[Mesa-dev] Merge of glsl2 branch to master

[Ian Romanick]

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Keith Whitwell wrote:
> On Mon, 2010-08-09 at 23:48 -0700, Eric Anholt wrote:
>> The previous compiler called _mesa_remove_output_reads unconditionally,
>> while the new compiler doesn't.  This should be left up to the backend
>> in my opinion, so perhaps a little flag like Shader.EmitNoIfs and
>> friends for r300g to set for now to call this for the Mesa IR generation
>> would be a good solution.
> 
> It seems like this represents a change in the semantics of the mesa
> driver interface -- previously mesa had the (perhaps implicit) rule that
> OUT registers were write-only.  The glsl2 branch appears not to be
> respecting that.
> 
> I don't really mind this as a follow-on change, but it seems like it
> would be cleaner to keep the glsl2 merge as purely a change on one side
> of the interface, not one which requires a change to drivers or the
> interface itself.

I has always thought that ARB_vp/fp allowed this, but it seems that I
was wrong.  I see two options two resolve this:

1. Always generate duplicate writes to shadow copies of shader outputs.
 Generate all reads from the shadow copies.  Rely on the optimizer to
remove the dead writes.

2. Add a lowering pass that inserts the shadow copies only for the
shader outputs that are actually read.

I'm leaning towards option #2 for a couple reasons.  I suspect that the
optimizer may not eliminate some of the extra writes.  Option #2 also
allows backends that don't need the shadow copies (i.e., i965) to skip
the extra pass.

On the flip side, I think option #1 may be easier to implement.

In either case, I've submitted a bug to track this issue.  Someone
should hack up a validator for Mesa IR and TGSI (like the ir_validate
pass).  I'm sure there other implicit, undocumented invariant
assumptions sprinkled through the code.  Catching deviations from those
sooner rather than later is the winning plan.

https://bugs.freedesktop.org/show_bug.cgi?id=29490
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