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

[Erik Faye-Lund]

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?

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.