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[clang] Fix nondeterminism in MemberPointerType #137910

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Merged
merged 4 commits into from
May 5, 2025
Merged

[clang] Fix nondeterminism in MemberPointerType #137910

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b31a3f8
[clang] Fix nondeterminism in MemberPointerType
emaxx-google Apr 30, 2025
a8c958e
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emaxx-google Apr 30, 2025
21c7ab1
make new accessor private
emaxx-google May 5, 2025
a4bbcc3
add FIXME
emaxx-google May 5, 2025
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11 changes: 10 additions & 1 deletion clang/include/clang/AST/Type.h
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Original file line number Diff line number Diff line change
Expand Up @@ -3602,6 +3602,9 @@ class MemberPointerType : public Type, public llvm::FoldingSetNode {
}

NestedNameSpecifier *getQualifier() const { return Qualifier; }
/// Note: this can trigger extra deserialization when external AST sources are
/// used. Prefer `getCXXRecordDecl()` unless you really need the most recent
/// decl.
CXXRecordDecl *getMostRecentCXXRecordDecl() const;

bool isSugared() const;
Expand All @@ -3610,7 +3613,10 @@ class MemberPointerType : public Type, public llvm::FoldingSetNode {
}

void Profile(llvm::FoldingSetNodeID &ID) {
Profile(ID, getPointeeType(), getQualifier(), getMostRecentCXXRecordDecl());
// FIXME: `getMostRecentCXXRecordDecl()` should be possible to use here,
// however when external AST sources are used it causes nondeterminism
// issues (see https://github.com/llvm/llvm-project/pull/137910).
Profile(ID, getPointeeType(), getQualifier(), getCXXRecordDecl());
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Is the problem that we run this profile in the 'middle' of a TU and the most recent decl changes by the end? If so, this makes sense.

That said, I'd expect this to be the above suggestion in case this member pointer type has a redeclaration that perhaps changes what the actual referenced decl is.

Suggested change
Profile(ID, getPointeeType(), getQualifier(), getCXXRecordDecl());
Profile(ID, getPointeeType(), getQualifier(), getCXXRecordDecl()->getCanonicalDecl());

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Yes, but this suggestion seems not needed because this getCanonicalDecl() call is already made inside the Profile() method:

llvm-project/clang/lib/AST/Type.cpp

Line 5279 in e8ae779

ID.AddPointer(Cls->getCanonicalDecl());

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Hmm... then I'm surprised that this patch is needed at all for the purposes of non-determinism. getMostRecentCXXRecordDecl()->getCanonicalDecl() will get the same declaration as getCXXRecordDecl()->getCanonicalDecl().

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The issue is transient - the reproducibility rate on the reproducer (https://pastebin.com/6aL6rmBe) is only 1%. Also it's not the pointer we have here that's problematic, but rather the fact of extra AST node loading inside the FoldingSet hash calculation operation.

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Hmm... then I'm surprised that this patch is needed at all for the purposes of non-determinism

My theory is that non-determinism is caused by side-effects in the deserialization logic that runs behind getMostRecentCXXRecordDecl(). We probably just get unlucky with rehashing points at different times for the hash table (FoldingSet) that makes us either call or not call getMostRecentCXXRecordDecl.

It is not a new issue, just rare and now getting surfaced by the new place where getMostRecentCXXRecordDecl is called.

}

static void Profile(llvm::FoldingSetNodeID &ID, QualType Pointee,
Expand All @@ -3620,6 +3626,9 @@ class MemberPointerType : public Type, public llvm::FoldingSetNode {
static bool classof(const Type *T) {
return T->getTypeClass() == MemberPointer;
}

private:
CXXRecordDecl *getCXXRecordDecl() const;
};

/// Capture whether this is a normal array (e.g. int X[4])
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10 changes: 7 additions & 3 deletions clang/lib/AST/Type.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5275,10 +5275,14 @@ void MemberPointerType::Profile(llvm::FoldingSetNodeID &ID, QualType Pointee,
ID.AddPointer(Cls->getCanonicalDecl());
}

CXXRecordDecl *MemberPointerType::getCXXRecordDecl() const {
return dyn_cast<MemberPointerType>(getCanonicalTypeInternal())
->getQualifier()
->getAsRecordDecl();
}

CXXRecordDecl *MemberPointerType::getMostRecentCXXRecordDecl() const {
auto *RD = dyn_cast<MemberPointerType>(getCanonicalTypeInternal())
->getQualifier()
->getAsRecordDecl();
auto *RD = getCXXRecordDecl();
if (!RD)
return nullptr;
return RD->getMostRecentNonInjectedDecl();
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