[Mesa-dev] [PATCH 0/4] glsl: Builtins, now!

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> Hello,
>
> This series reworks the GLSL compiler to inline built-in functions
> immediately, at compile time, rather than importing prototypes and
> linking against an extra shader later on.
>
> Without this, compile time optimizations are nearly useless.  For example,
> trying to use a dot product would generate:
>
>    (call dot ...)
>
> which would not be turned into
>
>    (expression dot ...)
>
> until link time, when we actually import the functions, and finally inline.
>
> Function calls are opaque, and most optimization passes freak out and bail
> when they see them.  For example, loop unrolling wouldn't happen at compile
> time at all if a loop contains a built-in function call.  This is pretty bad.
>
> Most built-in functions turn into opcodes, or tiny expression trees.
> It makes sense to simply inline them immediately.  Some are larger
> (such as atan).  But we eventually inline them all today anyway, so
> I don't see any downside to doing it sooner rather than later.
>
> Ian recently sent out a patch series that adds a lot of flags for
> "do this optimization anyway, but only for built-ins", in an attempt
> to clean up this (call ...) mess.  I think this series should also
> solve those problems, but it deletes a bunch of code, rather than
> adding more code.
>
> The downside is that the shader-db statistics aren't great today,
> but I don't think there's a lot we can do about that.  This changes
> the optimization order significantly.  Overall, the difference was
> pretty small, so I think I'm OK with it.

FWIW, I like this series a lot.
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