GLSL discard behavior

[Valentin-Sarthou]

Hello,

Since 9323100 (merged in 15.4.0), GLSL's discard now maps to OpDemoteToHelperInvocation instead of OpTerminateInvocation when targeting SPIR-V >= 1.6.

This change has already been discussed in #3954, but I would like to add a few more elements.

From the GLSL 4.60 specification, section 6.4. Jumps, about the discard keyword:

Control flow exits the shader, and subsequent implicit or explicit derivatives are undefined when this control flow is non-uniform

This sentence seems to describe the behavior of OpTerminateInvocation, as opposed to OpDemoteToHelperInvocation.

Also, from the specification of VK_KHR_shader_terminate_invocation:

OpTerminateInvocation provides the behavior required by the GLSL discard statement, and should be used when available by GLSL compilers and applications that need the GLSL discard behavior.

So from what I understand, mapping discard to OpTerminateInvocation was indeed the expected behavior, at least according to the specification.

I understand that the ambiguity of discard may cause bugs in applications, but isn't that why GLSL_EXT_demote_to_helper_invocation and GL_EXT_terminate_invocation were introduced ? The latter states:

It is recommended that programmers use demote [...] or terminateInvocation instead of discard.

Is it reasonable for glslang to deviate from the GLSL, VK and EXTs specifications on this point? Am I missing something?