[Mesa-dev] [PATCH 2/4] i965/fs: Disable CSE on instructions writing to HW_REG.

[Matt Turner]

CSE would otherwise combine the two mul(8) emitted by [iu]mulExtended:

	mul(8)  acc0 x y
	mach(8) null x y
	mov(8)  lsb  acc0
	...
	mul(8)  acc0 x y
	mach(8) msb  x y
Into:
	mul(8)  temp x y
	mov(8)  acc0 temp
	mach(8) null x y
	mov(8)  lsb  acc0
	...
	mov(8)  acc0 temp
	mach(8) msb  x y

But mul(8) into the accumulator produces more than 32-bits of precision,
which is required and lost if multiplying into a general register and
moving to the accumulator.
---
 src/mesa/drivers/dri/i965/brw_fs_cse.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
index ccd4e5e..61b3aeb 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_cse.cpp
@@ -98,7 +98,8 @@ fs_visitor::opt_cse_local(bblock_t *block, exec_list *aeb)
       if (is_expression(inst) &&
           !inst->predicate &&
           !inst->is_partial_write() &&
-          !inst->conditional_mod)
+          !inst->conditional_mod &&
+          inst->dst.file != HW_REG)
       {
 	 bool found = false;
 
-- 
1.8.3.2