PING^2: [PATCH] rtl-optimization/110939 Really fix narrow comparison of memory and constant

[Xi Ruoyao <xry111@xry111.site>]

Ping again.

On Fri, 2023-08-18 at 13:04 +0200, Stefan Schulze Frielinghaus via Gcc-patches wrote:
> Ping.  Since this fixes bootstrap problem PR110939 for Loongarch I'm
> pingen this one earlier.
> 
> On Thu, Aug 10, 2023 at 03:04:03PM +0200, Stefan Schulze Frielinghaus wrote:
> > In the former fix in commit 41ef5a34161356817807be3a2e51fbdbe575ae85 I
> > completely missed the fact that the normal form of a generated constant for a
> > mode with fewer bits than in HOST_WIDE_INT is a sign extended version of the
> > actual constant.  This even holds true for unsigned constants.
> > 
> > Fixed by masking out the upper bits for the incoming constant and sign
> > extending the resulting unsigned constant.
> > 
> > Bootstrapped and regtested on x64 and s390x.  Ok for mainline?
> > 
> > While reading existing optimizations in combine I stumbled across two
> > optimizations where either my intuition about the representation of
> > unsigned integers via a const_int rtx is wrong, which then in turn would
> > probably also mean that this patch is wrong, or that the optimizations
> > are missed sometimes.  In other words in the following I would assume
> > that the upper bits are masked out:
> > 
> > diff --git a/gcc/combine.cc b/gcc/combine.cc
> > index 468b7fde911..80c4ff0fbaf 100644
> > --- a/gcc/combine.cc
> > +++ b/gcc/combine.cc
> > @@ -11923,7 +11923,7 @@ simplify_compare_const (enum rtx_code code, machine_mode mode,
> >        /* (unsigned) < 0x80000000 is equivalent to >= 0.  */
> >        else if (is_a <scalar_int_mode> (mode, &int_mode)
> >                && GET_MODE_PRECISION (int_mode) - 1 < HOST_BITS_PER_WIDE_INT
> > -              && ((unsigned HOST_WIDE_INT) const_op
> > +              && (((unsigned HOST_WIDE_INT) const_op & GET_MODE_MASK (int_mode))
> >                    == HOST_WIDE_INT_1U << (GET_MODE_PRECISION (int_mode) - 1)))
> >         {
> >           const_op = 0;
> > @@ -11962,7 +11962,7 @@ simplify_compare_const (enum rtx_code code, machine_mode mode,
> >        /* (unsigned) >= 0x80000000 is equivalent to < 0.  */
> >        else if (is_a <scalar_int_mode> (mode, &int_mode)
> >                && GET_MODE_PRECISION (int_mode) - 1 < HOST_BITS_PER_WIDE_INT
> > -              && ((unsigned HOST_WIDE_INT) const_op
> > +              && (((unsigned HOST_WIDE_INT) const_op & GET_MODE_MASK (int_mode))
> >                    == HOST_WIDE_INT_1U << (GET_MODE_PRECISION (int_mode) - 1)))
> >         {
> >           const_op = 0;
> > 
> > For example, while bootstrapping on x64 the optimization is missed since
> > a LTU comparison in QImode is done and the constant equals
> > 0xffffffffffffff80.
> > 
> > Sorry for inlining another patch, but I would really like to make sure
> > that my understanding is correct, now, before I come up with another
> > patch.  Thus it would be great if someone could shed some light on this.
> > 
> > gcc/ChangeLog:
> > 
> >         * combine.cc (simplify_compare_const): Properly handle unsigned
> >         constants while narrowing comparison of memory and constants.
> > ---
> >  gcc/combine.cc | 19 ++++++++++---------
> >  1 file changed, 10 insertions(+), 9 deletions(-)
> > 
> > diff --git a/gcc/combine.cc b/gcc/combine.cc
> > index e46d202d0a7..468b7fde911 100644
> > --- a/gcc/combine.cc
> > +++ b/gcc/combine.cc
> > @@ -12003,14 +12003,15 @@ simplify_compare_const (enum rtx_code code, machine_mode mode,
> >        && !MEM_VOLATILE_P (op0)
> >        /* The optimization makes only sense for constants which are big enough
> >          so that we have a chance to chop off something at all.  */
> > -      && (unsigned HOST_WIDE_INT) const_op > 0xff
> > -      /* Bail out, if the constant does not fit into INT_MODE.  */
> > -      && (unsigned HOST_WIDE_INT) const_op
> > -        < ((HOST_WIDE_INT_1U << (GET_MODE_PRECISION (int_mode) - 1) << 1) - 1)
> > +      && ((unsigned HOST_WIDE_INT) const_op & GET_MODE_MASK (int_mode)) > 0xff
> >        /* Ensure that we do not overflow during normalization.  */
> > -      && (code != GTU || (unsigned HOST_WIDE_INT) const_op < HOST_WIDE_INT_M1U))
> > +      && (code != GTU
> > +         || ((unsigned HOST_WIDE_INT) const_op & GET_MODE_MASK (int_mode))
> > +            < HOST_WIDE_INT_M1U)
> > +      && trunc_int_for_mode (const_op, int_mode) == const_op)
> >      {
> > -      unsigned HOST_WIDE_INT n = (unsigned HOST_WIDE_INT) const_op;
> > +      unsigned HOST_WIDE_INT n
> > +       = (unsigned HOST_WIDE_INT) const_op & GET_MODE_MASK (int_mode);
> >        enum rtx_code adjusted_code;
> >  
> >        /* Normalize code to either LEU or GEU.  */
> > @@ -12051,15 +12052,15 @@ simplify_compare_const (enum rtx_code code, machine_mode mode,
> >                 HOST_WIDE_INT_PRINT_HEX ") to (MEM %s "
> >                 HOST_WIDE_INT_PRINT_HEX ").\n", GET_MODE_NAME (int_mode),
> >                 GET_MODE_NAME (narrow_mode_iter), GET_RTX_NAME (code),
> > -               (unsigned HOST_WIDE_INT)const_op, GET_RTX_NAME (adjusted_code),
> > -               n);
> > +               (unsigned HOST_WIDE_INT) const_op & GET_MODE_MASK (int_mode),
> > +               GET_RTX_NAME (adjusted_code), n);
> >             }
> >           poly_int64 offset = (BYTES_BIG_ENDIAN
> >                                ? 0
> >                                : (GET_MODE_SIZE (int_mode)
> >                                   - GET_MODE_SIZE (narrow_mode_iter)));
> >           *pop0 = adjust_address_nv (op0, narrow_mode_iter, offset);
> > -         *pop1 = GEN_INT (n);
> > +         *pop1 = gen_int_mode (n, narrow_mode_iter);
> >           return adjusted_code;
> >         }
> >      }
> > -- 
> > 2.41.0
> > 

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University