Hi Jeff,
Jeff wrote:
> Just to make sure I understand. Are you saying the addresses for the
> MEMs are equal or the contents of the memory location are equal.
>
> For the former the alignment has to be the same, plain and simple, even
> if GCC isn't aware the alignments have to be the same.
>
> For the latter we'd be better off loading the data into a REG, then
> using the REG for the source of both memory stores.
The addresses are the same (they should probably have been canonicalized
earlier, that might be a separate bug). I'm not sure why the unaligned stores
have a lower alignment/no alias set, but it's certainly possible that a memset
expansion of a subfield of a structure has a lower alignment.
The known alignment of otherwise identical loads in different blocks could be
different, ie:
__attribute__((aligned(1)) struct { int x; } *p;
if (((intptr_t)p & 3) == 0)
x = p->x; // align 4
else
y = p->x; // align 1
It would be very wrong to change the alignment of the 2nd load to be 4-byte
aligned.
Wilco
