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i965/fs: Disable CSE on instructions writing to HW_REG.

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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.

Reviewed-by: Eric Anholt <eric@anholt.net>
This commit is contained in:
Matt Turner 2013-09-26 13:38:11 -07:00
parent 06e41a02a3
commit 6ff8f06308
1 changed files with 2 additions and 1 deletions
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3
src/mesa/drivers/dri/i965/brw_fs_cse.cpp
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@ -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;