond = select i1 %cmp, i8 %val, i8 6
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switch i8 %cond, label %bb1 [
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i8 0, label %bb2
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i8 10, label %bb3
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i8 13, label %bb3
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]
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bb1:
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call void @func1()
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unreachable
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bb2:
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call void @func2()
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unreachable
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bb3:
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call void @func3()
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unreachable
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}
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define void @test_ult_rhsc_fail(i8 %x) {
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; CHECK-LABEL: define void @test_ult_rhsc_fail(
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; CHECK-SAME: i8 [[X:%.*]]) {
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; CHECK-NEXT: [[VAL:%.*]] = add nsw i8 [[X]], -2
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; CHECK-NEXT: [[CMP:%.*]] = icmp ult i8 [[VAL]], 11
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; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i8 [[VAL]], i8 6
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; CHECK-NEXT: switch i8 [[COND]], label [[BB1:%.*]] [
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; CHECK-NEXT: i8 0, label [[BB2:%.*]]
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; CHECK-NEXT: i8 10, label [[BB3:%.*]]
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; CHECK-NEXT: i8 13, label [[BB3]]
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; CHECK-NEXT: ]
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; CHECK: bb1:
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; CHECK-NEXT: call void @func1()
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; CHECK-NEXT: unreachable
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; CHECK: bb2:
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; CHECK-NEXT: call void @func2()
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; CHECK-NEXT: unreachable
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; CHECK: bb3:
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; CHECK-NEXT: call void @func3()
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; CHECK-NEXT: unreachable
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;
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%val = add nsw i8 %x, -2
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%cmp = icmp ult i8 %val, 11
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%cond = select i1 %cmp, i8 %val, i8 6
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switch i8 %cond, label %bb1 [
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i8 0, label %bb2
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i8 10, label %bb3
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i8 13, label %bb3
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]
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bb1:
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call void @func1()
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unreachable
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bb2:
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call void @func2()
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unreachable
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bb3:
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call void @func3()
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unreachable
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}
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declare void @func1()
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declare void @func2()
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declare void @func3()

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