Richard Biener <richard.guent...@gmail.com> writes: > On Mon, Jul 27, 2020 at 11:09 AM Richard Sandiford > <richard.sandif...@arm.com> wrote: >> >> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes: >> > On Wed, Jul 22, 2020 at 5:18 PM Stefan Schulze Frielinghaus via >> > Gcc-patches <gcc-patches@gcc.gnu.org> wrote: >> >> >> >> This is a follow up to commit 5c9669a0e6c respectively discussion >> >> https://gcc.gnu.org/pipermail/gcc-patches/2020-June/549132.html >> >> >> >> In case that an alignment constraint is less than the size of a >> >> corresponding scalar type, ensure that we advance at least by one >> >> iteration. For example, on s390x we have for a long double an alignment >> >> constraint of 8 bytes whereas the size is 16 bytes. Therefore, >> >> TARGET_ALIGN / DR_SIZE equals zero resulting in an infinite loop which >> >> can be reproduced by the following MWE: >> > >> > But we guard this case with vector_alignment_reachable_p, so we shouldn't >> > have ended up here and the patch looks bogus. >> >> The above sounds like it ought to count as reachable alignment though. >> If a type requires a lower alignment than its size, then that's even >> more easily reachable than a type that requires the same alignment as >> the size. I guess at one extreme, a target alignment of 1 is always >> reachable. > > Well, if the element alignment is 8 but its size is 16 then when presumably > the desired vector alignment is a multiple of 16 we can never reach it. > Isn't this the case here?
If the desired vector alignment (TARGET_ALIGN) is a multiple of 16 then TARGET_ALIGN / DR_SIZE will be nonzero and the problem the patch is fixing wouldn't occur. I agree that we might never be able to reach that alignment if the pointer starts out misaligned by 8 bytes. But I think that's why it makes sense for the target to only ask for 8-byte alignment for vectors too, if it can cope with it. 8-byte alignment should always be achievable if the scalars are ABI-aligned. And if the target does ask for only 8-byte alignment, TARGET_ALIGN / DR_SIZE would be zero and the loop would never progress, which is the problem that the patch is fixing. It would even make sense for the target to ask for 1-byte alignment, if the target doesn't care about alignment at all. Thanks, Richard
