[Mesa-dev] [PATCH 1/2] i965/gs: Allow src0 immediates in GS_OPCODE_SET_WRITE_OFFSET.

[Matt Turner]

On Fri, Sep 25, 2015 at 10:47 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> GS_OPCODE_SET_WRITE_OFFSET is a MUL with a constant src[1] and special
> strides.  We can easily make the generator handle constant src[0]
> arguments by instead generating a MOV with the product of both operands.
>
> This isn't necessarily a win in and of itself - instead of a MUL, we
> generate a MOV, which should be basically the same cost.  However, we
> can probably avoid the earlier MOV to put src[0] into a register.
>
> shader-db statistics for geometry shaders only:
>
> total instructions in shared programs: 3207 -> 3173 (-1.06%)
> instructions in affected programs:     3207 -> 3173 (-1.06%)
> helped:                                11
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp | 7 +++++++
>  src/mesa/drivers/dri/i965/brw_vec4_generator.cpp        | 9 +++++++--
>  2 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp b/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
> index 5b6444e..610caef 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
> @@ -202,6 +202,13 @@ try_constant_propagate(const struct brw_device_info *devinfo,
>          return true;
>        }
>        break;
> +   case GS_OPCODE_SET_WRITE_OFFSET:
> +      /* This is just a multiply by a constant with special strides.
> +       * The generator will handle immediates in both arguments (generating
> +       * a single MOV of the product).  So feel free to propagate in src0.
> +       */
> +      inst->src[arg] = value;
> +      return true;
>
>     case BRW_OPCODE_CMP:
>        if (arg == 1) {
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp b/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
> index e69c067..620167d 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
> @@ -541,8 +541,13 @@ vec4_generator::generate_gs_set_write_offset(struct brw_reg dst,
>            src1.file == BRW_IMMEDIATE_VALUE &&
>            src1.type == BRW_REGISTER_TYPE_UD &&
>            src1.dw1.ud <= USHRT_MAX);
> -   brw_MUL(p, suboffset(stride(dst, 2, 2, 1), 3), stride(src0, 8, 2, 4),
> -           retype(src1, BRW_REGISTER_TYPE_UW));
> +   if (src0.file == IMM) {
> +      brw_MOV(p, suboffset(stride(dst, 2, 2, 1), 3),
> +              brw_imm_ud(src0.dw1.ud * src1.dw1.ud));

Alternatively, we could make opt_algebraic() constant-evaluate this at
a higher level. I'm not sure if that would help generate better code,
but it seems a little cleaner.