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[ValueTracking] Try to infer range of select from true and false values. #68256

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Merged
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Oct 5, 2023
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[ValueTracking] Try to infer range of select from true and false values. #68256

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b605237
[ValueTracking] Try to infer range of select from true and false values.
mgudim Oct 4, 2023
da18398
Fixed failing test.
mgudim Oct 4, 2023
c499f0d
Addressed review comments.
mgudim Oct 5, 2023
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11 changes: 8 additions & 3 deletions llvm/lib/Analysis/ValueTracking.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -8841,9 +8841,14 @@ ConstantRange llvm::computeConstantRange(const Value *V, bool ForSigned,
CR = ConstantRange::getNonEmpty(Lower, Upper);
} else if (auto *II = dyn_cast<IntrinsicInst>(V))
CR = getRangeForIntrinsic(*II);
else if (auto *SI = dyn_cast<SelectInst>(V))
CR = getRangeForSelectPattern(*SI, IIQ);
else if (isa<FPToUIInst>(V) || isa<FPToSIInst>(V)) {
else if (auto *SI = dyn_cast<SelectInst>(V)) {
ConstantRange CRTrue = computeConstantRange(
SI->getTrueValue(), ForSigned, UseInstrInfo, AC, CtxI, DT, Depth + 1);
ConstantRange CRFalse = computeConstantRange(
SI->getFalseValue(), ForSigned, UseInstrInfo, AC, CtxI, DT, Depth + 1);
CR = CRTrue.unionWith(CRFalse);
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code wise looks fine.
Maybe add a todo for (or just implement) trying to use select condition. I.e we have something like:
select (icmp ule x, 10), x, 10
we can bound to [0, 10].
Something like:

const APInt * C;
if(match(SelectCond, m_ICmp(Pred, m_Specific(SelectTrue), m_APInt(C))) {
    CRTrue = CRTrue.intersectWith(ConstantRange::makeExactICmpRegion(Pred), *C);
}
if(match(SelectCond, m_ICmp(Pred, m_Specific(SelectFalse), m_APInt(C))) {
   CRFalse = CRFalse.intersectWith(ConstantRange::makeExactICmpRegion(getInverse(Pred), *C));
}

You could get a bit more sophisticated and do non-constants of just m_APInt(C) and build a CR for the other operand.

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@mgudim mgudim Oct 4, 2023 •
edited
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@goldsteinn I think setLimitsForSelectPattern which is called below does what you describe.

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not quite, but the logic i'm proposing looks like it belongs better there. I'll make a patch for that.

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Still think the logic belongs here actually, but feel free to commit w.o adding, I'll likely add a follow up shortly after.

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@goldsteinn Maybe I didn't understand what you mean. But I tried the following test without my patch:

define i1 @foo(i32 %x)  {
entry:
  %cmp0_ = icmp ule i32 %x, 10
  %select_ = select i1 %cmp0_, i32 %x, i32 10
  %cmp1_ = icmp ule i32 %select_, 10
  ret i1 %cmp1_
}

This simplifies to:

define i1 @foo(i32 %x) {
entry:
  ret i1 true
}

I put a print statement right after a call to setLimitsForSelectPattern(*SI, Lower, Upper, IIQ), the return range is [0, 11).
Can you please demonstrate what's missing with another test case?

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@mgudim I believe the case @goldsteinn has in mind is where it's not a min/max pattern. So e.g. if you do icmp ule i32 %x, 20 then you know the result is <= 20, but it's not a min/max pattern.

CR = CR.intersectWith(getRangeForSelectPattern(*SI, IIQ));
} else if (isa<FPToUIInst>(V) || isa<FPToSIInst>(V)) {
APInt Lower = APInt(BitWidth, 0);
APInt Upper = APInt(BitWidth, 0);
// TODO: Return ConstantRange.
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14 changes: 13 additions & 1 deletion llvm/test/Analysis/BasicAA/range.ll
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Expand Up @@ -2,6 +2,7 @@

%struct.S = type { i32, [2 x i32], i32 }
%struct.S2 = type { i32, [4 x i32], [4 x i32] }
@G = global [10 x i32] zeroinitializer, align 4

; CHECK: Function: t1
; CHECK: NoAlias: i32* %gep1, i32* %gep2
Expand Down Expand Up @@ -258,8 +259,19 @@ join:
ret void
}

declare void @llvm.assume(i1)

; CHECK-LABEL: Function: select_in_gep
; CHECK: NoAlias: i32* %arrayidx, i32* getelementptr inbounds ([10 x i32], ptr @G, i64 0, i64 3)
define i32 @select_in_gep(i1 %c) {
entry:
%select_ = select i1 %c, i64 2, i64 1
%arrayidx = getelementptr inbounds [10 x i32], ptr @G, i64 0, i64 %select_
store i32 42, ptr %arrayidx, align 4
%load_ = load i32, ptr getelementptr inbounds ([10 x i32], ptr @G, i64 0, i64 3), align 4
ret i32 %load_
}

declare void @llvm.assume(i1)

!0 = !{ i32 0, i32 2 }
!1 = !{ i32 0, i32 1 }
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4 changes: 2 additions & 2 deletions llvm/test/Transforms/InstCombine/binop-select.ll
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Expand Up @@ -324,12 +324,12 @@ define i32 @sub_sel_op1_use(i1 %b) {
; CHECK-LABEL: @sub_sel_op1_use(
; CHECK-NEXT: [[S:%.*]] = select i1 [[B:%.*]], i32 42, i32 41
; CHECK-NEXT: call void @use(i32 [[S]])
; CHECK-NEXT: [[R:%.*]] = sub nsw i32 42, [[S]]
; CHECK-NEXT: [[R:%.*]] = sub nuw nsw i32 42, [[S]]
; CHECK-NEXT: ret i32 [[R]]
;
%s = select i1 %b, i32 42, i32 41
call void @use(i32 %s)
%r = sub nsw i32 42, %s
%r = sub nuw nsw i32 42, %s
ret i32 %r
}

Expand Down