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[InstCombine] Simplify switch with selects #84143

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Merged
merged 3 commits into from
Apr 15, 2024
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[InstCombine] Simplify switch with selects #84143

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0c4c262
[InstCombine] Add pre-commit tests. NFC.
dtcxzyw Mar 6, 2024
28ed09c
[InstCombine] Simplify switch with selects
dtcxzyw Mar 6, 2024
1828c72
[InstCombine] Address comments.
dtcxzyw Mar 22, 2024
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42 changes: 42 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3572,6 +3572,38 @@ Instruction *InstCombinerImpl::visitBranchInst(BranchInst &BI) {
return nullptr;
}

// Replaces (switch (select cond, X, C)/(select cond, C, X)) with (switch X) if
// we can prove that both (switch C) and (switch X) go to the default when cond
// is false/true.
static Value *simplifySwitchOnSelectUsingRanges(SwitchInst &SI,
SelectInst *Select,
bool IsTrueArm) {
unsigned CstOpIdx = IsTrueArm ? 1 : 2;
auto *C = dyn_cast<ConstantInt>(Select->getOperand(CstOpIdx));
if (!C)
return nullptr;

BasicBlock *CstBB = SI.findCaseValue(C)->getCaseSuccessor();
if (CstBB != SI.getDefaultDest())
return nullptr;
Value *X = Select->getOperand(3 - CstOpIdx);
ICmpInst::Predicate Pred;
const APInt *RHSC;
if (!match(Select->getCondition(),
m_ICmp(Pred, m_Specific(X), m_APInt(RHSC))))
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Is there a reason to only handle RHSC?

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We can improve this by using KnownBits/ConstantRange in the future. But I guess this patch covers most of the cases in rust applications.

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Think you could also technically generalize to RHSC (or Other arm really) implying any single case and not condition implying the same condition for the arm, although not sure if that would practically ever come up. Think we would just simplify the select.

return nullptr;
if (IsTrueArm)
Pred = ICmpInst::getInversePredicate(Pred);

// See whether we can replace the select with X
ConstantRange CR = ConstantRange::makeExactICmpRegion(Pred, *RHSC);
for (auto Case : SI.cases())
if (!CR.contains(Case.getCaseValue()->getValue()))
return nullptr;

return X;
}

Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
Value *Cond = SI.getCondition();
Value *Op0;
Expand Down Expand Up @@ -3645,6 +3677,16 @@ Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
}
}

// Fold switch(select cond, X, Y) into switch(X/Y) if possible
if (auto *Select = dyn_cast<SelectInst>(Cond)) {
if (Value *V =
simplifySwitchOnSelectUsingRanges(SI, Select, /*IsTrueArm=*/true))
return replaceOperand(SI, 0, V);
if (Value *V =
simplifySwitchOnSelectUsingRanges(SI, Select, /*IsTrueArm=*/false))
return replaceOperand(SI, 0, V);
}

KnownBits Known = computeKnownBits(Cond, 0, &SI);
unsigned LeadingKnownZeros = Known.countMinLeadingZeros();
unsigned LeadingKnownOnes = Known.countMinLeadingOnes();
Expand Down
159 changes: 159 additions & 0 deletions llvm/test/Transforms/InstCombine/switch-select.ll
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@@ -0,0 +1,159 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
; RUN: opt < %s -passes=instcombine -S | FileCheck %s

define void @test_ult_rhsc(i8 %x) {
; CHECK-LABEL: define void @test_ult_rhsc(
; CHECK-SAME: i8 [[X:%.*]]) {
; CHECK-NEXT: switch i8 [[X]], label [[BB1:%.*]] [
; CHECK-NEXT: i8 2, label [[BB2:%.*]]
; CHECK-NEXT: i8 12, label [[BB3:%.*]]
; CHECK-NEXT: ]
; CHECK: bb1:
; CHECK-NEXT: call void @func1()
; CHECK-NEXT: unreachable
; CHECK: bb2:
; CHECK-NEXT: call void @func2()
; CHECK-NEXT: unreachable
; CHECK: bb3:
; CHECK-NEXT: call void @func3()
; CHECK-NEXT: unreachable
;
%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
}

define void @test_eq_lhsc(i8 %x) {
; CHECK-LABEL: define void @test_eq_lhsc(
; CHECK-SAME: i8 [[X:%.*]]) {
; CHECK-NEXT: switch i8 [[X]], label [[BB1:%.*]] [
; CHECK-NEXT: i8 0, label [[BB2:%.*]]
; CHECK-NEXT: i8 10, label [[BB3:%.*]]
; CHECK-NEXT: ]
; CHECK: bb1:
; CHECK-NEXT: call void @func1()
; CHECK-NEXT: unreachable
; CHECK: bb2:
; CHECK-NEXT: call void @func2()
; CHECK-NEXT: unreachable
; CHECK: bb3:
; CHECK-NEXT: call void @func3()
; CHECK-NEXT: unreachable
;
%cmp = icmp eq i8 %x, 4
%cond = select i1 %cmp, i8 6, i8 %x
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
}

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

bb1:
call void @func1()
unreachable
bb2:
call void @func2()
unreachable
bb3:
call void @func3()
unreachable
}

define void @test_ult_rhsc_fail(i8 %x) {
; CHECK-LABEL: define void @test_ult_rhsc_fail(
; CHECK-SAME: i8 [[X:%.*]]) {
; CHECK-NEXT: [[VAL:%.*]] = add nsw i8 [[X]], -2
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i8 [[VAL]], 11
; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i8 [[VAL]], i8 6
; CHECK-NEXT: switch i8 [[COND]], label [[BB1:%.*]] [
; CHECK-NEXT: i8 0, label [[BB2:%.*]]
; CHECK-NEXT: i8 10, label [[BB3:%.*]]
; CHECK-NEXT: i8 13, label [[BB3]]
; CHECK-NEXT: ]
; CHECK: bb1:
; CHECK-NEXT: call void @func1()
; CHECK-NEXT: unreachable
; CHECK: bb2:
; CHECK-NEXT: call void @func2()
; CHECK-NEXT: unreachable
; CHECK: bb3:
; CHECK-NEXT: call void @func3()
; CHECK-NEXT: unreachable
;
%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
i8 13, label %bb3
]

bb1:
call void @func1()
unreachable
bb2:
call void @func2()
unreachable
bb3:
call void @func3()
unreachable
}

declare void @func1()
declare void @func2()
declare void @func3()