[Mesa-dev] [PATCH 2/6] i965/fs: Unrestrict constant propagation into integer multiply.

[Matt Turner]

Gen8+'s MUL instruction doesn't ignore the high 16-bits of one source
like on earlier platforms, so we can constant propagate into it without
worry. Integer multiplies (not into the accumulator, which is done for
imul_high) are lowered in lower_integer_multiplication(), so it's safe
there as well.

On Broadwell, fragment shaders only:
total instructions in shared programs: 4377769 -> 4377451 (-0.01%)
instructions in affected programs:     48064 -> 47746 (-0.66%)
helped:                                156

On Broadwell, vertex shaders only:
total instructions in shared programs: 2858885 -> 2856313 (-0.09%)
instructions in affected programs:     26380 -> 23808 (-9.75%)
helped:                                134

On Broadwell, vertex shaders only (with INTEL_USE_NIR=1):
total instructions in shared programs: 2911688 -> 2865984 (-1.57%)
instructions in affected programs:     1421715 -> 1376011 (-3.21%)
helped:                                6186
---
 src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
index 52bfa92..c92aae4 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
@@ -541,8 +541,16 @@ fs_visitor::try_constant_propagate(fs_inst *inst, acp_entry *entry)
             /* Fit this constant in by commuting the operands.
              * Exception: we can't do this for 32-bit integer MUL/MACH
              * because it's asymmetric.
+             *
+             * The BSpec says for Broadwell that
+             *
+             *    "When multiplying DW x DW, the dst cannot be accumulator."
+             *
+             * Integer MUL with a non-accumulator destination will be lowered
+             * by lower_integer_multiplication(), so don't restrict it.
              */
-            if ((inst->opcode == BRW_OPCODE_MUL ||
+            if (((inst->opcode == BRW_OPCODE_MUL &&
+                  inst->dst.is_accumulator()) ||
                  inst->opcode == BRW_OPCODE_MACH) &&
                 (inst->src[1].type == BRW_REGISTER_TYPE_D ||
                  inst->src[1].type == BRW_REGISTER_TYPE_UD))
-- 
2.3.6