[cfe-dev] Diagnostic for under-aligned pointers

Vedant Kumar via cfe-dev cfe-dev at lists.llvm.org
Fri Sep 16 14:29:03 PDT 2016


Thanks for the pointer!

I tried out my example with -Wcast-align -Weverything but couldn't trigger a
diagnostic. Based on test/Sema/warn-cast-align.c, it seems like this produces
exactly the kind of diagnostic I'd like. Maybe it's only tested for explicit
casts to avoid false positives?

vedant

> On Sep 16, 2016, at 2:25 PM, Kim Gräsman <kim.grasman at gmail.com> wrote:
> 
> 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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