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[msan] Fix "Add optional flag to improve instrumentation of disjoint OR (#145990)" #146799

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Merged
merged 3 commits into from
Jul 3, 2025
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[msan] Fix "Add optional flag to improve instrumentation of disjoint OR (#145990)" #146799

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7a7aeb0
[msan] Fix "Add optional flag to improve instrumentation of disjoint …
thurstond Jul 2, 2025
86a1df3
Recast operands as ints instead, per Florian2
thurstond Jul 2, 2025
7eb398e
Refactor to NotV1 / NotV2
thurstond Jul 2, 2025
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7 changes: 6 additions & 1 deletion llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2512,6 +2512,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
// S = S | (V1 & V2)
Value *S1 = getShadow(&I, 0);
Value *S2 = getShadow(&I, 1);
// Gotcha: V1 and V2 are NOT'ed here
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@fmayer fmayer Jul 2, 2025

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change the names instead?

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@thurstond thurstond Jul 2, 2025

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The composite names would get clunky and possibly ambiguous:

    // Name is ambiguous: it's ((NotV1) & S2), as opposed to (Not (V1 & S2))
    Value *NotV1S2 = IRB.CreateAnd(NotV1, S2);
    Value *S1NotV2 = IRB.CreateAnd(S1, NotV2);

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@fmayer fmayer Jul 2, 2025

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maybe give them more meaningless names then, so people have to look up the definition? idk tho

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@thurstond thurstond Jul 3, 2025

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I changed the order to make it unambiguous

Value *V1 = IRB.CreateNot(I.getOperand(0));
Value *V2 = IRB.CreateNot(I.getOperand(1));
if (V1->getType() != S1->getType()) {
Expand All @@ -2524,7 +2525,11 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {

Value *S = IRB.CreateOr({S1S2, V1S2, S1V2});
if (ClPreciseDisjointOr && cast<PossiblyDisjointInst>(&I)->isDisjoint()) {
Value *V1V2 = IRB.CreateAnd(V1, V2);
// "V1" and "V2" were NOT'ed above, but we still want to reuse them
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@fmayer fmayer Jul 2, 2025

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Is that really better than just int casting them again?

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@thurstond thurstond Jul 2, 2025

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I'll do that

// because they were IntCast'ed to the same type as the shadows.
//
// (V1 & V2) == ~(~V1 | ~V2) (de Morgan)
Value *V1V2 = IRB.CreateNot(IRB.CreateOr(V1, V2));
S = IRB.CreateOr({S, V1V2});
}

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5 changes: 3 additions & 2 deletions llvm/test/Instrumentation/MemorySanitizer/or.ll
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Expand Up @@ -45,8 +45,9 @@ define i8 @test_disjoint_or(i8 %a, i8 %b) sanitize_memory {
; CHECK-IMPRECISE: [[C:%.*]] = or disjoint i8 [[A]], [[B]]
; CHECK-IMPRECISE-NEXT: store i8 [[TMP11]], ptr @__msan_retval_tls, align 8
;
; CHECK-PRECISE: [[TMP10:%.*]] = and i8 [[TMP3]], [[TMP4]]
; CHECK-PRECISE-NEXT: [[TMP12:%.*]] = or i8 [[TMP11]], [[TMP10]]
; CHECK-PRECISE: [[TMP10:%.*]] = or i8 [[TMP3]], [[TMP4]]
; CHECK-PRECISE-NEXT: [[TMP11:%.*]] = xor i8 [[TMP10]], -1
; CHECK-PRECISE-NEXT: [[TMP12:%.*]] = or i8 [[TMP9]], [[TMP11]]
; CHECK-PRECISE-NEXT: [[C:%.*]] = or disjoint i8 [[A]], [[B]]
; CHECK-PRECISE-NEXT: store i8 [[TMP12]], ptr @__msan_retval_tls, align 8
;
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