[Mesa-dev] [PATCH] GLSL: fix too eager constant variable optimization

[Ian Romanick]

On 03/01/2013 02:02 AM, Aras Pranckevicius wrote:
> Hi,
>
> opt_constant_variable was marking a variable as constant as long as
> there was exactly one constant assignment to it, but did not take into
> account that this assignment might be in a dynamic branch or a loop.
>
> Was happening on a fragment shader like this:
>
> uniform float mode;
> float func (float c) {
>      if (mode == 2.0)
>          return c; // was returning 0.1 after optimization!
>      if (mode == 3.0)
> discard;
>      if (mode == 10.0)
>          c = 0.1;
>      return c;
> }
> void main() {
>      vec4 c = gl_FragCoord;
>      c.x = func(c.x);
>      gl_FragColor = c;
> }
>
>
> Now, looking further this optimization pass should also not mark
> variables as const if there was a dereference of them before that first
> assignment. I had code to do this (a hashtable that would track
> dereferences before assignment is done). But couldn't come up with a
> test case that would break the whole set of optimizations that Mesa does
> (lower jumps, or inlining, ... were getting in the way and hide the bug).

Good catch!  There are a couple white-space issues in the patch, but I 
can fix those when I push it.  I'll wait a bit for other people to give 
review comments.  I'll probably also copy the above text into the commit 
message.

> --
> Aras Pranckevičius
> work: http://unity3d.com
> home: http://aras-p.info
>
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