Re: [PATCH v2] Add TARGET_MOVE_WITH_MODE_P

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Mar 9, 2022 at 7:04 PM Richard Sandiford
<richard.sandiford@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.tools@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.

>
> Thanks,
> Richard