Accept mismatching auxiliary interpolations.

The ES3.1 spec, and discussion on Khronos.org, confirm that dEQP is
correct in accepting mismatching centroid specifiers in shader
linkage. Mismatching flat/smooth is still a link error.

Fixes:
shaders.linkage.varying.rules.differing_interpolation_2

Change-Id: I3016f4147e7c1b16b02371ee95866c8daf826212
Reviewed-on: https://chromium-review.googlesource.com/251205
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>

include/GLSLANG/ShaderVars.h

@@ -28,6 +28,9 @@ enum InterpolationType
     INTERPOLATION_FLAT
 };
 
+// Validate link & SSO consistency of interpolation qualifiers
+COMPILER_EXPORT bool InterpolationTypesMatch(InterpolationType a, InterpolationType b);
+
 // Uniform block layout qualifier, see section 4.3.8.3 of the ESSL 3.00.4 spec
 enum BlockLayoutType
 {

src/compiler/translator/ShaderVars.cpp

@@ -14,6 +14,23 @@
 namespace sh
 {
 
+namespace
+{
+
+InterpolationType GetNonAuxiliaryInterpolationType(InterpolationType interpolation)
+{
+    return (interpolation == INTERPOLATION_CENTROID ? INTERPOLATION_SMOOTH : interpolation);
+}
+
+}
+// The ES 3.0 spec is not clear on this point, but the ES 3.1 spec, and discussion
+// on Khronos.org, clarifies that a smooth/flat mismatch produces a link error,
+// but auxiliary qualifier mismatch (centroid) does not.
+bool InterpolationTypesMatch(InterpolationType a, InterpolationType b)
+{
+    return (GetNonAuxiliaryInterpolationType(a) == GetNonAuxiliaryInterpolationType(b));
+}
+
 ShaderVariable::ShaderVariable()
     : type(0),
       precision(0),

src/libANGLE/Program.cpp

@@ -1543,7 +1543,7 @@ bool Program::linkValidateVaryings(InfoLog &infoLog, const std::string &varyingN
         return false;
     }
 
-    if (vertexVarying.interpolation != fragmentVarying.interpolation)
+    if (!sh::InterpolationTypesMatch(vertexVarying.interpolation, fragmentVarying.interpolation))
     {
         infoLog.append("Interpolation types for %s differ between vertex and fragment shaders", varyingName.c_str());
         return false;