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. Thanks, Richard
