Fix support for SV_Coverage on GLSL path (#1239)

There were two overlapping issues here:

1. We always mapped `SV_Coverage` to `gl_SampleMask`, even though `gl_SampleMaskIn` is the correct built-in variable to use for an input varying.

2. We treated `gl_SampleMask` like it was a scalar shader input, when it and `gl_SampleMaskIn` are actually arrays of indeterminate size (as a byproduct of trying to future-proof for implementations that might support hundreds or thousands of samples per pixel...)

The fix here is simple: map to either `gl_SampleMask[0]` or `gl_SampleMaskIn[0]` as appropriate. I suppose that this approach doesn't handle the possibility of eventually supporting >32 samples per pixel by having something like `uint2 coverage : SV_Coverage`, but I think we can cross that bridge when we come to it.

1 files changed, 13 insertions, 5 deletions

diff --git a/source/slang/slang-ir-glsl-legalize.cpp b/source/slang/slang-ir-glsl-legalize.cpp
index 2c889f341..170027405 100644
--- a/source/slang/slang-ir-glsl-legalize.cpp
+++ b/source/slang/slang-ir-glsl-legalize.cpp
@@ -300,16 +300,24 @@ GLSLSystemValueInfo* getGLSLSystemValueInfo(
     }
     else if(semanticName == "sv_coverage")
     {
-        // TODO: deal with `gl_SampleMaskIn` when used as an input.
-
-        // TODO: type conversion is required here.
-
         // uint in hlsl, int in glsl
         // https://www.opengl.org/sdk/docs/manglsl/docbook4/xhtml/gl_SampleMask.xml
 
         requiredType = builder->getBasicType(BaseType::Int);
 
-        name = "gl_SampleMask";
+        // Note: `gl_SampleMask` is actually an *array* of `int`,
+        // rather than a single scalar. Because HLSL `SV_Coverage`
+        // on allows for a 32 bits worth of coverage, we will
+        // only use the first array element in the generated GLSL.
+
+        if( kind == LayoutResourceKind::VaryingInput )
+        {
+            name = "gl_SampleMaskIn[0]";
+        }
+        else
+        {
+            name = "gl_SampleMask[0]";
+        }
     }
     else if(semanticName == "sv_depth")
     {