rs6000: Disassemble opaque modes using subregs to allow optimizations [PR109116]

PR109116 exposes an issue where using unspecs to access each vector component
of an opaque mode variable leads to unneeded register copies, because our rtl
optimizers cannot handle unspecs.  Instead, use subregs to access each vector
component of the opaque mode variable, which our optimizers know how to handle.

I did not include a test case with the patch, since writing a test case that
attempts to ensure we don't emit unneeded register copies is nearly impossible
since those copies can still be generated for reasons other than the causes
in this patch.  I have verified that this patch does improve code generation
for some unit tests and our AI libraries team has confirmed that performance
of their tests improved when using this patch.

This passed bootstrap and regtesting with no regressions on powerpc64le-linux
and powerpc64-linux.  Ok for trunk?

Peter


gcc/
	PR target/109116
	* config/rs6000/mma.md (vsx_disassemble_pair): Expand into a vector
	register sized subreg.
	* config/rs6000/mma.md (*vsx_disassemble_pair): Delete.
	(mma_disassemble_acc): Expand into a vector register sized subreg.
	(*mma_disassemble_acc): Delete.
	* config/rs6000/rs6000.cc (rs6000_modes_tieable_p): Allow vector modes
	to tie with OOmode.

Hi Peter,

on 2023/11/16 07:50, Peter Bergner wrote:
> PR109116 exposes an issue where using unspecs to access each vector component
> of an opaque mode variable leads to unneeded register copies, because our rtl
> optimizers cannot handle unspecs.  Instead, use subregs to access each vector
> component of the opaque mode variable, which our optimizers know how to handle.
> 
> I did not include a test case with the patch, since writing a test case that
> attempts to ensure we don't emit unneeded register copies is nearly impossible
> since those copies can still be generated for reasons other than the causes
> in this patch.  I have verified that this patch does improve code generation
> for some unit tests and our AI libraries team has confirmed that performance
> of their tests improved when using this patch.
> 
> This passed bootstrap and regtesting with no regressions on powerpc64le-linux
> and powerpc64-linux.  Ok for trunk?
> 
> Peter
> 
> 
> gcc/
> 	PR target/109116
> 	* config/rs6000/mma.md (vsx_disassemble_pair): Expand into a vector
> 	register sized subreg.
> 	* config/rs6000/mma.md (*vsx_disassemble_pair): Delete.
> 	(mma_disassemble_acc): Expand into a vector register sized subreg.
> 	(*mma_disassemble_acc): Delete.
> 	* config/rs6000/rs6000.cc (rs6000_modes_tieable_p): Allow vector modes
> 	to tie with OOmode.
> 
> diff --git a/gcc/config/rs6000/mma.md b/gcc/config/rs6000/mma.md
> index 575751d477e..2ca405469e2 100644
> --- a/gcc/config/rs6000/mma.md
> +++ b/gcc/config/rs6000/mma.md
> @@ -398,29 +398,8 @@ (define_expand "vsx_disassemble_pair"
>     (match_operand 2 "const_0_to_1_operand")]
>    "TARGET_MMA"
>  {
> -  rtx src;
> -  int regoff = INTVAL (operands[2]);
> -  src = gen_rtx_UNSPEC (V16QImode,
> -			gen_rtvec (2, operands[1], GEN_INT (regoff)),
> -			UNSPEC_MMA_EXTRACT);
> -  emit_move_insn (operands[0], src);
> -  DONE;
> -})
> -
> -(define_insn_and_split "*vsx_disassemble_pair"
> -  [(set (match_operand:V16QI 0 "mma_disassemble_output_operand" "=mwa")
> -       (unspec:V16QI [(match_operand:OO 1 "vsx_register_operand" "wa")
> -		      (match_operand 2 "const_0_to_1_operand")]
> -		      UNSPEC_MMA_EXTRACT))]
> -  "TARGET_MMA
> -   && vsx_register_operand (operands[1], OOmode)"
> -  "#"
> -  "&& reload_completed"
> -  [(const_int 0)]
> -{
> -  int reg = REGNO (operands[1]);
> -  int regoff = INTVAL (operands[2]);
> -  rtx src = gen_rtx_REG (V16QImode, reg + regoff);
> +  int regoff = INTVAL (operands[2]) * GET_MODE_SIZE (V16QImode);

Is it intentional to keep GET_MODE_SIZE (V16QImode) instead of 16?
I think if one day NUM_POLY_INT_COEFFS isn't 1 on rs6000 any more,
we have to add one explicit .to_constant () here.  So I prefer this
to use 16 directly, maybe one comment above to indicate what's for
the value 16.

