On Fri, Sep 25, 2015 at 10:47 AM, Kenneth Graunke <kenn...@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 <kenn...@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.
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