Re: [PATCH v2] Store_bit_field_1: Use SUBREG instead of REG if possible

[YunQiang Su <wzssyqa@gmail.com>]

Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> 于2023年7月19日周三 14:27写道：
>
> On Wed, 19 Jul 2023, YunQiang Su wrote:
>
> > PR #104914
> >
> > When work with
> >   int val;
> >   ((unsigned char*)&val)[3] = *buf;
> >   if (val > 0) ...
> > The RTX mode is obtained from REG instead of SUBREG, which make
> > D<INS> is used instead of <INS>.  Thus something wrong happens
> > on sign-extend default architectures, like MIPS64.
> >
> > Let's use str_rtx and mode of str_rtx as the parameters for
> > store_integral_bit_field if:
> >   modes of op0 and str_rtx are INT;
> >   length of op0 is greater than str_rtx.
> >
> > This patch has been tested on aarch64-linux-gnu, x86_64-linux-gnu,
> > mips64el-linux-gnuabi64 without regression.
>
> I still think you are "fixing" this in the wrong place.  The bugzilla
> audit trail points to combine and later notes an eventual expansion
> issue (but for another testcase/target).
>
> You have to explain in more detail on what is wrong with the initial
> RTL on mips.
>

In the first RTL file, aka xx.c.256r.expand, the zero_extract RTX is like

(insn 10 9 11 2 (set (zero_extract:DI (reg/v:DI 200 [ val ])
            (const_int 8 [0x8])
            (const_int 0 [0]))
        (subreg:DI (reg:QI 202) 0)) "../xx.c":4:29 -1
     (nil))

Not, all of the REG are in DImode. On MIPS64, it will expand to `DINS`
instructions.
While in fact here, we expect an SImode operation, due to `val` in C
code is `int`.

With my patch, the RTX will be like:

(insn 10 9 11 2 (set (zero_extract:SI (subreg:SI (reg/v:DI 200 [ val ]) 0)
            (const_int 8 [0x8])
            (const_int 0 [0]))
        (subreg:SI (reg:QI 202) 0)) "xx.c":4:29 -1
     (nil))

So the operation will be SImode, aka `INS` instruction for MIPS64.

The problem is based on 2 fact/root cause:
1. MIPS's `INS` instruction will be always to sign-extension, while `DINS` won't
    li $7, 0xff
    li $8, 0
    ins $8,$7,24,8  # set the 24-32 bits of $8 to 0xff.
The value of $8 will be 0xff ff ff ff ff 00 00 00.

    li $7, 0xff
    li $8, 0
    dins $8,$7,24,8  # set the 24-32 bits of $8 to 0xff.
The value of $8 will be 0x 00 00 00 00 ff 00 00 00.

2. Due to most of MIPS instructions work with 32bit value, aka instructions
without `d` as its first char (in fact with few exception), are sign-extension,
the MIPS backend just ignore `extendsidi2`, aka RTX

(insn 14 13 15 2 (set (reg/v:DI 200 [ val ])
        (sign_extend:DI (subreg:SI (reg/v:DI 200 [ val ]) 0))) "xx.c":5:29 -1
     (nil))



> Richard.
>
> > gcc/ChangeLog:
> >         PR: 104914.
> >         * expmed.cc(store_bit_field_1): Pass str_rtx and its mode
> >       to store_integral_bit_field if the length of op0 is greater
> >       than str_rtx.
> >
> > gcc/testsuite/ChangeLog:
> >         PR: 104914.
> >       * gcc.target/mips/pr104914.c: New testcase.
> > ---
> >  gcc/expmed.cc                            | 20 +++++++++++++++++---
> >  gcc/testsuite/gcc.target/mips/pr104914.c | 17 +++++++++++++++++
> >  2 files changed, 34 insertions(+), 3 deletions(-)
> >  create mode 100644 gcc/testsuite/gcc.target/mips/pr104914.c
> >
> > diff --git a/gcc/expmed.cc b/gcc/expmed.cc
> > index fbd4ce2d42f..5531c19e891 100644
> > --- a/gcc/expmed.cc
> > +++ b/gcc/expmed.cc
> > @@ -850,6 +850,7 @@ store_bit_field_1 (rtx str_rtx, poly_uint64 bitsize, poly_uint64 bitnum,
> >       since that case is valid for any mode.  The following cases are only
> >       valid for integral modes.  */
> >    opt_scalar_int_mode op0_mode = int_mode_for_mode (GET_MODE (op0));
> > +  opt_scalar_int_mode str_mode = int_mode_for_mode (GET_MODE (str_rtx));
> >    scalar_int_mode imode;
> >    if (!op0_mode.exists (&imode) || imode != GET_MODE (op0))
> >      {
> > @@ -881,9 +882,22 @@ store_bit_field_1 (rtx str_rtx, poly_uint64 bitsize, poly_uint64 bitnum,
> >       op0 = gen_lowpart (op0_mode.require (), op0);
> >      }
> >
> > -  return store_integral_bit_field (op0, op0_mode, ibitsize, ibitnum,
> > -                                bitregion_start, bitregion_end,
> > -                                fieldmode, value, reverse, fallback_p);
> > +  /* If MODEs of str_rtx and op0 are INT, and the length of op0 is greater than
> > +     str_rtx, it means that str_rtx has a shorter SUBREG: int32 on 64 mach/ABI
> > +     is an example.  For this case, we should use the mode of SUBREG, otherwise
> > +     bad code will generate for sign-extension ports, like MIPS.  */
> > +  bool use_str_mode = false;
> > +  if (GET_MODE_CLASS (GET_MODE (str_rtx)) == MODE_INT
> > +      && GET_MODE_CLASS (GET_MODE (op0)) == MODE_INT
> > +      && known_gt (GET_MODE_SIZE (GET_MODE (op0)),
> > +                GET_MODE_SIZE (GET_MODE (str_rtx))))
> > +    use_str_mode = true;
> > +
> > +  return store_integral_bit_field (use_str_mode ? str_rtx : op0,
> > +                                use_str_mode ? str_mode : op0_mode,
> > +                                ibitsize, ibitnum, bitregion_start,
> > +                                bitregion_end, fieldmode, value,
> > +                                reverse, fallback_p);
> >  }
> >
> >  /* Subroutine of store_bit_field_1, with the same arguments, except
> > diff --git a/gcc/testsuite/gcc.target/mips/pr104914.c b/gcc/testsuite/gcc.target/mips/pr104914.c
> > new file mode 100644
> > index 00000000000..fd6ef6af446
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/mips/pr104914.c
> > @@ -0,0 +1,17 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-march=mips64r2 -mabi=64" } */
> > +
> > +/* { dg-final { scan-assembler-not "\tdins\t" } } */
> > +
> > +NOMIPS16 int test (const unsigned char *buf)
> > +{
> > +  int val;
> > +  ((unsigned char*)&val)[0] = *buf++;
> > +  ((unsigned char*)&val)[1] = *buf++;
> > +  ((unsigned char*)&val)[2] = *buf++;
> > +  ((unsigned char*)&val)[3] = *buf++;
> > +  if(val > 0)
> > +    return 1;
> > +  else
> > +    return 0;
> > +}
> >
>
> --
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
> HRB 36809 (AG Nuernberg)



-- 
YunQiang Su