[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:

You've checked that this is the right rule for unions?  We can just record the union as a containing aggregate that happens to have all the different union members at offset 0?

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