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

[YunQiang Su <wzssyqa@gmail.com>]

Richard Biener <rguenther@suse.de> 于2023年7月19日周三 17:23写道：
>
> On Wed, 19 Jul 2023, YunQiang Su wrote:
>
> > Richard Biener <rguenther@suse.de> ?2023?7?19??? 15:22???
> > >
> > > On Wed, 19 Jul 2023, YunQiang Su wrote:
> > >
> > > > Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> ?2023?7?19??? 14:27???
> > > > >
> > > > > 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))
> > >
> > > But if this RTL is correct then the above with DImode is correct as
> > > well and the issue is in the backend definition of the instruction
> > > defining 'DINS'?
> > >
> >
> > I don't think so.
> >
> > (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))
> >
> > This RTL has only info about DI. It doesn't has any info about the
> > real length of
> > `val`. For backend, it has no other choice instead of `DINS`.
> >
> > > > 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.
> > >
> > > Bit that's wrong.  (set (zero_extract:SI ...) should not affect
> > > bits outside of the indicated range.
> > >
> >
> > In fact, it is how sign-extension arch work.
> > No matter wrong or right, the ISA was/is defined like this.
> >
> > In fact, one MIPS 32 ABI, the same C code will generate the RTL like this,
> > and the 32bit object can still workable on 64bit CPU.
> > That's a smart (or brain-damaged) design.
> >
> > > @findex zero_extract
> > > @item (zero_extract:@var{m} @var{loc} @var{size} @var{pos})
> > > Like @code{sign_extract} but refers to an unsigned or zero-extended
> > > bit-field.  The same sequence of bits are extracted, but they
> > > are filled to an entire word with zeros instead of by sign-extension.
> > >
> >
> > That's depending on the definition of `word` here.
> > For `(zero_extract:SI`, I think that the word is limit to the low 32bit of
> > hardware register.
> > Anyway, it won't break ISA without sign-extension by default.
> >
> > Due to the nature of sign-extension ISA, if we don't sign-extension the
> > `int` variable, it will make something wrong.
> >
> > To make it clear: the word `sign extension` here means:
> >        the the value of 31bit will be copied to bits [32-63], and
> >        the value of bits[0-30] won't be copied.
> > Here is the examples:
> >     li $7, 0xff
> >     li $8, 0x00 00 ff 00
> >     ins $8,$7,16,8
> >                     ^^
> > The value of $8 will be: 0x 00 00 00 00 00 ff ff 00
> >
> >     li $7, 0xff
> >     li $8, 0x00 00 ff 00
> >     ins $8,$7,24,8
> >                     ^^
> > The value of $8 will be: 0x ff ff ff ff ff 00 ff 00
>
> But that's INS.
>
> > > Unlike @code{sign_extract}, this type of expressions can be lvalues
> > > in RTL; they may appear on the left side of an assignment, indicating
> > > insertion of a value into the specified bit-field.
> > > @end table
>
> Note ^^^ applies for (zero_extract ..) as a SET destination.  The
> issue is probably that MIPS is WORD_REGISTER_OPERATIONS which has
> proved "interesting" (and under-documented).  I suppose word_mode
> is DImode which is likely why the bitfield operations work on it.
>
> I see theres DINS, DINSM and DINSU in some mips ISA document
> I found on the internet but I didn't see where that would
> cause sign or zero extension off bit 31.
>

Yes. DINS/DINSM/DINSU is not sign-extension, as they worked on 64bit values.
If MIPS is extend to an 128bit arch, they will be sign-extension to 128bits ;)

INS is sign-extension. See
https://s3-eu-west-1.amazonaws.com/downloads-mips/documents/MD00087-2B-MIPS64BIS-AFP-6.06.pdf
Page 267,
    GPR[rt] <- sign_extend(GPR[rt]31..msb+1 || GPR[rs]msb-lsb..0 ||
GPR[rt]lsb-1..0)
For the definition about `sign_extend`,  please see Page 38.

> > >
> > > >     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.
> > >
> > > which isn't correct either.
> > >
> >
> > It is not correct or not-correct: The ISA manual just state like this,
> > and the hardwares are working like this.
>
> I don't see that.  That's definitely not what GCC expects here,
> the left-most word of the doubleword should be unchanged.
>

See the above documents, you can note that almost all instructions has
`sign_extend `.
Note, please read the MIPS64 documents instead of MIPS32 ones,
as MIPS32 assumes the the GPR is 32bit, so there is no `sign_extend`.

https://www.mips.com/products/architectures/mips64/

> Your testcase should be a dg-do-run and probably more like
>
> NOMIPS16 int __attribute__((noipa)) 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++;
>   return val;
> }
> int main()
> {
>   int val = 0x01020304;
>   val = test (&val);
>   if (val != 0x01020304)
>     abort ();
> }
>

No, the reason is that you code will always work well, with or without my patch.

> not sure if I got endianess correct.  Now, the question is what
> WORD_REGISTER_OPERATIONS implies for a bitfield insert and what
> the MIPS ABI says for returning SImode.
>
> Others might be able to answer this.
>
> > > If you look a few dumps further you'll see which instruction was
> > > recognized, I suspect the machine description is simply wrong here?
> > >
> >
> > The design of initial RTL may has expect that the backend may expand
> >
> > (insn 14 13 15 2 (set (reg/v:DI 201 [ val ])
> >         (sign_extend:DI (subreg:SI (reg/v:DI 201 [ val ]) 0))) "xx.c":5:29 -1
> >      (nil))
> >
> > to an `SLL` instruction, which can fix what `DINS` do, aka
> >      0x 00 00 00 00 ff 00 00 00 ---> 0x ff ff ff ff ff 00 00 00
> >
> > I guess this is what you mean about the mistake of machine description.
> > While MIPS md believes that it's sign-extension by default, so it is
> > not needed at all.
> >
> > > > 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))
> > > >
> > > >
> >
> > This is just background info about MIPS:
> >
> > On a MIPS32 hardware, the value -1 is  0x ff ff ff ff, which is same
> > with other arch.
> > On a MIPS64 hardware, the value of (int32_t)-1 is
> >      0x ff ff ff ff ff ff ff ff
> > which is same with (int64_t)-1.
> > So the single compare-and-branch instruction can work with both
> > int32_t and int64_t.
> >
> > On none sign-extension arch, like ARM64, (int32_t)-1 is
> >    0x 00 00 00 00 ff ff ff ff
> > and (int64_t)-1 is
> >    0x ff ff ff ff ff ff ff ff
> > That's why the `CMP` instructions for X and W have different encoding:
> > the 31bit of the encoding: `sf` bit.
> >
> >
> > ================================
> > For this problem, we have 2 choice to fix:
> > 1. This patch
> > 2.
>
> 2. disallow INS affecting bit 31 of a double-word - there's
> mips_use_ins_ext_p that could be used (maybe also pass in whether it's
> an INS).
>
> Richard.
>
> >
> > > >
> > > > > 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)
> > > >
> > > >
> > > >
> > > >
> > >
> > > --
> > > 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)
> >
> >
> >
> >
>
> --
> 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