ARB_gl_spirv: mapping of atomics

[jozefkucia]

The ARB_gl_spirv spec contains the following mapping:

 atomicCounterIncrement -> OpAtomicIIncrement
 atomicCounterDecrement -> OpAtomicIDecrement
 atomicCounter          -> OpAtomicLoad

The mapping seems to be inaccurate for atomicCounterDecrement. atomicCounterDecrement returns the value resulting from the decrement operation while the OpAtomicIDecrement returns the original value.

[pdaniell-nv]

@johnkslang to do some research on this then assign @dgkoch to make the extension spec update.

[johnkslang]

From glslang generator number:

//return 2; // EOpAtomicCounterDecrement gets a post decrement, to map between GLSL -> SPIR-V

[dgkoch]

@jozefkucia in general the "Mapping to SPIR-V" section isn't considered normative, but included to aid in understanding. As @johnkslang noted, we've already accounted for these differences between GLSL/HLSL and SPIRV in the glslang producer. I've also proposed a note in the extension spec to call this out: https://github.com/KhronosGroup/OpenGL-Registry/pull/185

[pdaniell-nv]

This should be resolved now by https://github.com/KhronosGroup/OpenGL-Registry/pull/185. Closing.