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

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

In D124571#3489362 <https://reviews.llvm.org/D124571#3489362>, @brenoguim wrote:

>> 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!" ?

I'm thinking something more like: you pass a PunnedPointer as an argument to a function.  It's passed in a register, and that register can be marked up with alignment.  There was a proposed clang patch for this at one point, but it didn't land because we were concerned about breaking things.

> 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.

I was thinking a `union { Ptr Data; void* VoidPtr }` or something like that.  If you access the union via memcpy, it shouldn't matter which member of the union is active, I think?

An aligned char array is probably also fine, though.


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