Is --source-entrypoint intended to work with GLSL?

[dneto0]

This is prompted by google/shaderc#860

With this GLSL shader:

#version 450
layout(location = 0) out vec4 o;
void a() { o = vec4(1.0); }
void b() { o = vec4(2.0); }

I tried:

  glslangValidator -V a.frag --sep a -e main -o a.spv

and got this error:

     a.frag
     ERROR: Source entry point must be "main"

If I use -H instead of -o a.spv then it does emit sensible output:

                          Capability Shader
           1:             ExtInstImport  "GLSL.std.450"
                          MemoryModel Logical GLSL450
                          EntryPoint Fragment 4  "main" 9
                          ExecutionMode 4 OriginUpperLeft
                          Source GLSL 450
                          Name 4  "main"
                          Name 9  "o"
                          Decorate 9(o) Location 0
           2:             TypeVoid
           3:             TypeFunction 2
           6:             TypeFloat 32
           7:             TypeVector 6(float) 4
           8:             TypePointer Output 7(fvec4)
        9(o):      8(ptr) Variable Output
          10:    6(float) Constant 1065353216
          11:    7(fvec4) ConstantComposite 10 10 10 10
     4(main):           2 Function None 3
           5:             Label
                          Store 9(o) 11
                          Return
                          FunctionEnd

So there's a discrepancy between -H and -o.

But also the person in the Shaderc issue was suggesting --sep would be useful for GLSL compilation.

[johnkslang]

There are multiple subjects. (See #605 as a related issue.)

Legal GLSL

It is illegal GLSL to not have the source code have "void main()" as the entry point. SPIR-V for Vulkan/OpenGL did not change that rule.

Renaming the SPIR-V entry point

You should be able to use the following to rename the GLSL entry point to a different SPIR-V entry point:

glslangValidator -e myEntry --source-entrypoint main ...

Compile-time versus link-time with multiple compilation units

Lacking main as an entry point is a link-time error, not a compile-time error, and your shader uses a desktop version, which allows multiple compilation units (as in, the entry point could still be in another compilation).

Both the -H and -V options implicitly request the link for you, but you need to provide all the compilation units together, as SPIR-V for graphics has only one compilation unit per stage, so multiple GLSL compilation units are turned into a single SPIR-V compilation unit in one go.

-o

It looks like when -o was added, it did not also implicitly link. I can see that going either way, depending on whether you want it to act like other tools making just one of many compilation units, or do what SPIR-V for graphics actually needs today.