How to align stack at 32 byte boundary in GCC?

Question

I\'m using MinGW64 build based on GCC 4.6.1 for Windows 64bit target. I\'m playing around with the new Intel\'s AVX instructions. My command line arguments are -march=

Answer 1

I just ran in the same issue of having segmentation faults when using AVX inside my functions. And it was also due to the stack misalignment. Given the fact that this is a compiler issue (and the options that could help are not available in Windows), I worked around the stack usage by:

1. Using static variables (see this issue). Given the fact that they are not stored in the stack, you can force their alignment by using __attribute__((align(32))) in your declaration. For example: static __m256i r __attribute__((aligned(32))).

2. Inlining the functions/methods receiving/returning AVX data. You can force GCC to inline your function/method by adding inline and __attribute__((always_inline)) to your function prototype/declaration. Inlining your functions increase the size of your program, but they also prevent the function from using the stack (and hence, avoids the stack-alignment issue). Example: inline __m256i myAvxFunction(void) __attribute__((always_inline));.

Be aware that the usage of static variables is no thread-safe, as mentioned in the reference. If you are writing a multi-threaded application you may have to add some protection for your critical paths.