OpSelect does not follow SPIR-V spec

[amdrexu]

We have encountered an issue of OpSelect, here is the shader:

#version 450

layout(location = 0) in  vec2 uv;
layout(location = 0) out vec4 o;

bool verify()
{
    return uv == vec2(0.0);
}

void main()
{
    o = verify() ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
}

This is the SPIR-V

// Module Version 10000
// Generated by (magic number): 80001
// Id's are bound by 30

                              Capability Shader
               1:             ExtInstImport  "GLSL.std.450"
                              MemoryModel Logical GLSL450
                              EntryPoint Fragment 4  "main" 13 24
                              ExecutionMode 4 OriginUpperLeft
                              Source GLSL 450
                              Name 4  "main"
                              Name 8  "verify("
                              Name 13  "uv"
                              Name 24  "o"
                              Decorate 13(uv) Location 0
                              Decorate 24(o) Location 0
               2:             TypeVoid
               3:             TypeFunction 2
               6:             TypeBool
               7:             TypeFunction 6(bool)
              10:             TypeFloat 32
              11:             TypeVector 10(float) 2
              12:             TypePointer Input 11(fvec2)
          13(uv):     12(ptr) Variable Input
              15:   10(float) Constant 0
              16:   11(fvec2) ConstantComposite 15 15
              17:             TypeVector 6(bool) 2
              22:             TypeVector 10(float) 4
              23:             TypePointer Output 22(fvec4)
           24(o):     23(ptr) Variable Output
              26:   10(float) Constant 1065353216
              27:   22(fvec4) ConstantComposite 15 26 15 26
              28:   22(fvec4) ConstantComposite 26 15 15 26
         4(main):           2 Function None 3
               5:             Label
              25:     6(bool) FunctionCall 8(verify()
              29:   22(fvec4) Select 25 27 28
                              Store 24(o) 29
                              Return
                              FunctionEnd
      8(verify():     6(bool) Function None 7
               9:             Label
              14:   11(fvec2) Load 13(uv)
              18:   17(bvec2) FOrdEqual 14 16
              19:     6(bool) All 18
                              ReturnValue 19
                              FunctionEnd

Please pay attention to these instructions:

25:     6(bool) FunctionCall 8(verify()
29:   22(fvec4) Select 25 27 28

25 is a boolean while 29, 27, 28 are all floating-point vectors. In SPIR-V spec, it says:
Condition must be a scalar or vector of Boolean type. It must have the same number of components as Result Type.

So apparently, it violates the spec. Do I miss something? In LLVM spec, the condition could be a scalar (perform vector selection) or a vector (perform component-wise selection). I want to confirm whether it is a spec bug or a glslang bug.

Thank you.

[johnkslang]

It would be valuable to have a selection instruction that took a bool and selected between two composite objects of the same type. SPIR-V is quite clear that OpSelect does not do that (as you quoted), but I think that's what the implementation expected.

So, for now the implementation needs to be fixed in glslang, but I'd also like a spec issue to request the new behavior; it has come up for selecting between two structures as well, without having to use control flow.

[johnkslang]

Clarifying: my "implementation", I meant glslang.

So, I'll fix glslang, and also request that we have an instruction added to SPIR-V.

[amdrexu]

Thank you. It is clear now.