From: Richard Biener <richard.guenther@gmail.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: Drew Ross <drross@redhat.com>, Jeff Law <jeffreyalaw@gmail.com>,
Andrew Pinski <pinskia@gmail.com>,
gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] match.pd: Implement missed optimization (x << c) >> c -> -(x & 1) [PR101955]
Date: Tue, 25 Jul 2023 08:54:12 +0200 [thread overview]
Message-ID: <CAFiYyc2KUXeMTFKZMpEk6+7b0v1VdqNrjYto=niMn_T3H4s-XQ@mail.gmail.com> (raw)
In-Reply-To: <ZL7UEN6KoI24weZG@tucnak>
On Mon, Jul 24, 2023 at 9:42 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Mon, Jul 24, 2023 at 03:29:54PM -0400, Drew Ross via Gcc-patches wrote:
> > So would something like
> >
> > (simplify
> > (rshift (nop_convert? (lshift @0 INTEGER_CST@1)) @@1)
> > (with { tree stype = build_nonstandard_integer_type (1, 0); }
> > (if (INTEGRAL_TYPE_P (type)
> > && !TYPE_UNSIGNED (type)
> > && wi::eq_p (wi::to_wide (@1), element_precision (type) - 1))
> > (convert (convert:stype @0)))))
> >
> > work?
>
> Certainly swap the if and with and the (with then should be indented by 1
> column to the right of (if and (convert one further (the reason for the
> swapping is not to call build_nonstandard_integer_type when it will not be
> needed, which will be probably far more often then an actual match).
With that fixed I think for non-vector integrals the above is the most suitable
canonical form of a sign-extension. Note it should also work for any other
constant shift amount - just use the appropriate intermediate precision for
the truncating type. You might also want to verify what RTL expansion
produces before/after - it at least shouldn't be worse. We _might_ want
to consider to only use the converts when the intermediate type has
mode precision (and as a special case allow one bit as in your above case)
so it can expand to (sign_extend:<outer> (subreg:<inner> reg)).
> As discussed privately, the above isn't what we want for vectors and the 2
> shifts are probably best on most arches because even when using -(x & 1) the
> { 1, 1, 1, ... } vector would often needed to be loaded from memory.
I think for vectors a vpcmpgt {0,0,0,..}, %xmm is the cheapest way of
producing the result. Note that to reflect this on GIMPLE you'd need
_2 = _1 < { 0,0...};
res = _2 ? { -1, -1, ...} : { 0, 0,...};
because whether the ISA has a way to produce all-ones masks isn't known.
For scalars using -(T)(_1 < 0) would also be possible.
That said - do you have any testcase where the canonicalization is an enabler
for further transforms or was this requested stand-alone?
Thanks,
Richard.
> Jakub
>
next prev parent reply other threads:[~2023-07-25 6:54 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-21 15:08 Drew Ross
2023-07-21 17:27 ` Andrew Pinski
2023-07-22 6:09 ` Jeff Law
2023-07-24 7:16 ` Richard Biener
2023-07-24 19:29 ` Drew Ross
2023-07-24 19:42 ` Jakub Jelinek
2023-07-25 6:54 ` Richard Biener [this message]
2023-07-25 19:25 ` Drew Ross
2023-07-25 19:43 ` Jakub Jelinek
2023-07-26 8:39 ` Richard Biener
2023-07-26 18:18 ` Drew Ross
2023-07-28 6:30 ` Richard Biener
2023-08-01 19:20 ` [PATCH] match.pd: Canonicalize (signed x << c) >> c [PR101955] Drew Ross
2023-08-01 21:36 ` Jakub Jelinek
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