[Mesa-dev] [RFC PATCH 5/5] i965: Skip execution size adjustment for instructions of width 4

[Iago Toral Quiroga]

This code in brw_set_dest adjusts the execution size of any instruction
with a dst.width < 8. However, we don't want to do this with instructions
operating on doubles, since these will have a width of 4, but still
need an execution size of 8 (for SIMD8). Unfortunately, we can't just check
the size of the operands involved to detect if we are doing an operation on
doubles, because we can have instructions that do operations on double
operands interpreted as UD, operating on any of its 2 32-bit components.

Previous commits have made it so we never emit instructions with a horizontal
width of 4 that don't have the correct execution size set for gen7/gen8, so
we can skip it in this case, avoiding the conflicts with fp64 requirements.

Expanding the same fix to other hardware generations requires many more
changes but since we are not targetting fp64 support on them
wer don't really care for now.
---
 src/mesa/drivers/dri/i965/brw_eu_emit.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_eu_emit.c b/src/mesa/drivers/dri/i965/brw_eu_emit.c
index 78f2c8c..50a8771 100644
--- a/src/mesa/drivers/dri/i965/brw_eu_emit.c
+++ b/src/mesa/drivers/dri/i965/brw_eu_emit.c
@@ -202,8 +202,20 @@ brw_set_dest(struct brw_codegen *p, brw_inst *inst, struct brw_reg dest)
    /* Generators should set a default exec_size of either 8 (SIMD4x2 or SIMD8)
     * or 16 (SIMD16), as that's normally correct.  However, when dealing with
     * small registers, we automatically reduce it to match the register size.
+    *
+    * In platforms that support fp64 we can emit instructions with a width of
+    * 4 that need two SIMD8 registers and an exec_size of 8 or 16. In these
+    * cases we need to make sure that these instructions have their exec sizes
+    * set properly when they are emitted and we can't rely on this code to fix
+    * it.
     */
-   if (dest.width < BRW_EXECUTE_8)
+   bool fix_exec_size;
+   if (devinfo->gen == 7 || devinfo->gen == 8)
+      fix_exec_size = dest.width < BRW_EXECUTE_4;
+   else
+      fix_exec_size = dest.width < BRW_EXECUTE_8;
+
+   if (fix_exec_size)
       brw_inst_set_exec_size(devinfo, inst, dest.width);
 }
 
-- 
2.1.4