[spirv] opphi generation about glslang HOT 4 CLOSED

Generally eliminating a bunch of load/stores ("mem -> reg") is future work. It does make an OpPhi for a do-while loop to handle continue, though that very special case might go away with an upcoming control-flow change to SPIR-V.

from glslang.

from glslang.

No, not significantly. Still the same CFG with the same concept of annotating nested structure on top of it. I shouldn't have made a future-looking statement.

The minor point is the current OpPhi generated is pretty special case and might change, and the major point is how/when/should glslang generally generate "in register" form of data flow, which would involve OpPhi in general when control flow is present. I don't have answers right now.

from glslang.

Update: The recent change (7c1aa10) to handle && and || short circuiting was done using OpPhi, so it is now pretty easy to get OpPhi to show up (use a non-trivial right operand to && or ||).

Example pseudo code:

r1 <- left operand
if (r1)
    r2 <- right operand
r3 = phi(r1, r2)

from glslang.