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

[Aras Pranckevicius]

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).


-- 
Aras Pranckevičius
work: http://unity3d.com
home: http://aras-p.info
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/mesa-dev/attachments/20130301/1a1347a9/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: glsl-constant-variable-fix.diff
Type: application/octet-stream
Size: 2927 bytes
Desc: not available
URL: <http://lists.freedesktop.org/archives/mesa-dev/attachments/20130301/1a1347a9/attachment.obj>