[Mesa-dev] [PATCH 2/8] glsl: Fix constant evaluation of the rcp op.

[Ian Romanick]

On 01/24/2017 03:26 PM, Francisco Jerez wrote:
> Will avoid a regression in a future commit that introduces some
> additional rcp operations.

When I converted GLSL IR to ir_expression_operation.py, I was careful to
keep all the expressions the same.  rcp and div had these weird guards.
GLSL doesn't require that NaN be generated, and quite a few old GPUs
don't.  If the atan2 implementation depends on NaN being generated by
rcp, it may have problems on i915, r300, and similar GPUs.  I don't know
what they generate, but it's not NaN and it's probably not 0.0.

That said, this matches NIR, and it's probably fine.

> ---
>  src/compiler/glsl/ir_expression_operation.py | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/compiler/glsl/ir_expression_operation.py b/src/compiler/glsl/ir_expression_operation.py
> index f91ac9b..4ac1ffb 100644
> --- a/src/compiler/glsl/ir_expression_operation.py
> +++ b/src/compiler/glsl/ir_expression_operation.py
> @@ -422,7 +422,7 @@ ir_expression_operation = [
>     operation("neg", 1, source_types=numeric_types, c_expression={'u': "-((int) {src0})", 'default': "-{src0}"}),
>     operation("abs", 1, source_types=signed_numeric_types, c_expression={'i': "{src0} < 0 ? -{src0} : {src0}", 'f': "fabsf({src0})", 'd': "fabs({src0})", 'i64': "{src0} < 0 ? -{src0} : {src0}"}),
>     operation("sign", 1, source_types=signed_numeric_types, c_expression={'i': "({src0} > 0) - ({src0} < 0)", 'f': "float(({src0} > 0.0F) - ({src0} < 0.0F))", 'd': "double(({src0} > 0.0) - ({src0} < 0.0))", 'i64': "({src0} > 0) - ({src0} < 0)"}),
> -   operation("rcp", 1, source_types=real_types, c_expression={'f': "{src0} != 0.0F ? 1.0F / {src0} : 0.0F", 'd': "{src0} != 0.0 ? 1.0 / {src0} : 0.0"}),
> +   operation("rcp", 1, source_types=real_types, c_expression={'f': "1.0F / {src0}", 'd': "1.0 / {src0}"}),
>     operation("rsq", 1, source_types=real_types, c_expression={'f': "1.0F / sqrtf({src0})", 'd': "1.0 / sqrt({src0})"}),
>     operation("sqrt", 1, source_types=real_types, c_expression={'f': "sqrtf({src0})", 'd': "sqrt({src0})"}),
>     operation("exp", 1, source_types=(float_type,), c_expression="expf({src0})"),         # Log base e on gentype
>