opengl - Shader execution after writing to gl_FragDepth -

given fragment shader modifies original depth of fragment , writes gl_fragdepth. code after writing gl_fragdepth still executed if depth test fails @ point?

example:

in float depth; layout(depth_less) out float gl_fragdepth; void main() {   float newdepth = depth - somemodification;   gl_fragdepth = newdepth;   a(); } 

will a executed if newdepth greater current value in gl_fragdepth?

if so, alternative stop shader doing unneccessary computations in a - without using custom depth buffer?

in example, a() executed long contributes output value, e.g. color (otherwise, compiler remove optimization). depth test per-sample operation performed after fragment shader. under special circumstances, possible test before fragment shader, requires fragment shader not write gl_fragdepth.

is modification uniform 1 or different each fragment? if uniform, in geometry shader - apply depth modification whole primitive. use early-z. if it's on per-fragment basis, try binding current depth render target readonly image, fetch stored depth value, perform manual comparison in shader , discard fragment if fails. however, neither know whether can bind bound framebuffer's render targets images, readonly, nor whether more or less performant executing a().