i965/gs: Allow src0 immediates in GS_OPCODE_SET_WRITE_OFFSET.

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 <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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 5b6444e3210..610caef7dce 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 50f96632f7f..dcacc900540 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));
+   } else {
+      brw_MUL(p, suboffset(stride(dst, 2, 2, 1), 3), stride(src0, 8, 2, 4),
+              retype(src1, BRW_REGISTER_TYPE_UW));
+   }
    brw_set_default_access_mode(p, BRW_ALIGN_16);
    brw_pop_insn_state(p);
 }