WARNING: Calls to any function that may require a gradient calculation inside a conditional block may return undefined results

GLES2.0:

Some device will give a warning on compling shaders(yet the compling will succeed), and the rendering result is incorrect with blink & artifacts.

the problems is gradient calculation(interpolation) relies on the neighbor pixels,

if the neighbor pixels are not in the same conditional branch,  the gradient will be invalid and result may be undifed.

I guess it is an un-defined behavior, so some devices/drivers works good, some don't.

//like this
if( test )
    color = texture2D(sampler,UV);
else
   color = vec4(0,0,0,0);

One workaround is use mix() or step() instead of conditional test. 

color = mix(vec4(0,0,0,0), texture2D(...), testVal);

When using conditional branch, the texture sampling may be not actually executed, which will increase graphics performance.

One draw back on using mix/step is that the sampling is always performed, then interpolated/multiplied.

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udpate:

Actually to many conditonal test & branch blocks(even without texture sampling in it) in fragment shader will case rendering halt, no response, and even system restart.

this result is based on testing on Galaxy S4 (PowerVR SGX544), compared to testing on iPad Air(PowerVR G6430, no such problems).

maybe it's only a driver issue, since both graphics chips are PowerVR family.