[PATCH] Simplify mult where both arguments are 0 or 1 (PR tree-optimization/87954).

[Richard Biener]

On Wed, Jun 5, 2019 at 3:35 PM Martin Liška <mliska@suse.cz> wrote:
>
> On 6/5/19 3:04 PM, Richard Biener wrote:
> > On Wed, Jun 5, 2019 at 2:09 PM Martin Liška <mliska@suse.cz> wrote:
> >>
> >> On 6/5/19 1:13 PM, Richard Biener wrote:
> >>> On Wed, Jun 5, 2019 at 12:56 PM Martin Liška <mliska@suse.cz> wrote:
> >>>>
> >>>> Hi.
> >>>>
> >>>> I'm suggesting one multiplication simplification pattern.
> >>>>
> >>>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
> >>>>
> >>>> Ready to be installed?
> >>>
> >>> +  (if (INTEGRAL_TYPE_P (type)
> >>> +       && wi::eq_p (get_nonzero_bits (@1), wi::one (TYPE_PRECISION (type)))
> >>> +       && wi::eq_p (get_nonzero_bits (@2), wi::one (TYPE_PRECISION (type))))
> >>>
> >>>   && wi::eq_p (wi::bit_or (get_nonzero_bits (@1), get_nonzero_bits  (@2)), 1))
> >>>
> >>> (I think literal 1 still works)?
> >>
> >> Yep, I can confirm that.
> >>
> >>> How does it behave for  singed/unsigned 1-bit
> >>> bitfields?  A gimple testcase maybe necessary to see.
> >>
> >> Can we really have a mult that will have a bitfield type?
> >
> > As said you probably need a GIMPLE testcase to avoid
> > promoting to int.  Oh, and that doesn't work yet because
> > we cannot "parse" bit-precision types for temporaries.
> >
> > struct X { int a : 1; int b : 1; };
> >
> > int foo (struct X *p)
> > {
> >   return p->a;
> > }
> >
> > produces
> >
> > int __GIMPLE (ssa)
> > foo (struct X * p)
> > {
> >   int D_1913;
> >   <unnamed-signed:1> _1;
> > ...
> >
> > we have similar issues with dumping of vector types but
> > there at least one can use a typedef and manual editing.
> > For bit-precision types we need to invent a "C" extension
> > (thus also for vectors).
> >
> > Anyway...
>
> I see, I'm sending updated version of the patch I've been just testing.
> It's addressing Richard Sandifords's note.
>
> May I install it after testing?

Yes.

Thanks,
Richard.

> >
> >> $ cat gcc/testsuite/gcc.dg/pr87954-2.c
> >> #define __GFP_DMA 1u
> >> #define __GFP_RECLAIM 0x10u
> >>
> >> struct bt
> >> {
> >>   unsigned int v:1;
> >> };
> >>
> >> unsigned int
> >> imul(unsigned int flags)
> >> {
> >>   struct bt is_dma, is_rec;
> >>
> >>   is_dma.v = !!(flags & __GFP_DMA);
> >>   is_rec.v = !!(flags & __GFP_RECLAIM);
> >>
> >>   return is_rec.v * !is_dma.v;
> >> }
> >>
> >> $ ./xgcc -B. /home/marxin/Programming/gcc/gcc/testsuite/gcc.dg/pr87954-2.c -fdump-tree-optimized=/dev/stdout -O2
> >>
> >> ;; Function imul (imul, funcdef_no=0, decl_uid=1909, cgraph_uid=1, symbol_order=0)
> >>
> >> imul (unsigned int flags)
> >> {
> >>   struct bt is_dma;
> >>   _Bool _1;
> >>   unsigned int _2;
> >>   _Bool _3;
> >>   unsigned char _4;
> >>   _Bool _6;
> >>   unsigned int _9;
> >>   <unnamed-unsigned:1> _11;
> >>   unsigned char _14;
> >>
> >>   <bb 2> [local count: 1073741824]:
> >>   _1 = (_Bool) flags_7(D);
> >>   _2 = flags_7(D) & 16;
> >>   _3 = _2 != 0;
> >>   is_dma.v = _1;
> >>   _4 = BIT_FIELD_REF <is_dma, 8, 0>;
> >>   _14 = ~_4;
> >>   _6 = (_Bool) _14;
> >>   _11 = _3 & _6;
> >>   _9 = (unsigned int) _11;
> >>   is_dma ={v} {CLOBBER};
> >>   return _9;
> >> }
> >>
> >>>
> >>> Does this mean we want to turn plus into bit_ior when
> >>> get_nonzero_bits() & get_nonzero_bits() is zero?
> >>
> >> That's quite interesting transformation, I'll add it as a follow up patch.
> >
> > I was just curious - maybe we should do the reverse instead?
> > For mult vs. bit-and I think the latter will be "faster" (well, probably not
> > even that...).  But for plus vs or?
>
> Hmm, expected speed up will be probably very small.
>
> Martin
>
> >
> >
> >>>
> >>> X * [0, 1] -> X & sign-extend-from-bit-1 also works I guess, but multiplication
> >>> looks more canonical.
> >> Ok here.
> >>
> >> Martin
> >>
> >>>
> >>> Thanks,
> >>> Richard.
> >>>
> >>>> Thanks,
> >>>> Martin
> >>>>
> >>>> gcc/ChangeLog:
> >>>>
> >>>> 2019-06-05  Martin Liska  <mliska@suse.cz>
> >>>>
> >>>>         PR tree-optimization/87954
> >>>>         * match.pd: Simplify mult where both arguments are 0 or 1.
> >>>>
> >>>> gcc/testsuite/ChangeLog:
> >>>>
> >>>> 2019-06-05  Martin Liska  <mliska@suse.cz>
> >>>>
> >>>>         PR tree-optimization/87954
> >>>>         * gcc.dg/pr87954.c: New test.
> >>>> ---
> >>>>  gcc/match.pd                   |  8 ++++++++
> >>>>  gcc/testsuite/gcc.dg/pr87954.c | 21 +++++++++++++++++++++
> >>>>  2 files changed, 29 insertions(+)
> >>>>  create mode 100644 gcc/testsuite/gcc.dg/pr87954.c
> >>>>
> >>>>
> >>
>