Swizzled l-values as output arguments not implemented.

[paulthomson]

bug_report.zip

Issue found using GraphicsFuzz.

Tool versions:

• glslangValidator commit hash: e383c5f

To reproduce:

glslangValidator -V shader.frag -o shader.frag.spv

The following shader files are included in the attached archive, some of which are also shown inline below:

• 0_glsl/shader.frag

0_glsl/shader.frag:

#version 310 es
precision highp float;

mediump vec4 GLF_dead0gl_FragCoord;

void main()
{
 modf(vec4(1.0), GLF_dead0gl_FragCoord.gabr);
}

[cnorthrop]

We just hit this issue using ANGLE with the Vulkan backend when running OpenGL ES 3.1 conformance tests.

ANGLE bug: https://bugs.chromium.org/p/angleproject/issues/detail?id=4402

deqp tests:

  KHR-GLES31.core.shader_bitfield_operation.frexp.*
  KHR-GLES31.core.shader_bitfield_operation.uaddCarry.*
  KHR-GLES31.core.shader_bitfield_operation.usubBorrow.*
  KHR-GLES31.core.shader_bitfield_operation.umulExtended.*
  KHR-GLES31.core.shader_bitfield_operation.imulExtended.*

Different built-ins, but same assert.

Here's an example shader:

#version 450 core

layout (local_size_x=1, local_size_y=1, local_size_z=1) in;

layout(set=0, binding=0, std430, column_major) buffer _uData
{
    uvec4 _uinUvec4;
    ivec4 _uinIvec4;
    vec4 _uinVec4;
    uvec4 _uin2Uvec4;
    ivec4 _uin2Ivec4;
    vec4 _uin2Vec4;
    int _uoffset;
    int _ubits;
    uvec4 _uoutUvec4;
    ivec4 _uoutIvec4;
    vec4 _uoutVec4;
    uvec4 _uout2Uvec4;
    ivec4 _uout2Ivec4;
    vec4 _uout2Vec4;
};

layout(set=0, binding=1) uniform ANGLEUniformBlock{
    uvec4 acbBufferOffsets;
} ANGLEUniforms;

void main()
{
    (_uoutVec4.xy = frexp(_uinVec4.xy, _uoutIvec4.xy));
}

From the comments, it seems this is missing functionality:

    // If swizzle exists, it is out-of-order or not full, we must load the target vector,
    // extract and insert elements to perform writeMask and/or swizzle.  This does not
    // go with getting a direct l-value pointer.
    assert(accessChain.swizzle.size() == 0);
    assert(accessChain.component == NoResult);

Similar to bug #773 as well.

Let us know if you need any more example shaders.

[johnkslang]

What was missing was the case of built-in function-output parameters to a swizzled argument l-value. SPIR-V cannot represent these and so needs additional instructions.