[Mesa-dev] [PATCH 11/16] glsl: Add ir_binop_pow to get_range

[Ian Romanick]

On 11/16/2014 05:51 PM, Thomas Helland wrote:
> The spec states that pow is undefined for x < 0.
> Just set the range to correspond to a constant 0
> if this is the case.

It is technically undefined, but I think all hardware either follows the
SM2.0 rules[1] or the SM5 rules[2].  Since a lot of the applications run
on Mesa are, alas, ports of DX apps, they may depend on one of these
behaviors.

A SM2.0 app will have gone through the HLSL compiler, which I *think*
just lowers pow(x,y) to exp(y, log(abs(x)), so it may not matter too
much there.

I'm not sure what the right answer is.

1:
http://msdn.microsoft.com/en-us/library/windows/desktop/bb147286(v=vs.85).aspx

2:
http://msdn.microsoft.com/en-us/library/windows/desktop/bb509636(v=vs.85).aspx

> ---
>  src/glsl/opt_minmax.cpp | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/src/glsl/opt_minmax.cpp b/src/glsl/opt_minmax.cpp
> index 9852dd9..ad8c88a 100644
> --- a/src/glsl/opt_minmax.cpp
> +++ b/src/glsl/opt_minmax.cpp
> @@ -335,6 +335,17 @@ get_range(ir_rvalue *rval)
>              high = add(r0.high, r1.high)->constant_expression_value();
>           return minmax_range(low, high);
>  
> +      case ir_binop_pow:
> +         r0 = get_range(expr->operands[0]);
> +         if (is_greater_than_or_equal_zero(r0.low))
> +            low = new(mem_ctx) ir_constant(0.0f);
> +         // Result is undefined so we can set the range to bikeshed.
> +         if (is_less_than_zero(r0.high)) {
> +            low = new(mem_ctx) ir_constant(0.0f);
> +            high = new(mem_ctx) ir_constant(0.0f);
> +         }
> +         return minmax_range(low, high);
> +
>        default:
>           break;
>        }
>