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GLSL hardware PCF code generation issue

Question asked by lukef on Nov 5, 2015

I'm experiencing a performance concern when implementing shadow PCF in GLSL - it seems that for every call to texture(), the compiler emits a branch between IMAGE_SAMPLE and IMAGE_SAMPLE_C, presumably depending on the sampler params.


This would be fair enough if there was only 1 call to texture(), but I'm implementing a 4x4 box filter (excluding the hardware 2x2), so this results in 32 branches(!)


You can see what I mean below, this pattern of 2 branches and the various boilerplate to query the sampler state repeats 16 times. Definitely not good for performance.




For GLSL to reproduce the issue, see this example:



So my question is: is there any hint I can give the compiler that the shadow map sampler will always be compatible with IMAGE_SAMPLE_C?