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.
