[PATCH] D124571: Avoid strict aliasing violation on type punning inside llvm::PointerIntPair

[Breno Rodrigues Guimaraes via Phabricator via llvm-commits]

brenoguim added a comment.

> That looks like a globalopt bug.  I remember there was a patch in LLVM 14 related to this; I don't think it's something you can reasonably work around.

I thought of conditionally making PunnedPointer use aligned storage, which works even on clangs 11,12,13.  Something in the lines of: https://godbolt.org/z/hv8jPceKK
That is ugly though.

> I think strictly speaking, to avoid UB you might need to declare "Data" as a union: otherwise, the compiler might assume the value it contains is aligned.  I don't think clang does, in practice, but probably you want to be conservative.

Do you mean that, in the current code, the compiler could see that I'm reading `Data` through a memcpy into `intptr_t` and then checking the last bits and say: "Hey, if these bits came from Data, and Data is a pointer to something that must be aligned, then those bits are certainly zero!" ?

In that case my conclusion would be to use an aligned char array instead of a pointer. I didn't understand how using a union (union with which members?) would help. I would also have to track the active member of the union to avoid tripping in another UB as well.


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  https://reviews.llvm.org/D124571/new/

https://reviews.llvm.org/D124571