[Mesa-dev] [PATCH] glsl/constant propagation: kill whole var if LHS involves array indexing.

[Eric Anholt]

On Sat, 19 May 2012 10:08:20 -0700, Paul Berry <stereotype441 at gmail.com> wrote:
> When considering which components of a variable were killed by an
> assignment, constant propagation would previously just use the write
> mask of the assignment.  This worked if the LHS of the assignment was
> simple, e.g.:
> 
> v.xy = ...; // (assign (xy) (var_ref v) ...)
> 
> But it did the wrong thing if the LHS of the assignment involved an
> array indexing operator, since in this case the write mask is always
> (x):
> 
> v[i] = ...; // (assign (x) (deref_array (var_ref v) (var_ref i)) ...)
> 
> In general, we can't predict which vector component will be selected
> by array indexing, so the only safe thing to do in this case is to
> kill the entire variable.
> 
> Fixes piglit tests {fs,vs}-vector-indexing-kills-all-channels.shader_test.

Reviewed-by: Eric Anholt <eric at anholt.net>
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