[Mesa-dev] [PATCH 30/59] intel/compiler: document MAD algebraic optimization

[Pohjolainen, Topi]

On Tue, Dec 04, 2018 at 08:16:54AM +0100, Iago Toral Quiroga wrote:
> This optimization depends on two other optimization passes: the
> constant propagation pass, which allows immediate propagation
> on MAD/LRP instructions even though the hardware can't do it,
> and the combine constants pass to fix this up afterwards for the
> cases that we could not optimize here.
> 
> Also, the optimization can generate cases for MUL/ADD that we
> should not find otherwise, which are then implemented building
> on that assumption, so better documenting these is useful.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen at intel.com>

> ---
>  src/intel/compiler/brw_fs.cpp | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
> index 509c6febf38..a9ddafc05d1 100644
> --- a/src/intel/compiler/brw_fs.cpp
> +++ b/src/intel/compiler/brw_fs.cpp
> @@ -2461,6 +2461,11 @@ fs_visitor::opt_algebraic()
>           }
>  
>           if (inst->src[0].file == IMM) {
> +            /* We produce these from the MAD optimization below, which
> +             * should only be happening for 32-bit float because we
> +             * prevent constant propagation to MAD sources for other
> +             * bit-sizes.
> +             */
>              assert(inst->src[0].type == BRW_REGISTER_TYPE_F);
>              inst->opcode = BRW_OPCODE_MOV;
>              inst->src[0].f *= inst->src[1].f;
> @@ -2482,6 +2487,11 @@ fs_visitor::opt_algebraic()
>           }
>  
>           if (inst->src[0].file == IMM) {
> +            /* We produce these from the MAD optimization below, which
> +             * should only be happening for 32-bit float because we
> +             * prevent constant propagation to MAD sources for other
> +             * bit-sizes.
> +             */
>              assert(inst->src[0].type == BRW_REGISTER_TYPE_F);
>              inst->opcode = BRW_OPCODE_MOV;
>              inst->src[0].f += inst->src[1].f;
> @@ -2565,6 +2575,11 @@ fs_visitor::opt_algebraic()
>           }
>           break;
>        case BRW_OPCODE_MAD:
> +         /* ALign16 MAD can't do immediate sources, however we allow constant
> +          * propagation to these instructions to enable these algebraic
> +          * optimizations. For the cases that we can't optmize here, we
> +          * rely on the combine constants pass to fix it up later.
> +          */
>           if (inst->src[1].is_zero() || inst->src[2].is_zero()) {
>              inst->opcode = BRW_OPCODE_MOV;
>              inst->src[1] = reg_undef;
> @@ -2585,6 +2600,13 @@ fs_visitor::opt_algebraic()
>              inst->src[2] = reg_undef;
>              progress = true;
>           } else if (inst->src[1].file == IMM && inst->src[2].file == IMM) {
> +            /* We should not be getting here for anything other than 32-bit
> +             * float since we prevent constant-propagation to MAD instructions
> +             * for everything else.
> +             */
> +            assert(inst->src[1].type == inst->src[2].type &&
> +                   inst->src[1].type == BRW_REGISTER_TYPE_F);
> +
>              inst->opcode = BRW_OPCODE_ADD;
>              inst->src[1].f *= inst->src[2].f;
>              inst->src[2] = reg_undef;
> -- 
> 2.17.1
> 
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