On Mon, Oct 28, 2013 at 11:08 PM, Eric Anholt <e...@anholt.net> wrote:
> Matt Turner <matts...@gmail.com> writes:
>
>> Since compare instructions write the flag register, they should not be
>> considered dead even if their destination is never read. Instead of
>> removing them if found to be dead, set their destination to null to free
>> a register.
>>
>> ---
>> src/mesa/drivers/dri/i965/brw_fs.cpp | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp
>> b/src/mesa/drivers/dri/i965/brw_fs.cpp
>> index 7e7ced0..b985251 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
>> @@ -1903,10 +1903,12 @@ fs_visitor::dead_code_eliminate()
>>
>> if (dead) {
>> /* Don't dead code eliminate instructions that write to the
>> - * accumulator as a side-effect. Instead just set the
>> destination
>> - * to the null register to free it.
>> + * accumulator or flag register as a side effect. Instead just
>> set
>> + * the destination to the null register to free it.
>> */
>> switch (inst->opcode) {
>> + case BRW_OPCODE_CMP:
>> + case BRW_OPCODE_CMPN:
>> case BRW_OPCODE_ADDC:
>> case BRW_OPCODE_SUBB:
>> case BRW_OPCODE_MACH:
>
> I don't think this one is needed in the series any more after the rework
> to not generate new CMP.whatever.f0 to a non-null destination in CSE,
> but I think it should still land anyway. I'd prefer the test for
> "writes to the flag register" to be inst->writes_flag() rather than just
> a couple of opcodes that always write the flag, though.
That's a good point and a good idea. I'll drop it from this series and
send it using inst->writes_flag() separately.
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