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

[Erik Faye-Lund]

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.