[Mesa-dev] Precision qualifiers in the IR

[Iago Toral]

On Thu, 2015-01-08 at 13:54 +0200, Aras Pranckevicius wrote:
>         I see precision qualifiers being parsed and stored in the AST,
>         but I
>         don't see where this information is passed to the IR:
>         ir_variable or
>         glsl_type don't have this info, in fact,
>         apply_type_qualifier_to_variable() in ast_to_hir.cpp seems to
>         ignore the
>         ast precision qualifier data completely.
> 
> 
> Correct, precision is blissfully ignored in Mesa IR.
> 
> 
>  
>         So I am guessing that we should add precision information to
>         the
>         glsl_type, but I wonder if there is a reason why this hasn't
>         been done
>         yet
> 
> 
> I did add precision support for glsl-optimizer
> (https://github.com/aras-p/glsl-optimizer) which is a fork of Mesa.
> 
> 
> I forgot why I did not make precision be part of the type to be
> honest, but I think there was some reason. Maybe because a ton of
> places in Mesa do things like "get me a float type", and changing all
> these places to deal with 4x more types (unspecified, low, medium,
> high) did not sound attactive.
> 
> 
> So what I did is ir make ir_rvalues and ir_variables have precision
> (and a few other oddball things, like return type of
> ir_function_signature). And then in places that construct rvalues or
> variables, the precision is determined. In some cases that comes
> directly from AST, in other cases (e.g. results of optimization
> passes) it comes from "higher precision of things being operated
> upon" (e.g. result of a+b is higher precision of the arguments), etc.
> 
> 
> And then I tweaked some optimization passes to stop applying
> optimizations across different precision. For example, tree grafting
> should not paste subtrees into other places, if precision is different
> there.
> 
> 
> Not sure if my approach (precision as not part of type, but rvalues
> +variables) is a good one, but seems to work so far in glsl-optimizer.

Hi Aras, thanks for the input.

For now I am not really trying to fully incorporate precision qualifiers
into the IR, what I am trying to do is to get enough info in the IR so I
can do compile and link time checks for variable definitions, so I am
not going down to the level of ir_rvalue for example. I think that part
can be added later, once we decide to put precision qualifiers into
actual use in Mesa.

BTW, in the end I did not add the precision info to glsl_type, I am also
adding this in ir_variable. That approach complicated things more than
necessary and there are also various mentions in the spec that state
that precision qualifiers do change a variable's type, so that approach
seemed less consistent with the spirit of the spec too.

Iago