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[msan] Sharpen instrumentation of Intrinsic::{ctlz,cttz} #145609

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Merged
merged 6 commits into from
Jun 25, 2025
Merged

[msan] Sharpen instrumentation of Intrinsic::{ctlz,cttz} #145609

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13736a6
[msan] Sharpen instrumentation of Intrinsic::{ctlz,cttz}
thurstond Jun 24, 2025
3cee7d1
Add comment about possible further improvement
thurstond Jun 24, 2025
358dc26
Typo
thurstond Jun 24, 2025
3656c9e
Remove stray change
thurstond Jun 24, 2025
23e3b65
Clarify little-endian (and ctlz)
thurstond Jun 25, 2025
fb7c2a7
Clarify zero-poison comment
thurstond Jun 25, 2025
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6 changes: 4 additions & 2 deletions llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
//===- MemorySanitizer.cpp - detector of uninitialized reads --------------===//
.//===- MemorySanitizer.cpp - detector of uninitialized reads --------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
Expand Down Expand Up @@ -3285,9 +3285,11 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
// e.g., if 0/1 are initialized bits with concrete value 0/1, and ? is an
// uninitialized bit:
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for completeness, note that this is little nedian?

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Done

// - 0001 0??? is fully initialized
// - 000? ???? is fully uninitialized
// - 000? ???? is fully uninitialized (*)
// - ???? ???? is fully uninitialized
// - 0000 0000 is fully initialized iff !is_zero_poison
// (*) TODO: arguably, since the number of zeros is in the range [0, 8], we
// only need to poison 4 bits.
//
// OutputShadow =
// ((ConcreteZerosCount >= ShadowZerosCount) && !AllZeroShadow)
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