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[ValueTracking] Implement sdiv/udiv support for isKnownNonNullFromDominatingCondition #67282

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Merged
merged 4 commits into from
Oct 20, 2023
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[ValueTracking] Implement sdiv/udiv support for isKnownNonNullFromDominatingCondition #67282

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2b8d93f
[ValueTracking] Implement udiv support for isKnownNonNullFromDominati…
dc03 Sep 24, 2023
0d1a47f
Use pattern matching
dc03 Sep 25, 2023
ee12907
Add sdiv support + Add more tests, clarifying terminology
dc03 Sep 25, 2023
9ffa883
Replace UDiv/SDiv matchers by IDiv
dc03 Sep 28, 2023
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4 changes: 4 additions & 0 deletions llvm/lib/Analysis/ValueTracking.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2264,6 +2264,10 @@ static bool isKnownNonNullFromDominatingCondition(const Value *V,
return true;
}

if (match(U, m_IDiv(m_Value(), m_Specific(V))) &&
isValidAssumeForContext(cast<Instruction>(U), CtxI, DT))
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@dtcxzyw dtcxzyw Sep 25, 2023

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isValidAssumeForContext assumes the first argument is an assume intrinsic. It is incorrect to pass a udiv.

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@dc03 dc03 Sep 25, 2023

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Hmm, I am not sure of any other way to check for post-dominance.

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@dtcxzyw dtcxzyw Sep 25, 2023

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Maybe DT->dominates(cast<Instruction>(U), CtxI)? It guarantees we execute udiv before select. Then we can simplify the pattern in simplifySelectWithICmpCond.

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@dc03 dc03 Sep 25, 2023 •
edited
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In this case, CtxI is always the icmp, so the dominates call will only work when the udiv dominates the icmp and not the other way round.

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@nikic nikic Sep 25, 2023

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isValidAssumeForContext() should be fine to use for non-assumes as well. The key property of assumes here is that violating the fact results in immediate undefined behavior, which is also true for divisions.

Though I guess the isEphemeralValueOf() part of the function is only relevant for assumes...

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@dc03 dc03 Oct 19, 2023

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@nikic Any issues that could occur with isEphemeralValueOf()?

return true;

// Consider only compare instructions uniquely controlling a branch
Value *RHS;
CmpInst::Predicate Pred;
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54 changes: 54 additions & 0 deletions llvm/test/Analysis/ValueTracking/select-known-non-zero.ll
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Expand Up @@ -393,3 +393,57 @@ define i1 @inv_select_v_sle_nonneg(i8 %v, i8 %C, i8 %y) {
%r = icmp eq i8 %s, 0
ret i1 %r
}

; Check udiv/sdiv occuring before icmp.
define i64 @incorrect_safe_div_1(i64 %n, i64 %d) {
; CHECK-LABEL: @incorrect_safe_div_1(
; CHECK-NEXT: [[TMP1:%.*]] = udiv i64 [[N:%.*]], [[D:%.*]]
; CHECK-NEXT: ret i64 [[TMP1]]
;
%1 = udiv i64 %n, %d
%2 = icmp eq i64 %d, 0
%3 = select i1 %2, i64 -1, i64 %1
ret i64 %3
}

; Check icmp occuring before udiv/sdiv.
define i64 @incorrect_safe_div_2(i64 %n, i64 %d) {
; CHECK-LABEL: @incorrect_safe_div_2(
; CHECK-NEXT: [[TMP1:%.*]] = sdiv i64 [[N:%.*]], [[D:%.*]]
; CHECK-NEXT: ret i64 [[TMP1]]
;
%1 = icmp eq i64 %d, 0
%2 = sdiv i64 %n, %d
%3 = select i1 %1, i64 -1, i64 %2
ret i64 %3
}

define i64 @incorrect_safe_div_call_1(i64 %n, i64 %d) {
; CHECK-LABEL: @incorrect_safe_div_call_1(
; CHECK-NEXT: [[TMP1:%.*]] = sdiv i64 [[N:%.*]], [[D:%.*]]
; CHECK-NEXT: tail call void @use(i64 [[D]])
; CHECK-NEXT: ret i64 [[TMP1]]
;
%1 = sdiv i64 %n, %d
tail call void @use(i64 %d)
%2 = icmp eq i64 %d, 0
%3 = select i1 %2, i64 -1, i64 %1
ret i64 %3
}

define i64 @incorrect_safe_div_call_2(i64 %n, i64 %d) {
; CHECK-LABEL: @incorrect_safe_div_call_2(
; CHECK-NEXT: [[TMP1:%.*]] = icmp eq i64 [[D:%.*]], 0
; CHECK-NEXT: tail call void @use(i64 [[D]])
; CHECK-NEXT: [[TMP2:%.*]] = udiv i64 [[N:%.*]], [[D]]
; CHECK-NEXT: [[TMP3:%.*]] = select i1 [[TMP1]], i64 -1, i64 [[TMP2]]
; CHECK-NEXT: ret i64 [[TMP3]]
;
%1 = icmp eq i64 %d, 0
tail call void @use(i64 %d)
%2 = udiv i64 %n, %d
%3 = select i1 %1, i64 -1, i64 %2
ret i64 %3
}

declare void @use(i64)