[Mesa-dev] [PATCH] mesa/glsl: add ir_unop_round

[Kenneth Graunke]

On 07/03/2012 09:00 AM, Brian Paul wrote:
> The GLSL round() and roundEven() functions were both generating the
> ir_unop_round_even instruction but the GLSL spec allows some leeway for
> implementing round().  This change allows drivers to take advantage of
> that.  For i965, they still equate to the same thing.
> 
> v2: implement ir_unop_round for constant expressions.
[snip]
> +   case ir_unop_round:
> +      assert(op[0]->type->base_type == GLSL_TYPE_FLOAT);
> +      for (unsigned c = 0; c < op[0]->type->components(); c++) {
> +	 data.f[c] = roundf(op[0]->value.f[c]);
> +      }
> +      break;

This really has the same problem: ir_unop_round now has two different
meanings.  On the CPU, you've chosen to make it round away from zero.
On i965, it rounds toward the nearest even number.  On other drivers, it
does...something...

The real constraint is that it needs to be consistent on both the CPU
(constants) and GPU (non-constants).  The only way I see to solve this
is to have drivers set a flag indicating how they'd like round to
behave, and to check that in the constant expression evaluation code.

Or, just pick one meaning and make everybody do that.