[PATCH] D33328: [CodeGen] Pessimize aliasing for union members (and may-alias) objects

[John McCall via Phabricator via cfe-commits]

rjmccall added inline comments.


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Comment at: lib/CodeGen/CGExpr.cpp:1436
+    if (BaseInfo.getMayAlias())
+      TBAAInfo = CGM.getTBAAInfo(getContext().CharTy);
     llvm::MDNode *TBAAPath = CGM.getTBAAStructTagInfo(TBAABaseType, TBAAInfo,
----------------
kparzysz wrote:
> rjmccall wrote:
> > Hmm.  Should we be constructing a struct-path TBAA at all if the base may alias, as opposed to just using 'char'?
> I did that and got a verifier error: "Old-style TBAA is no longer allowed, use struct-path TBAA instead. fatal error: error in backend: Broken function found, compilation aborted!"
Oh, I haven't been paying enough attention to LLVM's TBAA changes.  It looks like they've canonicalized on always using the struct-access-style TBAA metadata, which is fine; the point is that we should be using whatever tag we would use for dereferencing a simple char*, which I believe just means passing true for ConvertTypeToTag to DecorateInstructionWithTBAA .


Repository:
  rL LLVM

https://reviews.llvm.org/D33328