> +  rtx src = simplify_gen_subreg (V16QImode, operands[1], OOmode, regoff);
>    emit_move_insn (operands[0], src);
>    DONE;
>  })
> @@ -472,29 +451,8 @@ (define_expand "mma_disassemble_acc"
>     (match_operand 2 "const_0_to_3_operand")]
>    "TARGET_MMA"
>  {
> -  rtx src;
> -  int regoff = INTVAL (operands[2]);
> -  src = gen_rtx_UNSPEC (V16QImode,
> -			gen_rtvec (2, operands[1], GEN_INT (regoff)),
> -			UNSPEC_MMA_EXTRACT);
> -  emit_move_insn (operands[0], src);
> -  DONE;
> -})
> -
> -(define_insn_and_split "*mma_disassemble_acc"
> -  [(set (match_operand:V16QI 0 "mma_disassemble_output_operand" "=mwa")
> -       (unspec:V16QI [(match_operand:XO 1 "fpr_reg_operand" "d")
> -		      (match_operand 2 "const_0_to_3_operand")]
> -		      UNSPEC_MMA_EXTRACT))]
> -  "TARGET_MMA
> -   && fpr_reg_operand (operands[1], XOmode)"
> -  "#"
> -  "&& reload_completed"
> -  [(const_int 0)]
> -{
> -  int reg = REGNO (operands[1]);
> -  int regoff = INTVAL (operands[2]);
> -  rtx src = gen_rtx_REG (V16QImode, reg + regoff);
> +  int regoff = INTVAL (operands[2]) * GET_MODE_SIZE (V16QImode);

Likewise.

> +  rtx src = simplify_gen_subreg (V16QImode, operands[1], XOmode, regoff);
>    emit_move_insn (operands[0], src);
>    DONE;
>  })
> diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
> index 5f56c3ed85b..f2efa46c147 100644
> --- a/gcc/config/rs6000/rs6000.cc
> +++ b/gcc/config/rs6000/rs6000.cc
> @@ -1964,9 +1964,12 @@ rs6000_hard_regno_mode_ok (unsigned int regno, machine_mode mode)
>  static bool
>  rs6000_modes_tieable_p (machine_mode mode1, machine_mode mode2)
>  {
> -  if (mode1 == PTImode || mode1 == OOmode || mode1 == XOmode
> -      || mode2 == PTImode || mode2 == OOmode || mode2 == XOmode)
> -    return mode1 == mode2;
> +   if (mode1 == PTImode || mode1 == OOmode || mode1 == XOmode
> +       || mode2 == PTImode || mode2 == XOmode)
> +     return mode1 == mode2;
> + 
> +  if (mode2 == OOmode)
> +    return ALTIVEC_OR_VSX_VECTOR_MODE (mode1);

I vaguely remembered that Segher mentioned it's unexpected for opaque
modes to have tieable modes excepting for themselves, but if this is the
only way to get rid of those extra moves, I guess we can special-case
them here.  Looking forward to Segher's comments on this part.

BR,
Kewen

>  
>    if (ALTIVEC_OR_VSX_VECTOR_MODE (mode1))
>      return ALTIVEC_OR_VSX_VECTOR_MODE (mode2);

On 11/24/23 3:28 AM, Kewen.Lin wrote:
>> +  int regoff = INTVAL (operands[2]) * GET_MODE_SIZE (V16QImode);
> 
> Is it intentional to keep GET_MODE_SIZE (V16QImode) instead of 16?
> I think if one day NUM_POLY_INT_COEFFS isn't 1 on rs6000 any more,
> we have to add one explicit .to_constant () here.  So I prefer this
> to use 16 directly, maybe one comment above to indicate what's for
> the value 16.

I normally don't like hard coding constants in the code, so used
GET_MODE_SIZE (V16QImode) as the number of bytes of a vector register,
but if that's going to cause an issue in the future, I'm fine using 16.
Changed.



>> +  int regoff = INTVAL (operands[2]) * GET_MODE_SIZE (V16QImode);
> 
> Likewise.

Changed.




>> diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
>> index 5f56c3ed85b..f2efa46c147 100644
>> --- a/gcc/config/rs6000/rs6000.cc
>> +++ b/gcc/config/rs6000/rs6000.cc
>> @@ -1964,9 +1964,12 @@ rs6000_hard_regno_mode_ok (unsigned int regno, machine_mode mode)
>>  static bool
>>  rs6000_modes_tieable_p (machine_mode mode1, machine_mode mode2)
>>  {
>> -  if (mode1 == PTImode || mode1 == OOmode || mode1 == XOmode
>> -      || mode2 == PTImode || mode2 == OOmode || mode2 == XOmode)
>> -    return mode1 == mode2;
>> +   if (mode1 == PTImode || mode1 == OOmode || mode1 == XOmode
>> +       || mode2 == PTImode || mode2 == XOmode)
>> +     return mode1 == mode2;
>> + 
>> +  if (mode2 == OOmode)
>> +    return ALTIVEC_OR_VSX_VECTOR_MODE (mode1);
> 
> I vaguely remembered that Segher mentioned it's unexpected for opaque
> modes to have tieable modes excepting for themselves, but if this is the
> only way to get rid of those extra moves, I guess we can special-case
> them here.  Looking forward to Segher's comments on this part.

To be honest, my original patch didn't have this.  I think it was Mike who
said we want or need this.  Mike, why do we want/need this again?

That said, the documentation for TARGET_MODES_TIEABLE_P says:

  This hook returns true if a value of mode mode1 is accessible in
  mode mode2 without copying.

Given OOmode (ie, __vector_pair) under the covers is two consecutive
vector registers, and we use them/initialize them with two vectors,
then mode1 being a vector mode could be accesible from an OOmode mode2
without copying, meaning we could access it directly from the registers
holding mode2.

Segher, your input to the above an the subreg portion of the patch in general?

Peter