Question about vectorization

[Konstantin Vladimirov]

Hi,

Consider simple test:

#include <stdlib.h>

#ifdef FAILV

unsigned short* get_aa(void);
double* get_bb(void);

#else

extern unsigned short a[1024];
extern double b[1024];

#endif


unsigned short *foo()
{
    size_t i;

#ifdef FAILV
    unsigned short * restrict aa = get_aa();
    double * restrict bb = get_bb();
#else
    unsigned short * restrict aa = a;
    double * restrict bb = b;
#endif

    for (i = 0; i < 1024; ++i)
    {
        *bb = *aa;
        ++bb; ++aa;
    }

    return aa;
}

Compile it with latest gcc, 4.8 or 4.9:

gcc -O3 -ftree-vectorizer-verbose=2 --std=c99 -S test.c
gcc -O3 -ftree-vectorizer-verbose=2 --std=c99 -S test.c -DFAILV

In second case it outputs:

test.c:28: note: versioning for alias required: can't determine
dependence between *aa_22 and *bb_23

So loop is vectorized but there is unnecessary aliasing check inside.

But AFAIC, due to strong aliasing rules, compiler should statically
know, that aliasing is not possible in that case.

Is it a bug?

---
With best regards, Konstantin