SPV_KHR_maximal_reconvergence

If I is a dynamic instance of an instruction, let the tangle that executes I be T(I).

Let F be the first dynamically executed instruction in an entry point. T(F) will be converged.

Note: This is a restatement of the initial conditions in Uniform Control Flow.

A divergence occurs when executing I if the invocations in T(I) do not all have the same dynamic successor.

The tangles that execute the dynamic successors of a dynamic instruction instance I form a partition of those invocations in T(I) that have a dynamic successor. The tangles of the dynamic successors may include invocations not in T(I) if that dynamic successor reconverges.

The only instructions that can produce a divergence are:

Note: This means that invocations cannot spontaneously diverge, although demoting an invocation to a helper invocation may look like spontaneous divergence.

Note: All invocations in a tangle that are not terminated during the execution of an OpFunctionCall will remain tangled in the next dynamic instance executed in calling function. That is, function call return acts as reconvergence point.

Note: When the last invocation in a quad is demoted to a helper invocation, the whole quad may be terminated. Since the invocations in the quad may be diverged, the termination of a quad may give the appearance of spontaneous divergence of some tangles. The invocations that were already helper invocations might be in vastly different points in the program execution.

Invocations that diverged from each other are said to reconverge when they rejoin a common tangle. Reconvergence occurs at certain related dynamic instances. A dynamic instruction instance, L, is related to another dynamic instruction instance, I, if I executed before L and the invocations in T(I) are candiates for inclusion in T(L). The subset of T(I) required to reconverge depends on the instructions executed as detailed below.

With L related to I as above, an invocation escapes reconvergence when that invocation is in T(I), but not in T(L). This only occurs when:

Note: The common cases an invocation would escape reconvergence are breaking from a switch or loop, or continuing in a loop.

Note: OpKill will behave the same as either OpTerminateInvocation or OpDemoteToHelperInvocation depending on the implementation. It is recommended that shader authors use OpTerminateInvocation or OpDemoteToHelperInvocation instead of OpKill whenever possible to produce more predictable behavior.

The only related instances introduced during execution are the following:

Invocations will not reconverge except at Merge Blocks, Continue Targets, and case constructs or after OpFunctionCall is executed.

A tangle that executes an instance of a merge instruction, M, represents the maximal tangle for all of the invocations in T(M). That is, implementations will not merge tangles during execution except through reconvergence.

Note: This means that the instructions in a break block will execute as if they were still diverged according to the loop iteration. This restricts potential transformations an implementation may perform on the IR to match shader author expectations. Similarly, instructions in the loop construct cannot be moved into the continue construct unless it can be proven that invocations are always converged.