diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/mesa/drivers/dri/i965/brw_fs.cpp | 21 |
1 files changed, 15 insertions, 6 deletions
diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp index 935cbfa0973..053c4c32719 100644 --- a/src/mesa/drivers/dri/i965/brw_fs.cpp +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp @@ -1188,13 +1188,22 @@ fs_visitor::emit_frontfacing_interpolation(ir_variable *ir) { fs_reg *reg = new(this->mem_ctx) fs_reg(this, ir->type); - /* The frontfacing comes in as a bit in the thread payload. */ if (brw->gen >= 6) { - emit(BRW_OPCODE_SHL, *reg, - fs_reg(retype(brw_vec1_grf(0, 0), BRW_REGISTER_TYPE_D)), - fs_reg(16)); - emit(BRW_OPCODE_NOT, *reg, *reg); - emit(BRW_OPCODE_ASR, *reg, *reg, fs_reg(31)); + /* Bit 15 of g0.0 is 0 if the polygon is front facing. We want to create + * a boolean result from this (~0/true or 0/false). + * + * We can use the fact that bit 15 is the MSB of g0.0:W to accomplish + * this task in only one instruction: + * - a negation source modifier will flip the bit; and + * - a W -> D type conversion will sign extend the bit into the high + * word of the destination. + * + * An ASR 15 fills the low word of the destination. + */ + fs_reg g0 = fs_reg(retype(brw_vec1_grf(0, 0), BRW_REGISTER_TYPE_W)); + g0.negate = true; + + emit(ASR(*reg, g0, fs_reg(15))); } else { struct brw_reg r1_6ud = retype(brw_vec1_grf(1, 6), BRW_REGISTER_TYPE_UD); /* bit 31 is "primitive is back face", so checking < (1 << 31) gives |