[Mesa-dev] [PATCH 1/3] i965/vec4: Call opt_algebraic after opt_cse.

[Ian Romanick]

On 09/30/2014 02:16 PM, Matt Turner wrote:
> On Tue, Sep 30, 2014 at 2:10 PM, Ian Romanick <idr at freedesktop.org> wrote:
>> On 09/27/2014 12:12 PM, Matt Turner wrote:
>>> The next patch adds an algebraic optimization for the pattern
>>>
>>>    sqrt a, b
>>>    rcp  c, a
>>>
>>> and turns it into
>>>
>>>    sqrt a, b
>>>    rsq  c, b
>>>
>>> but many vertex shaders do
>>>
>>>    a = sqrt(b);
>>>    var1 /= a;
>>>    var2 /= a;
>>>
>>> which generates
>>>
>>>    sqrt a, b
>>>    rcp  c, a
>>>    rcp  d, a
>>>
>>> If we apply the algebraic optimization before CSE, we'll end up with
>>>
>>>    sqrt a, b
>>>    rsq  c, b
>>>    rcp  d, a
>>
>> Why doesn't a second pass through opt_algebraic turn this into
> 
> Because the addition in patch #2 just recognizes a consecutive sqrt+rcp pattern.

That makes sense.  Series is

Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>

>>    rsq  c, b
>>    rsq  d, b
>>
>> Seems like this could cause us to miss other optimization opportunities...
> 
> This seems pretty sufficient for the collection of shaders in
> shader-db -- no regressions, cuts vec4 instructions, and handles 410
> sqrt+rcp pairs.