[llvm] [clang] [Clang][IR] add TBAA metadata on pointer, union and array types. (PR #75177)

[John McCall via cfe-commits]

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@@ -4598,8 +4602,7 @@ LValue CodeGenFunction::EmitLValueForField(LValue base,
   if (base.getTBAAInfo().isMayAlias() ||
           rec->hasAttr<MayAliasAttr>() || FieldType->isVectorType()) {
     FieldTBAAInfo = TBAAAccessInfo::getMayAliasInfo();
-  } else if (rec->isUnion()) {
-    // TODO: Support TBAA for unions.
+  } else if (rec->isUnion() && !CGM.getCodeGenOpts().UnionTBAA) {
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rjmccall wrote:

Well, I understand your intent and was asking whether you've put any effort in analyzing whether this rule actually matches the standard's aliasing rules.  For what it's worth, I think it does, at least as currently written; IIRC, the committee has considered various weaker rules that permit union members to alias.  But it's also true that this is going to make us enforce a stricter rule than we have been.

This patch is both implementing the stricter rules and enabling them by default, right?  I think we should probably stage those separately, i.e. land this as an opt-in feature and then try turning it on by default in a  follow-up commit.  That way, if we see miscompiles from this, we can (temporarily) revert the small commit changing the default while we're analyzing that and not have to revert the whole implementation.

https://github.com/llvm/llvm-project/pull/75177