Mesa (master): i965/fs: Optimize gl_FrontFacing calculation on Gen4/5.
Matt Turner
mattst88 at kemper.freedesktop.org
Tue Aug 19 01:34:30 UTC 2014
Module: Mesa
Branch: master
Commit: 19c6617adfec618889bb52d5398b8ac3d5969c18
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=19c6617adfec618889bb52d5398b8ac3d5969c18
Author: Matt Turner <mattst88 at gmail.com>
Date: Sun Aug 10 10:28:34 2014 -0700
i965/fs: Optimize gl_FrontFacing calculation on Gen4/5.
Doesn't use fewer instructions, but it does avoid writing the flag
register and if we want to switch the representation of true for Gen4/5
in the future, we can just delete the AND instruction.
---
src/mesa/drivers/dri/i965/brw_fs.cpp | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)
diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 053c4c3..20485aa 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -1205,12 +1205,23 @@ fs_visitor::emit_frontfacing_interpolation(ir_variable *ir)
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
- * us front face
+ /* Bit 31 of g1.6 is 0 if the polygon is front facing. We want to create
+ * a boolean result from this (1/true or 0/false).
+ *
+ * Like in the above case, since the bit is the MSB of g1.6:UD we can use
+ * the negation source modifier to flip it. Unfortunately the SHR
+ * instruction only operates on UD (or D with an abs source modifier)
+ * sources without negation.
+ *
+ * Instead, use ASR (which will give ~0/true or 0/false) followed by an
+ * AND 1.
*/
- emit(CMP(*reg, fs_reg(r1_6ud), fs_reg(1u << 31), BRW_CONDITIONAL_L));
- emit(BRW_OPCODE_AND, *reg, *reg, fs_reg(1u));
+ fs_reg asr = fs_reg(this, ir->type);
+ fs_reg g1_6 = fs_reg(retype(brw_vec1_grf(1, 6), BRW_REGISTER_TYPE_D));
+ g1_6.negate = true;
+
+ emit(ASR(asr, g1_6, fs_reg(31)));
+ emit(AND(*reg, asr, fs_reg(1)));
}
return reg;
More information about the mesa-commit
mailing list