[Mesa-dev] [PATCH] i965: Allow CSE on Gen4-5 unary math.

Kenneth Graunke kenneth at whitecape.org
Mon Oct 13 23:45:07 PDT 2014

Due to the implicit move-from-GRF, unary math looks a lot like the Gen6+
math instruction: it's a single instruction (SEND) with a GRF source.
The difference is that it also implicitly clobbers a message register.

The only visible effect is that CSE will remove the MRF-clobbering from
later math operations.  This should be fine; compute_to_mrf and
remove_redundant_mrf_writes don't look at the values populated by
implied writes, so they can't rely on those values being present.
Less interference may actually help those passes make more progress.

Binary math is still problematic, since it involves a separate MOV
instruction to load the second operand.  We continue disallowing CSE for
binary math operations.

total instructions in shared programs: 3340303 -> 3340100 (-0.01%)
instructions in affected programs:     26927 -> 26724 (-0.75%)
Nothing hurt, gained, or lost.  ~6% reduction on a few shaders.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
 src/mesa/drivers/dri/i965/brw_fs_cse.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

No Piglit regressions on Ironlake.

diff --git a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
index 817fc1f..8012001 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
@@ -99,7 +99,7 @@ is_expression(const fs_inst *const inst)
-      return inst->mlen == 0;
+      return inst->mlen < 2;
       return !is_copy_payload(inst);

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