[cfe-dev] Diagnostic for under-aligned pointers

[Kim Gräsman via cfe-dev]

I was just reading about -Wcast-align the other day, isn't this what it
does? Or does it only trigger for explicit casts?

- Kim

Den 16 sep. 2016 10:09 em skrev "Vedant Kumar via cfe-dev" <
cfe-dev at lists.llvm.org>:

> Hi,
>
> Does it make sense to emit a diagnostic when we implicitly cast a pointer
> to
> another pointer type with a higher required alignment?
>
> Here's an example where this might be useful. The pointer "uiptr" is at
> least
> 1-byte aligned, but it's implicitly cast to "int *" (and treated as at
> least
> 8-byte aligned):
>
>     extern int printf(const char *S, ...);
>
>     int load_aligned(int *ptr) {
>       printf("Alignment of ptr: %lu\n", __alignof__(ptr));
>       return *ptr;
>     }
>
>     typedef int unaligned_int __attribute__((aligned(1)));
>
>     int main() {
>       unaligned_int ui = 0;
>       unaligned_int *__attribute__((aligned(1))) uiptr = &ui;
>       printf("Alignment of uiptr: %lu\n", __alignof__(uiptr));
>       return load_aligned(uiptr);
>     }
>
> When compiled with ToT clang, this program prints out:
>
>     Alignment of uiptr: 1
>     Alignment of ptr: 8
>
> IIUC, users should only see the warning if they opt into lowering the
> alignment
> of a pointer. I wouldn't expect this to be too noisy, but I don't have any
> data
> on this.
>
> It's true that we can catch this issue with ubsan, but only when the value
> of
> "uiptr" is actually not 8-byte aligned. Besides, it might be helpful to
> have a
> compile-time check for this (e.g in situations where it's tricky to deploy
> the
> sanitizer runtime).
>
> best,
> vedant
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