Add GL_EXT_function_control_attributes

[Karpov-Ruslan]

This extension allows to use [[inline]] and [[noinline]] attributes on functions to give a hint to the compiler.

One of the side effects of this extension is the ability to fine-tune function inlining when using spirv-opt. Let's say I have a GLSL compute shader:

#version 460

layout(set = 0, binding = 0, std430) buffer SSBO {
    uint data;
};

void func() {
    // Imagine that there is quite a lot of code here.
    atomicAdd(data, 1U);
    atomicAdd(data, 2U);
    atomicAdd(data, 3U);
    atomicAdd(data, 4U);
    atomicAdd(data, 5U);
    atomicAdd(data, 6U);
    atomicAdd(data, 7U);
    atomicAdd(data, 8U);
    atomicAdd(data, 9U);
}

void main() {
    // There are a lot of `func` calls here.
    func();
    func();
    func();
    func();
    func();
    func();
    func();
    func();
    func();
    func();
}

I'll take this shader and compile it to SPIR-V using glslang (https://godbolt.org/z/adfrb5soW), and then I'll want to use spirv-opt with the -Os flag, which should reduce the size of incoming SPIR-V. But in this case the size will be increased (https://godbolt.org/z/fxv5rjGo9). However, by adding the [[noinline]] attribute to the func function, the DontInline Function Control bit will be added to this function in SPIR-V, so spirv-opt will not inline this function and the size of the outgoing SPIR-V will indeed be reduced (https://godbolt.org/z/d6daKEK5n).

[spencer-lunarg]

tl;dr - agree that GLSL should just have a simple way to just expose this and 💯 support this idea

@Karpov-Ruslan I realize that the goal is to stop spirv-opt, but what is the goal after that

Basically I have tried doing this for GPU-AV inside VVL and found that NO ONE actually will honor your DontInline function control mask... the worst part is there is no way to query for support of it neither

NVIDIA 553.31+ does support it, but we have found periodic issues with it, for example, we have to always turn it off for Ray Tracing shaders

... so more of a "heads up"... if you only care about SPIR-V binary size, that is a VERY valid reason as well!

[Karpov-Ruslan]

Basically I have tried doing this for GPU-AV inside VVL and found that NO ONE actually will honor your DontInline function control mask... the worst part is there is no way to query for support of it neither

To be honest, I didn’t even consider it. Sad to hear that. However, SPIR-V Function Control is about hints to the compiler. If the driver developers don't take this into account, it's their full right. Perhaps in the future more and more drivers will support this, so we need to be prepared.