[Mesa-dev] [PATCH 09/10] i965/fs: Add a peephole pass to combine ADD with ADDC/SUBB.

[Eric Anholt]

Matt Turner <mattst88 at gmail.com> writes:

> On Fri, Oct 4, 2013 at 2:19 PM, Paul Berry <stereotype441 at gmail.com> wrote:
>> On 4 October 2013 13:51, Paul Berry <stereotype441 at gmail.com> wrote:
>>>
>>> On 3 October 2013 10:59, Matt Turner <mattst88 at gmail.com> wrote:
>>>>
>>>> v2: Check fixed_hw_reg.{file,nr} instead of dst.reg.
>>>> v3: Store the bool emitted_addc_or_subb in the class, not static.
>>>> ---
>>>>  src/mesa/drivers/dri/i965/brw_fs.h           |   3 +
>>>>  src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 104
>>>> +++++++++++++++++++++++++++
>>>>  2 files changed, 107 insertions(+)
>>>
>>>
>>> Reviewed-by: Paul Berry <stereotype441 at gmail.com>
>>
>>
>> Whoops, wait a minute.  I spoke too soon.
>>
>> I'm concerned because it looks like this peephole pass is too generous.  The
>> following instruction stream, for example, shoud not be peephole optimized:
>>
>> addc null, x, y
>> mov carry, acc0
>> mov x, z
>> add sum x, y
>>
>> But I don't see anything in fs_visitor::try_combine_add_with_addc_subb() to
>> prevent this.
>
> We're operating on virtual grfs at this point, so I don't think that
> case is possible. I.e., if there were an intervening write to x, it
> just would have been a different vgrf. I think Ken had this same
> concern at one point, although I don't think he ever said it via
> email.

Huh?  Why would an intervening write to x have a different vgrf?
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