Re: [PATCH v2] Add TARGET_MOVE_WITH_MODE_P

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Mar 14, 2022 at 8:44 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Richard Biener <richard.guenther@gmail.com> writes:
> > 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.
>
> Hmm, yeah.  Certainly we shouldn't require the target to support
> a scalar integer equivalent of every vector mode.
>

I'd like to resolve this before GCC 12 is released.

Thanks.


-- 
H.J.