https://llvm.org/bugs/show_bug.cgi?id=27449
Bug ID: 27449
Summary: Retire the non-deterministic ASTFileSignature
Product: clang
Version: unspecified
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Modules
Assignee: unassignedclangb...@nondot.org
Reporter: apra...@apple.com
CC: dgre...@apple.com, llvm-bugs@lists.llvm.org
Classification: Unclassified
Implicitly-built modules currently contain a random 32-bit ASTFileSignature.
This makes clang modules nondeterministic which is generally undesirable.
ASTReader has a comment that explains how we arrived at the status quo nicely:
// For implicit modules we output a signature that we can use to ensure
// duplicate module builds don't collide in the cache as their output order
// is non-deterministic.
// FIXME: Remove this when output is deterministic.
if (Context.getLangOpts().ImplicitModules) {
Signature = getSignature();
RecordData::value_type Record[] = {Signature};
Stream.EmitRecord(SIGNATURE, Record);
}
static ASTFileSignature getSignature() {
while (1) {
if (ASTFileSignature S = llvm::sys::Process::GetRandomNumber())
return S;
// Rely on GetRandomNumber to eventually return non-zero...
}
}
The existence of test/Modules/stress1.cpp seems to indicate that we were able
to remove all(?) sources of nondeterminism in the mean time, so we should be
able to retire this hack.
A minor complication is that module debug info currently uses the module
signature as the DWO_id to uniquely identify a module, but we should be able to
replace this by an actual hash of the serialized AST (which won't be
deterministic until after the ASTFileSignature is removed).
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