Richard Biener <richard.guent...@gmail.com> writes: > On Wed, Mar 9, 2022 at 7:04 PM Richard Sandiford > <richard.sandif...@arm.com> wrote: >> >> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes: >> > On Wed, Mar 2, 2022 at 10:18 PM H.J. Lu <hjl.to...@gmail.com> wrote: >> >> >> >> On Wed, Mar 02, 2022 at 09:51:26AM +0100, Richard Biener wrote: >> >> > On Tue, Mar 1, 2022 at 11:41 PM H.J. Lu via Gcc-patches >> >> > <gcc-patches@gcc.gnu.org> wrote: >> >> > > >> >> > > Add TARGET_FOLD_MEMCPY_MAX for the maximum number of bytes to fold >> >> > > memcpy. >> >> > > The default is >> >> > > >> >> > > MOVE_MAX * MOVE_RATIO (optimize_function_for_size_p (cfun)) >> >> > > >> >> > > For x86, it is MOVE_MAX to restore the old behavior before >> >> > >> >> > I know we've discussed this to death in the PR, I just want to repeat >> >> > here >> >> > that the GIMPLE folding expects to generate a single load and a single >> >> > store (that is what it does on the GIMPLE level) which is why MOVE_MAX >> >> > was chosen originally (it's documented to what a "single instruction" >> >> > does). >> >> > In practice MOVE_MAX does not seem to cover vector register sizes >> >> > so Richard pulled MOVE_RATIO which is really intended to cover >> >> > the case of using multiple instructions for moving memory (but then I >> >> > don't remember whether for the ARM case the single load/store GIMPLE >> >> > will be expanded to multiple load/store instructions). >> >> > >> >> > TARGET_FOLD_MEMCPY_MAX sounds like a stop-gap solution, >> >> > being very specific for memcpy folding (we also fold memmove btw). >> >> > >> >> > There is also MOVE_MAX_PIECES which _might_ be more appropriate >> >> > than MOVE_MAX here and still honor the idea of single instructions. >> >> > Now neither arm nor aarch64 define this and it defaults to MOVE_MAX, >> >> > not MOVE_MAX * MOVE_RATIO. >> >> > >> >> > So if we need a new hook then that hook should at least get the >> >> > 'speed' argument of MOVE_RATIO and it should get a better name. >> >> > >> >> > I still think that it should be possible to improve the insn check to >> >> > avoid use of "disabled" modes, maybe that's also a point to add >> >> > a new hook like .move_with_mode_p or so? To quote, we do >> >> >> >> Here is the v2 patch to add TARGET_MOVE_WITH_MODE_P. >> > >> > Again I'd like to shine light on MOVE_MAX_PIECES which explicitely >> > mentions "a load or store used TO COPY MEMORY" (emphasis mine) >> > and whose x86 implementation would already be fine (doing larger moves >> > and also not doing too large moves). But appearantly the arm folks >> > decided that that's not fit and instead (mis-?)used MOVE_MAX * MOVE_RATIO. >> >> It seems like there are old comments and old documentation that justify >> both interpretations, so there are good arguments on both sides. But >> with this kind of thing I think we have to infer the meaning of the >> macro from the way it's currently used, rather than trusting such old >> and possibly out-of-date and contradictory information. >> >> FWIW, I agree that (if we exclude old reload, which we should!) the >> only direct uses of MOVE_MAX before the patch were not specific to >> integer registers and so MOVE_MAX should include vectors if the >> target wants vector modes to be used for general movement. >> >> Even if people disagree that that's the current meaning, I think it's >> at least a sensible meaning. It provides information that AFAIK isn't >> available otherwise, and it avoids overlap with MAX_FIXED_MODE_SIZE. >> >> So FWIW, I think it'd be reasonable to change non-x86 targets if they >> want vector modes to be used for single-insn copies. > > Note a slight complication in the GIMPLE folding case is that we > do not end up using vector modes but we're using "fake" > integer modes like OImode which x86 has move patterns for. > If we'd use vector modes we could use existing target hooks to > eventually decide whether auto-using those is desired or not.
Hmm, yeah. Certainly we shouldn't require the target to support a scalar integer equivalent of every vector mode. Richard
