Re: [PATCH 2/3] arm: Remove unnecessary zero-extending of MVE predicates before use [PR 107674]

[Richard Sandiford <richard.sandiford@arm.com>]

"Andre Vieira (lists)" <andre.simoesdiasvieira@arm.com> writes:
> Hi,
>
> This patch teaches GCC that zero-extending a MVE predicate from 16-bits 
> to 32-bits and then only using 16-bits is a no-op.
> It does so in two steps:
> - it lets gcc know that it can access any MVE predicate mode using any 
> other MVE predicate mode without needing to copy it, using the 
> TARGET_MODES_TIEABLE_P hook,
> - it teaches simplify_subreg to optimize a subreg with a vector 
> outermode, by replacing this outermode with a same-sized integer mode 
> and trying the avalailable optimizations, then if successful it 
> surrounds the result with a subreg casting it back to the original 
> vector outermode.
>
> This removes the unnecessary zero-extending shown on PR 107674 (though 
> it's a sign-extend there), that was introduced in gcc 11.
>
> Bootstrapped on aarch64-none-linux-gnu and regression tested on 
> arm-none-eabi and armeb-none-eabi for armv8.1-m.main+mve.fp.
>
> OK for trunk?
>
> gcc/ChangeLog:
>
> 	PR target/107674
>          * conig/arm/arm.cc (arm_hard_regno_mode_ok): Use new MACRO.
>          (arm_modes_tieable_p): Make MVE predicate modes tieable.
> 	* config/arm/arm.h (VALID_MVE_PRED_MODE):  New define.
> 	* simplify-rtx.cc (simplify_context::simplify_subreg): Teach
> 	simplify_subreg to simplify subregs where the outermode is not scalar.
>
> gcc/testsuite/ChangeLog:
>
> 	* gcc.target/arm/mve/mve_vpt.c: Change to remove unecessary
> 	zero-extend.
>
> diff --git a/gcc/config/arm/arm.h b/gcc/config/arm/arm.h
> index 6f7ecf9128047647fc41677e634cd9612a13242b..4352c830cb6d2e632a225edea861b5ceb35dd035 100644
> --- a/gcc/config/arm/arm.h
> +++ b/gcc/config/arm/arm.h
> @@ -1091,6 +1091,10 @@ extern const int arm_arch_cde_coproc_bits[];
>     || (MODE) == V16QImode || (MODE) == V8HFmode || (MODE) == V4SFmode \
>     || (MODE) == V2DFmode)
>  
> +#define VALID_MVE_PRED_MODE(MODE) \
> +  ((MODE) == HImode							\
> +   || (MODE) == V16BImode || (MODE) == V8BImode || (MODE) == V4BImode)
> +
>  #define VALID_MVE_SI_MODE(MODE) \
>    ((MODE) == V2DImode ||(MODE) == V4SImode || (MODE) == V8HImode \
>     || (MODE) == V16QImode)
> diff --git a/gcc/config/arm/arm.cc b/gcc/config/arm/arm.cc
> index 3f171188de513e258369397e4726afe27bd9fdbf..18460ef5280be8c1df85eff424a1bf66d6019c0a 100644
> --- a/gcc/config/arm/arm.cc
> +++ b/gcc/config/arm/arm.cc
> @@ -25564,10 +25564,7 @@ arm_hard_regno_mode_ok (unsigned int regno, machine_mode mode)
>      return false;
>  
>    if (IS_VPR_REGNUM (regno))
> -    return mode == HImode
> -      || mode == V16BImode
> -      || mode == V8BImode
> -      || mode == V4BImode;
> +    return VALID_MVE_PRED_MODE (mode);
>  
>    if (TARGET_THUMB1)
>      /* For the Thumb we only allow values bigger than SImode in
> @@ -25646,6 +25643,10 @@ arm_modes_tieable_p (machine_mode mode1, machine_mode mode2)
>    if (GET_MODE_CLASS (mode1) == GET_MODE_CLASS (mode2))
>      return true;
>  
> +  if (TARGET_HAVE_MVE
> +      && (VALID_MVE_PRED_MODE (mode1) && VALID_MVE_PRED_MODE (mode2)))
> +    return true;
> +
>    /* We specifically want to allow elements of "structure" modes to
>       be tieable to the structure.  This more general condition allows
>       other rarer situations too.  */
> diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
> index 7fb1e97fbea4e7b8b091f5724ebe0cb61eee7ec3..a951272186585c0a5cc3e0155285e7a635865f42 100644
> --- a/gcc/simplify-rtx.cc
> +++ b/gcc/simplify-rtx.cc
> @@ -7652,6 +7652,22 @@ simplify_context::simplify_subreg (machine_mode outermode, rtx op,
>  	}
>      }
>  
> +  /* Try simplifying a SUBREG expression of a non-integer OUTERMODE by using a
> +     NEW_OUTERMODE of the same size instead, other simplifications rely on
> +     integer to integer subregs and we'd potentially miss out on optimizations
> +     otherwise.  */

How about:

  /* If the outer mode is not integral, try taking a subreg with the
     equivalent integer outer mode and then bitcasting the result.
     Other simplifications rely on integer to integer subregs and we'd
     potentially miss out on optimizations otherwise.  */

(Think it's easier to read as two sentences, and there's no
new_outermode in the code.)

> +  if (known_gt (GET_MODE_SIZE (innermode),
> +		GET_MODE_SIZE (outermode))
> +      && SCALAR_INT_MODE_P (innermode)
> +      && !SCALAR_INT_MODE_P (outermode)
> +      && int_mode_for_size (GET_MODE_BITSIZE (outermode),
> +			    0).exists (&int_outermode))
> +    {
> +      rtx tem = simplify_subreg (int_outermode, op, innermode, byte);
> +      if (tem)
> +	return simplify_gen_subreg (outermode, tem, GET_MODE (tem), byte);

Perhaps safer as s/GET_MODE (tem)/int_outermode/, in case TEM turns
out to be constant.

OK for the simplify-rtx.cc part with those changes.

Thanks,
Richard


> +    }
> +
>    /* If OP is a vector comparison and the subreg is not changing the
>       number of elements or the size of the elements, change the result
>       of the comparison to the new mode.  */
> diff --git a/gcc/testsuite/gcc.target/arm/mve/mve_vpt.c b/gcc/testsuite/gcc.target/arm/mve/mve_vpt.c
> index 26a565b79dd1348e361b3aa23a1d6e6d13bffce8..8e562a9f065eff157f63ebd5acf9af0a2155b5c5 100644
> --- a/gcc/testsuite/gcc.target/arm/mve/mve_vpt.c
> +++ b/gcc/testsuite/gcc.target/arm/mve/mve_vpt.c
> @@ -16,9 +16,6 @@ void test0 (uint8_t *a, uint8_t *b, uint8_t *c)
>  **	vldrb.8	q2, \[r0\]
>  **	vldrb.8	q1, \[r1\]
>  **	vcmp.i8	eq, q2, q1
> -**	vmrs	r3, p0	@ movhi
> -**	uxth	r3, r3
> -**	vmsr	p0, r3	@ movhi
>  **	vpst
>  **	vaddt.i8	q3, q2, q1
>  **	vpst