Re: [PATCH][AArch64] PR84114: Avoid reassociating FMA

[James Greenhalgh <james.greenhalgh@arm.com>]

On Thu, Feb 22, 2018 at 11:38:03AM +0000, Wilco Dijkstra wrote:
> As discussed in the PR, the reassociation phase runs before FMAs are formed
> and so can significantly reduce FMA opportunities.  Although reassociation
> could be switched off, it helps in many cases, so a better alternative is to
> only avoid reassociation of floating point additions.  This fixes the testcase
> and gives 1% speedup on SPECFP2017, fixing the performance regression.
> 
> OK for commit?

This is OK as a fairly safe fix for stage 4. We should fix reassociation
properly in GCC 9.

Thanks,
James

> 
> ChangeLog:
> 2018-02-23  Wilco Dijkstra  <wdijkstr@arm.com>
> 
> 	PR tree-optimization/84114
> 	* config/aarch64/aarch64.c (aarch64_reassociation_width)
> 	Avoid reassociation of FLOAT_MODE addition.
> --
> 
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index b3d5fde171920e5759046a4bd61cfcf9eb78d7dd..5f9541cf700aaf18c1f1ac73054614e2932781e4 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -1109,15 +1109,16 @@ aarch64_min_divisions_for_recip_mul (machine_mode mode)
>    return aarch64_tune_params.min_div_recip_mul_df;
>  }
>  
> +/* Return the reassociation width of treeop OPC with mode MODE.  */
>  static int
> -aarch64_reassociation_width (unsigned opc ATTRIBUTE_UNUSED,
> -			     machine_mode mode)
> +aarch64_reassociation_width (unsigned opc, machine_mode mode)
>  {
>    if (VECTOR_MODE_P (mode))
>      return aarch64_tune_params.vec_reassoc_width;
>    if (INTEGRAL_MODE_P (mode))
>      return aarch64_tune_params.int_reassoc_width;
> -  if (FLOAT_MODE_P (mode))
> +  /* Avoid reassociating floating point addition so we emit more FMAs.  */
> +  if (FLOAT_MODE_P (mode) && opc != PLUS_EXPR)
>      return aarch64_tune_params.fp_reassoc_width;
>    return 1;
>  }