On Wed, Jul 26, 2023 at 8:19 PM Drew Ross <drr...@redhat.com> wrote:
>
> Here is what I came up with for combining the two:
>
> /* For (x << c) >> c, optimize into x & ((unsigned)-1 >> c) for
> unsigned x OR truncate into the precision(type) - c lowest bits
> of signed x (if they have mode precision or a precision of 1) */
> (simplify
> (rshift (nop_convert? (lshift @0 INTEGER_CST@1)) @@1)
> (if (wi::ltu_p (wi::to_wide (@1), element_precision (type)))
> (if (TYPE_UNSIGNED (type))
> (bit_and @0 (rshift { build_minus_one_cst (type); } @1))
> (if (INTEGRAL_TYPE_P (type))
> (with {
> int width = element_precision (type) - tree_to_uhwi (@1);
> tree stype = build_nonstandard_integer_type (width, 0);
> }
> (if (TYPE_PRECISION (stype) == 1 || type_has_mode_precision_p (stype))
> (convert (convert:stype @0))))))))
>
> Let me know what you think.
Looks good to me.
Thanks,
Richard.
> > Btw, I wonder whether we can handle
> > some cases of widening/truncating converts between the shifts?
>
> I will look into this.
>
> Drew
>
> On Wed, Jul 26, 2023 at 4:40 AM Richard Biener <richard.guent...@gmail.com>
> wrote:
>>
>> On Tue, Jul 25, 2023 at 9:26 PM Drew Ross <drr...@redhat.com> wrote:
>> >
>> > > 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.
>> > > 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)).
>> >
>> > Here is a pattern that that only matches to truncations that result in
>> > mode precision (or precision of 1):
>> >
>> > (simplify
>> > (rshift (nop_convert? (lshift @0 INTEGER_CST@1)) @@1)
>> > (if (INTEGRAL_TYPE_P (type)
>> > && !TYPE_UNSIGNED (type)
>> > && wi::gt_p (element_precision (type), wi::to_wide (@1), TYPE_SIGN
>> > (TREE_TYPE (@1))))
>> > (with {
>> > int width = element_precision (type) - tree_to_uhwi (@1);
>> > tree stype = build_nonstandard_integer_type (width, 0);
>> > }
>> > (if (TYPE_PRECISION (stype) == 1 || type_has_mode_precision_p (stype))
>> > (convert (convert:stype @0))))))
>> >
>> > Look ok?
>>
>> I suppose so. Can you see to amend the existing
>>
>> /* Optimize (x << c) >> c into x & ((unsigned)-1 >> c) for unsigned
>> types. */
>> (simplify
>> (rshift (lshift @0 INTEGER_CST@1) @1)
>> (if (TYPE_UNSIGNED (type)
>> && (wi::ltu_p (wi::to_wide (@1), element_precision (type))))
>> (bit_and @0 (rshift { build_minus_one_cst (type); } @1))))
>>
>> pattern? You will get a duplicate pattern diagnostic otherwise. It
>> also looks like this
>> one has the (nop_convert? ..) missing. Btw, I wonder whether we can handle
>> some cases of widening/truncating converts between the shifts?
>>
>> Richard.
>>
>> > > You might also want to verify what RTL expansion
>> > > produces before/after - it at least shouldn't be worse.
>> >
>> > The RTL is slightly better for the mode precision cases and slightly worse
>> > for the precision 1 case.
>> >
>> > > That said - do you have any testcase where the canonicalization is an
>> > > enabler
>> > > for further transforms or was this requested stand-alone?
>> >
>> > No, I don't have any specific test cases. This patch is just in response
>> > to pr101955.
>> >
>> > On Tue, Jul 25, 2023 at 2:55 AM Richard Biener
>> > <richard.guent...@gmail.com> wrote:
>> >>
>> >> On Mon, Jul 24, 2023 at 9:42 PM Jakub Jelinek <ja...@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
>> >> >
>> >>
>>
