[RFC PATCH] Gather vectorization (PR tree-optimization/50789)

[Richard Guenther]

On Fri, 28 Oct 2011, Jakub Jelinek wrote:

> On Fri, Oct 28, 2011 at 12:59:48PM +0200, Richard Guenther wrote:
> > It is also because of re-use of memory via memcpy (yes, some dubious
> > TBAA case from C, but essentially we don't want to break that).  Thus
> > we can't use TBAA on anonymous memory.
> 
> No, IMHO we always use a ref_all mem access in that case.
> If you meant something like:
> 
> void
> foo (int *intptr, float *floatptr)
> {
>   int i;
>   for (i = 0; i < 256; ++i)
>     {
>       int tem;
>       __builtin_memcpy (&tem, &intptr[i], sizeof (tem));
>       floatptr[i] = (float) tem;
>     }
> }
> 
> which is valid C even if intptr == floatptr, we have:
> 
> <bb 2>:
> 
> <bb 3>:
>   # i_21 = PHI <i_14(4), 0(2)>
>   # ivtmp.12_27 = PHI <ivtmp.12_26(4), 256(2)>
>   D.2709_3 = (long unsigned int) i_21;
>   D.2710_4 = D.2709_3 * 4;
>   D.2711_6 = intptr_5(D) + D.2710_4;
>   D.2712_7 = MEM[(char * {ref-all})D.2711_6];
>   D.2713_11 = floatptr_10(D) + D.2710_4;
>   D.2715_13 = (float) D.2712_7;
>   *D.2713_11 = D.2715_13;
>   i_14 = i_21 + 1;
>   ivtmp.12_26 = ivtmp.12_27 - 1;
>   if (ivtmp.12_26 != 0)
>     goto <bb 4>;
>   else
>     goto <bb 5>;
> 
> <bb 4>:
>   goto <bb 3>;
> 
> which is just fine even with TBAA.
> And similarly for
> void
> bar (int *intptr, float *floatptr)
> {
>   int i;
>   for (i = 0; i < 256; ++i)
>     {
>       float tem;
>       tem = (float) intptr[i];
>       __builtin_memcpy (&floatptr[i], &tem, sizeof (tem));
>     }
> }
> 
> where the ref-all isn't used for load, but for store.

Well, yeah.  I said it's probably difficult to generate a
C testcase.  It's still valid middle-end IL (and well-defined) to have
intptr == floatptr and  MEM[(int *)..] and MEM[(float *)...].

Richard.