RE: [PATCH 1/2]middle-end Fold BIT_FIELD_REF and Shifts into BIT_FIELD_REFs alone

[Tamar Christina <Tamar.Christina@arm.com>]

> -----Original Message-----
> From: Andrew Pinski <pinskia@gmail.com>
> Sent: Saturday, September 24, 2022 8:57 PM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <nd@arm.com>; rguenther@suse.de
> Subject: Re: [PATCH 1/2]middle-end Fold BIT_FIELD_REF and Shifts into
> BIT_FIELD_REFs alone
> 
> On Fri, Sep 23, 2022 at 4:43 AM Tamar Christina via Gcc-patches <gcc-
> patches@gcc.gnu.org> wrote:
> >
> > Hi All,
> >
> > This adds a match.pd rule that can fold right shifts and
> > bit_field_refs of integers into just a bit_field_ref by adjusting the
> > offset and the size of the extract and adds an extend to the previous size.
> >
> > Concretely turns:
> >
> > #include <arm_neon.h>
> >
> > unsigned int foor (uint32x4_t x)
> > {
> >     return x[1] >> 16;
> > }
> >
> > which used to generate:
> >
> >   _1 = BIT_FIELD_REF <x_2(D), 32, 32>;
> >   _3 = _1 >> 16;
> >
> > into
> >
> >   _4 = BIT_FIELD_REF <x_1(D), 16, 48>;
> >   _2 = (unsigned int) _4;
> >
> > I currently limit the rewrite to only doing it if the resulting
> > extract is in a mode the target supports. i.e. it won't rewrite it to
> > extract say 13-bits because I worry that for targets that won't have a
> > bitfield extract instruction this may be a de-optimization.
> 
> It is only a de-optimization for the following case:
> * vector extraction
> 
> All other cases should be handled correctly in the middle-end when
> expanding to RTL because they need to be handled for bit-fields anyways.
> Plus SIGN_EXTRACT and ZERO_EXTRACT would be used in the integer case
> for the RTL.
> Getting SIGN_EXTRACT/ZERO_EXTRACT early on in the RTL is better than
> waiting until combine really.
> 

Fair enough, I've dropped the constraint.

> 
> >
> > Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu
> > and no issues.
> >
> > Testcase are added in patch 2/2.
> >
> > Ok for master?
> >
> > Thanks,
> > Tamar
> >
> > gcc/ChangeLog:
> >
> >         * match.pd: Add bitfield and shift folding.
> >
> > --- inline copy of patch --
> > diff --git a/gcc/match.pd b/gcc/match.pd index
> >
> 1d407414bee278c64c00d425d9f025c1c58d853d..b225d36dc758f1581502c8d03
> 761
> > 544bfd499c01 100644
> > --- a/gcc/match.pd
> > +++ b/gcc/match.pd
> > @@ -7245,6 +7245,23 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> >        && ANY_INTEGRAL_TYPE_P (type) && ANY_INTEGRAL_TYPE_P
> (TREE_TYPE(@0)))
> >    (IFN_REDUC_PLUS_WIDEN @0)))
> >
> > +/* Canonicalize BIT_FIELD_REFS and shifts to BIT_FIELD_REFS.  */ (for
> > +shift (rshift)
> > +     op (plus)
> > + (simplify
> > +  (shift (BIT_FIELD_REF @0 @1 @2) integer_pow2p@3)
> > +  (if (INTEGRAL_TYPE_P (type))
> > +   (with { /* Can't use wide-int here as the precision differs between
> > +             @1 and @3.  */
> > +          unsigned HOST_WIDE_INT size = tree_to_uhwi (@1);
> > +          unsigned HOST_WIDE_INT shiftc = tree_to_uhwi (@3);
> > +          unsigned HOST_WIDE_INT newsize = size - shiftc;
> > +          tree nsize = wide_int_to_tree (bitsizetype, newsize);
> > +          tree ntype
> > +            = build_nonstandard_integer_type (newsize, 1); }
> 
> Maybe use `build_nonstandard_integer_type (newsize, /* unsignedp = */
> true);` or better yet `build_nonstandard_integer_type (newsize,
> UNSIGNED);`

Ah, will do,
Tamar.

> 
> I had started to convert some of the unsignedp into enum signop but I never
> finished or submitted the patch.
> 
> Thanks,
> Andrew Pinski
> 
> 
> > +    (if (ntype)
> > +     (convert:type (BIT_FIELD_REF:ntype @0 { nsize; } (op @2
> > + @3))))))))
> > +
> >  (simplify
> >   (BIT_FIELD_REF (BIT_FIELD_REF @0 @1 @2) @3 @4)
> >   (BIT_FIELD_REF @0 @3 { const_binop (PLUS_EXPR, bitsizetype, @2, @4);
> > }))
> >
> >
> >
> >
> > --