Implicit derivatives, uniform control flow, and SPIR-V.

The SPIR-V specification has a number of definitions that deal with implicit derivatives. Let's start with 2.19: Derivatives:

In all cases, derivatives are well defined only if the derivative group has uniform control flow.

The terms "derivative group" and "uniform control flow" are defined as:

Derivative Group: Defined only for the Fragment Execution Model: The set of invocations collectively processing a single point, line, or triangle, including any helper invocations.

Uniform Control Flow: Uniform control flow (or converged control flow) occurs when all invocations in the invocation group or derivative group execute the same control-flow path (and hence the same sequence of dynamic instances of instructions).

Put simply, implicit derivatives are well-defined under SPIR-V's rules if you have uniform control flow among all fragment shader invocations that process a primitive.

GLSL 4.60 defines things a bit differently. Its definition of Uniform Control Flow is:

Uniform control flow (or converged control flow) occurs when all invocations in the invocation group execute the same control-flow path (and hence the same sequence of dynamic instances of instructions).

Notice the lack of a statement about "derivative groups". That's important. In OpenGL, like in Vulkan, an "invocation group" is defined as:

For graphics shaders, an invocation group is an implementation-dependent subset¹ of the set of shader invocations of a given shader stage which are produced by a single drawing command.

Incidentally, footnote ¹ effectively says that you have to assume the worst-case: that all invocations of a stage from a single draw command are in the same invocation group. Which means that an invocation group must be assumed to be larger than a single primitive.

That's unfortunate, because GLSL says in section 8.9:

Implicit derivatives are undefined within non-uniform control flow and for non-fragment shader texture fetches.

Because of the lack of a concept of "derivative groups", this can only refer to uniform control flow within an invocation group. Which is rather larger than a derivative group.

However, GLSL's section 6.4 says:

Control flow exits the shader, and subsequent implicit or explicit derivatives are undefined when this control flow is non-uniform (meaning different fragments within the primitive take different control paths).

Emphasis added. So it seems clear that the original intent is that uniformity only needs to be within a primitive, not within a full invocation group.

This all needs to be clarified. And as with prior cases, I suspect the best thing to do is just copy SPIR-V's wording. After all, since we can load SPIR-V shaders now, those shaders still have to be able to act under SPIR-V's rules. So hardware has to be able to have the concept of derivative groups and uniformity within them as distinct from invocation groups.