[cfe-dev] Diagnostic for under-aligned pointers

Akira Hatanaka via cfe-dev cfe-dev at lists.llvm.org
Fri Sep 16 15:02:47 PDT 2016

> On Sep 16, 2016, at 1:09 PM, Vedant Kumar via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 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;
>    }

Are we talking about the alignment of the pointer variable or the alignment of the data ptr points to? If it’s the latter, I’ve seen requests from people who wanted clang to issue a warning when compiling the following code, so perhaps it is something we should look into.

typedef uint64_t unaligned_u64 __attribute__((aligned(1)));
unaligned_u64 a;
uint64_t *b = &a; /* should warn with -Wcast-align? */

>    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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