Re: [ARM] Implement division using vrecpe, vrecps

[Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>]

On 26/10/2018 06:04, Prathamesh Kulkarni wrote:
> Hi,
> This is a rebased version of patch that adds a pattern to neon.md for
> implementing division with multiplication by reciprocal using
> vrecpe/vrecps with -funsafe-math-optimizations excluding -Os.
> The newly added test-cases are not vectorized on armeb target with
> -O2. I posted the analysis for that here:
> https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01765.html
> 
> Briefly, the difference between little and big-endian vectorizer is in
> arm_builtin_support_vector_misalignment() which calls
> default_builtin_support_vector_misalignment() for big-endian case, and
> that returns false because
> movmisalign_optab does not exist for V2SF mode. This isn't observed
> with -O3 because loop peeling for alignment gets enabled.
> 
> It seems that the test cases in patch appear unsupported on armeb,
> after r221677 thus this patch requires no changes to
> target-supports.exp to adjust for armeb (unlike last time which
> stalled the patch).
> 
> Bootstrap+tested on arm-linux-gnueabihf.
> Cross-tested on arm*-*-* variants.
> OK for trunk ?
> 
> Thanks,
> Prathamesh
> 
> 
> tcwg-319-3.txt
> 
> 2018-10-26  Prathamesh Kulkarni<prathamesh.kulkarni@linaro.org>
> 
> 	* config/arm/neon.md (div<mode>3): New pattern.
> 
> testsuite/
> 	* gcc.target/arm/neon-vect-div-1.c: New test.
> 	* gcc.target/arm/neon-vect-div-2.c: Likewise.
> 
> diff --git a/gcc/config/arm/neon.md b/gcc/config/arm/neon.md
> index 5aeee4b08c1..25ed45d381a 100644
> --- a/gcc/config/arm/neon.md
> +++ b/gcc/config/arm/neon.md
> @@ -620,6 +620,38 @@
>                       (const_string "neon_mul_<V_elem_ch><q>")))]
>   )
>   
> +/* Perform division using multiply-by-reciprocal.
> +   Reciprocal is calculated using Newton-Raphson method.
> +   Enabled with -funsafe-math-optimizations -freciprocal-math
> +   and disabled for -Os since it increases code size .  */ > +
> +(define_expand "div<mode>3"
> +  [(set (match_operand:VCVTF 0 "s_register_operand" "=w")
> +        (div:VCVTF (match_operand:VCVTF 1 "s_register_operand" "w")
> +		  (match_operand:VCVTF 2 "s_register_operand" "w")))]
> +  "TARGET_NEON && !optimize_size
> +   && flag_unsafe_math_optimizations && flag_reciprocal_math"

I would prefer this to be more granular than 
flag_unsafe_math_optimization && flag_reciprocal_math which really is 
flag_reciprocal_math as it is turned on by default with 
funsafe-math-optimizations.

I think this should really be just flag_reciprocal_math.


Otherwise ok.

regards
Ramana




> +  {
> +    rtx rec = gen_reg_rtx (<MODE>mode);
> +    rtx vrecps_temp = gen_reg_rtx (<MODE>mode);
> +
> +    /* Reciprocal estimate.  */
> +    emit_insn (gen_neon_vrecpe<mode> (rec, operands[2]));
> +
> +    /* Perform 2 iterations of newton-raphson method.  */
> +    for (int i = 0; i < 2; i++)
> +      {
> +	emit_insn (gen_neon_vrecps<mode> (vrecps_temp, rec, operands[2]));
> +	emit_insn (gen_mul<mode>3 (rec, rec, vrecps_temp));
> +      }
> +
> +    /* We now have reciprocal in rec, perform operands[0] = operands[1] * rec.  */
> +    emit_insn (gen_mul<mode>3 (operands[0], operands[1], rec));
> +    DONE;
> +  }
> +)
> +
> +
>   (define_insn "mul<mode>3add<mode>_neon"
>     [(set (match_operand:VDQW 0 "s_register_operand" "=w")
>           (plus:VDQW (mult:VDQW (match_operand:VDQW 2 "s_register_operand" "w")
> diff --git a/gcc/testsuite/gcc.target/arm/neon-vect-div-1.c b/gcc/testsuite/gcc.target/arm/neon-vect-div-1.c
> new file mode 100644
> index 00000000000..50d04b4175b
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/arm/neon-vect-div-1.c
> @@ -0,0 +1,16 @@
> +/* Test pattern div<mode>3.  */
> +/* { dg-do compile } */
> +/* { dg-require-effective-target arm_neon_ok } */
> +/* { dg-require-effective-target vect_hw_misalign } */
> +/* { dg-options "-O2 -ftree-vectorize -funsafe-math-optimizations -fdump-tree-vect-details" } */
> +/* { dg-add-options arm_neon } */
> +
> +void
> +foo (int len, float * __restrict p, float *__restrict x)
> +{
> +  len = len & ~31;
> +  for (int i = 0; i < len; i++)
> +    p[i] = p[i] / x[i];
> +}
> +
> +/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" } } */
> diff --git a/gcc/testsuite/gcc.target/arm/neon-vect-div-2.c b/gcc/testsuite/gcc.target/arm/neon-vect-div-2.c
> new file mode 100644
> index 00000000000..606f54b4e0e
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/arm/neon-vect-div-2.c
> @@ -0,0 +1,16 @@
> +/* Test pattern div<mode>3.  */
> +/* { dg-do compile } */
> +/* { dg-require-effective-target arm_neon_ok } */
> +/* { dg-require-effective-target vect_hw_misalign } */
> +/* { dg-options "-O3 -ftree-vectorize -funsafe-math-optimizations -fdump-tree-vect-details -fno-reciprocal-math" } */
> +/* { dg-add-options arm_neon } */
> +
> +void
> +foo (int len, float * __restrict p, float *__restrict x)
> +{
> +  len = len & ~31;
> +  for (int i = 0; i < len; i++)
> +    p[i] = p[i] / x[i];
> +}
> +
> +/* { dg-final { scan-tree-dump-not "vectorized 1 loops" "vect" } } */
>