[cfe-dev] Alignment assumptions

Stephen Canon via cfe-dev cfe-dev at lists.llvm.org
Thu Dec 14 07:38:27 PST 2017

Intel documents these types as follows:

"The compiler aligns the __m256, __m256d, and __m256i local and global data to 32-byte boundaries on the stack"

That’s awkwardly worded, but these types have 32B alignment; the result of casting a pointer that is not suitably aligned is undefined (i.e this is a bug in your code).

– Steve

> On Dec 14, 2017, at 7:30 AM, Marcel Keller via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> Hi,
> The attached code segfaults with clang 3.8 and 4.0:
> $ clang++ -g -O3 -mavx main.cpp A.cpp  ; ./a.out
> Segmentation fault
> The problem is in the following line:
>    memset((__m256i*)dest, 0, length);
> Removing the cast to __m256i* solves the problem. It seems that clang assumes that __m256i* is always aligned correctly, which is not the case in the code. Is that a bug in clang, or is that a mistake on my side? GCC does not cause a segfault.
> Best regards,
> Marcel
> <A.h><main.cpp><A.cpp>_______________________________________________
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