[Mesa-dev] [PATCH 3/3] glsl: Optimize division by two with a right shift.

[Ian Romanick]

On 02/24/2014 03:36 PM, Roland Scheidegger wrote:
> Am 25.02.2014 00:00, schrieb Matt Turner:
>> ---
>>   src/glsl/opt_algebraic.cpp | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
>> index 778638c..50342ac 100644
>> --- a/src/glsl/opt_algebraic.cpp
>> +++ b/src/glsl/opt_algebraic.cpp
>> @@ -421,6 +421,9 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
>>         }
>>         if (is_vec_one(op_const[1]))
>>   	 return ir->operands[0];
>> +      if (is_vec_two(op_const[1]) && ir->type->is_integer()) {
>> +         return rshift(ir->operands[0], new(mem_ctx) ir_constant(1));
>> +      }
>>         break;
>>
>>      case ir_binop_dot:
>>
>
> Does glsl require any particular rounding behavior for negative numbers
> (like c99 does, which wouldn't work with shift emulation)? A quick look
> didn't find anything but it might be implied somewhere...

What is a case by C99 rules where x / 2 is not the same as x >> 2?

> Also, if you do this for two, wouldn't it make sense to extend that to
> power-of-two numbers or is that too much cpu work?

I think it was more a case that shader-db didn't have any occurances, 
and he didn't feel like doing the human work. :)

> Roland
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