spirv-opt: changes variable precision from relaxed/mediump to highp in some cases

[pixelflinger]

I found a scenario where a mediump variable is turned in to a highp after running spirv-opt.

#version 310 es
precision mediump float;
precision mediump int;
uniform mediump sampler2D tex;

layout(location = 0) in highp vec4 vertex;
layout(location = 0) uniform vec4 value;
layout(location = 0) out vec4 fragColor;

void main() {
        vec4 x = textureLod(tex, vertex.xy, 0.0); // specifying mediump here doesn't fix the problem
	if (max(x.r, x.g) < 2.0) {
		fragColor = vec4(0.0);
		return;
	}
	fragColor = vec4(1.0);
}

will produce:

#version 310 es
precision mediump float;
precision highp int;

layout(binding = 0) uniform mediump sampler2D tex;

layout(location = 0) in highp vec4 vertex;
layout(location = 0) out vec4 fragColor;

void main()
{
    do
    {
        highp vec4 _21 = textureLod(tex, vertex.xy, 0.0); // Note the highp here!!!
        if (mod(_21.x, _21.y) < 2.0)
        {
            fragColor = vec4(0.0);
            break;
        }
        fragColor = vec4(1.0);
        break;
    } while(false);
}

It looks like the conditions for it to happen are that the variable must be from reading a texture (e.g. replacing with a mediump uniform doesn't cause the problem), and that variable is fed to a two-parameter build-in (e.g. this doesn't happen with abs()).

I checked that the highp is not there right after calling glslang, but appears after running spirv-opt:

  glslangValidator --target-env opengl  test.frag -o test.spv
  spirv-opt -O test.spv -o test.opt.spv
  spirv-cross --es test.opt.spv 

[alan-baker]

glslang isn't currently adding RelaxedPrecision to the result of textureLod and spirv-opt just eliminates x by promoting it to registers. That's why spirv-cross is making _21 highp.

[alan-baker]

Filed KhronosGroup/glslang#2298.