General protection fault on _mm_shuffle_epi8

Question

I have a function in C with prototype void f(__m128i *x). Inside this function I now call _mm_shuffle_epi8(*x, MASK) where MASK is some constant __m128i type. When I run the code I get a segmentation fault, and Valgrind reveals it is actually a general protection fault occurring on this line.

What might be the cause of this, and how do I get around it?

Small working example:

#include <wmmintrin.h>
#include <smmintrin.h>

#define BSWAP_MASK              _mm_set_epi8(0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15)
#define ALIGN(n)                __attribute__ ((aligned(n)))

static inline void g(const unsigned char *in, unsigned char *out) {
    __m128i tmp = _mm_load_si128 ((__m128i*)in);
    _mm_store_si128((__m128i*)out, tmp);
}

void f(__m128i *res) {
    g((unsigned char*)&res, (unsigned char*)&res);
    *res = _mm_shuffle_epi8(*res, BSWAP_MASK);
}

int main() {
    ALIGN(16) __m128i x = _mm_set_epi8(0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15);
    f(&x);
}

If you could post a small, reproducible example that we can use to replicate, that we help. — Sean Bright, Feb 07 '14 at 13:12
Yeah, I am trying to, but my small example does not reproduce bad stuff.. — Martin Lauridsen, Feb 07 '14 at 13:14
If `(x)` points to a single `__m128` rather than an array, wouldn't it be easier just to pass by value, and return by value? Probably uses an SSE register for argument and return value passing anyway, even if not inlined. — Brett Hale, Feb 07 '14 at 13:27

score 1 · Answer 1 · edited May 23 '17 at 12:22

As suspected, you had alignment problems.

If you do not properly align your data, or use functions to load and store at unaligned addresses, the result is crash.

If you want to use unaligned addresses, then this fixes the issue :

static inline void g(const unsigned char *in, unsigned char *out) {
    //__m128i tmp = _mm_load_si128 ((__m128i*)in);
    __m128i tmp = _mm_loadu_si128 ((__m128i*)in);
    //_mm_store_si128((__m128i*)out, tmp);
    _mm_storeu_si128((__m128i*)out, tmp);
}

As for why the linker fails to make that variable properly aligned, see Are stack variables aligned by the GCC __attribute__((aligned(x)))? and the answers there.

If you are on linux, you could use posix_memalign() function.

General protection fault on _mm_shuffle_epi8

1 Answers1