[Mesa-dev] [PATCH 1/6] glsl: Optimize pow(x, 2) into x * x.

[Eric Anholt]

Erik Faye-Lund <kusmabite at gmail.com> writes:

> On Tue, Mar 11, 2014 at 7:27 PM, Eric Anholt <eric at anholt.net> wrote:
>> Erik Faye-Lund <kusmabite at gmail.com> writes:
>>
>>> On Tue, Mar 11, 2014 at 2:50 PM, Erik Faye-Lund <kusmabite at gmail.com> wrote:
>>>> On Mon, Mar 10, 2014 at 11:54 PM, Matt Turner <mattst88 at gmail.com> wrote:
>>>>> Cuts two instructions out of SynMark's Gl32VSInstancing benchmark.
>>>>> ---
>>>>>  src/glsl/opt_algebraic.cpp | 8 ++++++++
>>>>>  1 file changed, 8 insertions(+)
>>>>>
>>>>> diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
>>>>> index 5c49a78..8494bd9 100644
>>>>> --- a/src/glsl/opt_algebraic.cpp
>>>>> +++ b/src/glsl/opt_algebraic.cpp
>>>>> @@ -528,6 +528,14 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
>>>>>        if (is_vec_two(op_const[0]))
>>>>>           return expr(ir_unop_exp2, ir->operands[1]);
>>>>>
>>>>> +      if (is_vec_two(op_const[1])) {
>>>>> +         ir_variable *x = new(ir) ir_variable(ir->operands[1]->type, "x",
>>>>> +                                              ir_var_temporary);
>>>>> +         base_ir->insert_before(x);
>>>>> +         base_ir->insert_before(assign(x, ir->operands[0]));
>>>>> +         return mul(x, x);
>>>>> +      }
>>>>> +
>>>>
>>>> Is this safe? Since many GPUs implement pow(x, y) as exp2(log2(x) *
>>>> y), this will give different results for if y comes from a uniform vs
>>>> if it's a constant, no?
>>
>> Yes, but that wouldn't be covered by the "invariant" keyword.
>>
>>> To be a bit more clear: I don't think this is valid for expressions
>>> writing to variables marked as invariant (or expressions taking part
>>> in the calculations that leads up to invariant variable writes).
>>>
>>> I can't find anything allowing variance like this in the invariance
>>> section of the GLSL 3.30 specifications. In particular, the list
>>> following "To guarantee invariance of a particular output variable
>>> across two programs, the following must also be true" doesn't seem to
>>> require the values to be passed from the same source, only that the
>>> same values are passed. And in this case, the value 2.0 is usually
>>> exactly representable no matter what path it took there.
>>>
>>> Perhaps I'm being a bit too pedantic here, though.
>>
>> This file would do the same thing on the same expression tree in two
>> different programs, so "invariant" is fine (we've probably got other
>> problems with invariant, though).  The keyword you're probably thinking
>> of is "precise", which isn't in GLSL we implement yet.
>
> Are you saying that this only rewrites "x = pow(y, 2.0)" and not
> "const float e = 2.0; x = pow(y, e);"? If so, my point is moot,
> indeed. But if that's *not* the case, then I think we're in trouble
> still.

The second would also get rewritten, because other passes will move the
2.0 into the pow.

I thought I understood your objection, but now I don't.  I think you'll
have to lay out the pair of shaders involving the invariant keyword that
you think that would be broken by this pass.
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