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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Retire the non-deterministic ASTFileSignature"
href="https://llvm.org/bugs/show_bug.cgi?id=27449">27449</a>
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<th>Summary</th>
<td>Retire the non-deterministic ASTFileSignature
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Modules
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>aprantl@apple.com
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<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>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).</pre>
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