[PATCH] D118350: [Clang][Sema][AIX][PowerPC] Emit byval alignment warning only when struct member is passed to a function

[Sean Fertile via Phabricator via cfe-commits]

sfertile added a comment.

Hey Zarko. I really like how we only issue the warning when the struct is used, however I think this is still overly broad because of where the incompatibility lies between xlc and clang. I believe the layout of the structures will be the same for both compilers, and globals of this type will have the same alignment restrictions. A function like `void baz(int *);` will be compatible between the 2 compilers since the argument is just a pointer. The difference occurs when passing the structure by value on the stack, where xlc doesn't align the struct to the expected alignment, and clang/llvm does.  Since the incompatibility is in the calling convention only when the struct is passed byval, that should be the only time we emit the diagnostic.

There are 2 other things we should verify since I'm not sure if they are compatible or not:

1. When passing these structures byval but in argument passing registers, I'm guessing that xlc doesn't skip registers whose image in the PSA is under-aligned while llvm will.
2. Whether xlc passes these misaligned when its done through va_args.

The answers to those 2 questions will determine if we need to warn for all byval arguments that have alignments from attribute that is greater than 8, or if we can limit it a bit further.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D118350/new/

https://reviews.llvm.org/D118350