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i965/gs: Optimize away the EOT write on Gen8+ with static vertex count.

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With static vertex counts, the final EOT write doesn't actually write
any data - it's just there to end the thread.  Typically, the last
thing before ending the thread will be an EmitVertex() call, resulting
in a URB write.  We can just set EOT on that.

Note that this isn't always possible - there might be an intervening
SSBO write/image store, or the URB write may have been in a loop.

shader-db statistics for geometry shaders only:

total instructions in shared programs: 3173 -> 3149 (-0.76%)
instructions in affected programs:     176 -> 152 (-13.64%)
helped:                                8

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
This commit is contained in:
Kenneth Graunke 2015-09-25 08:21:57 -07:00
parent 08fe5799e6
commit d6a41b5f70
1 changed files with 15 additions and 0 deletions
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15
src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
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@ -236,6 +236,21 @@ vec4_gs_visitor::emit_thread_end()
bool static_vertex_count = c->prog_data.static_vertex_count != -1;
/* If the previous instruction was a URB write, we don't need to issue
* a second one - we can just set the EOT bit on the previous write.
*
* Skip this on Gen8+ unless there's a static vertex count, as we also
* need to write the vertex count out, and combining the two may not be
* possible (or at least not straightforward).
*/
vec4_instruction *last = (vec4_instruction *) instructions.get_tail();
if (last && last->opcode == GS_OPCODE_URB_WRITE &&
!(INTEL_DEBUG & DEBUG_SHADER_TIME) &&
devinfo->gen >= 8 && static_vertex_count) {
last->urb_write_flags = BRW_URB_WRITE_EOT | last->urb_write_flags;
return;
}
current_annotation = "thread end";
dst_reg mrf_reg(MRF, base_mrf);
src_reg r0(retype(brw_vec8_grf(0, 0), BRW_REGISTER_TYPE_UD));