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Aras Pranckevicius wrote:
> Hi,
>
> Currently GLSL2 optimizer can't remove assignments to struct members
> that are never used. After inlining, the struct is often not passed to
> any other functions as a whole; and some assignments to the members
> might be useless. For example, in this fragment shader assignment to
> s.unused could be optimized away. I guess then whole structure (of which
> only s.used is left) could be replaced with just a float temporary:
>
> struct foo {
> float used;
> float unused;
> };
> void main() {
> float f = 1.0;
> foo s;
> s.used = f;
> s.unused = f;
> gl_FragColor = vec4(s.used);
> }
>
> Right now GLSL2 optimizer optimizes the above into this (GLSL output):
>
> struct foo {
> float used;
> float unused;
> };
> void main ()
> {
> foo s;
> s .used = 1.000000;
> s .unused = 1.000000;
> gl_FragColor = s .used .xxxx;
> }
>
>
> From the code, it seems that ir_variable_refcount only tracks references
> to "whole variables" (in this case, whole struct), and not to individual
> members. Any quick ideas / pitfalls how that can be extended, before I
> try to figure it out myself?
I believe the plan is to eventually break structures and arrays that are
not variably indexed into individual variables. Your structure above
would be broken into s_used and s_unused. The existing dead code paths
would take care of s_unused. We'll need to do this break-up anyway to
do proper register assignment.
We'll have similar issue with code like:
uniform vec4 angles;
void main()
{
vec4 v;
v.x = sin(angles.x);
v.y = cos(angles.y);
v.z = tan(angles.z);
v.w = 1/tan(angles.w);
gl_Position = v.xyxy;
}
In this case v.zw is never used, and the (expensive) assignments should
be killed.
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