Metal: Prevent vertex shader's inactive varying from being removed

src/compiler/translator/TranslatorMetal.cpp

@@ -18,7 +18,12 @@
 #include "common/utilities.h"
 #include "compiler/translator/OutputVulkanGLSLForMetal.h"
 #include "compiler/translator/StaticType.h"
+#include "compiler/translator/tree_ops/InitializeVariables.h"
 #include "compiler/translator/tree_util/BuiltIn.h"
+#include "compiler/translator/tree_util/FindMain.h"
+#include "compiler/translator/tree_util/FindSymbolNode.h"
+#include "compiler/translator/tree_util/IntermNode_util.h"
+#include "compiler/translator/tree_util/ReplaceVariable.h"
 #include "compiler/translator/tree_util/RunAtTheEndOfShader.h"
 #include "compiler/translator/util.h"
 
@@ -79,6 +84,41 @@ ANGLE_NO_DISCARD bool AppendVertexShaderPositionYCorrectionToMain(TCompiler *com
     return RunAtTheEndOfShader(compiler, root, assignment, symbolTable);
 }
 
+// Initialize unused varying outputs.
+ANGLE_NO_DISCARD bool InitializeUnusedOutputs(TIntermBlock *root,
+                                              TSymbolTable *symbolTable,
+                                              const InitVariableList &unusedVars)
+{
+    if (unusedVars.empty())
+    {
+        return true;
+    }
+
+    TIntermSequence *insertSequence = new TIntermSequence;
+
+    for (const sh::ShaderVariable &var : unusedVars)
+    {
+        ASSERT(!var.active);
+        const TIntermSymbol *symbol = FindSymbolNode(root, var.name);
+        ASSERT(symbol);
+
+        TIntermSequence *initCode = CreateInitCode(symbol, false, false, symbolTable);
+
+        insertSequence->insert(insertSequence->end(), initCode->begin(), initCode->end());
+    }
+
+    if (insertSequence)
+    {
+        TIntermFunctionDefinition *main = FindMain(root);
+        TIntermSequence *mainSequence   = main->getBody()->getSequence();
+
+        // Insert init code at the start of main()
+        mainSequence->insert(mainSequence->begin(), insertSequence->begin(), insertSequence->end());
+    }
+
+    return true;
+}
+
 }  // anonymous namespace
 
 TranslatorMetal::TranslatorMetal(sh::GLenum type, ShShaderSpec spec) : TranslatorVulkan(type, spec)
@@ -119,6 +159,26 @@ bool TranslatorMetal::translate(TIntermBlock *root,
         }
     }
 
+    // Initialize unused varying outputs to avoid spirv-cross dead-code removing them in later
+    // stage. Only do this if SH_INIT_OUTPUT_VARIABLES is not specified.
+    if ((getShaderType() == GL_VERTEX_SHADER || getShaderType() == GL_GEOMETRY_SHADER_EXT) &&
+        !(compileOptions & SH_INIT_OUTPUT_VARIABLES))
+    {
+        InitVariableList list;
+        for (const sh::ShaderVariable &var : mOutputVaryings)
+        {
+            if (!var.active)
+            {
+                list.push_back(var);
+            }
+        }
+
+        if (!InitializeUnusedOutputs(root, &getSymbolTable(), list))
+        {
+            return false;
+        }
+    }
+
     // Write translated shader.
     root->traverse(&outputGLSL);
 

src/tests/gl_tests/GLSLTest.cpp

@@ -4624,6 +4624,34 @@ TEST_P(GLSLTest, StructsWithSameMembersDisambiguatedByName)
     EXPECT_PIXEL_COLOR_EQ(0, 0, GLColor::green);
 }
 
+// Test that an inactive varying in vertex shader but used in fragment shader can be linked
+// successfully.
+TEST_P(GLSLTest, InactiveVaryingInVertexActiveInFragment)
+{
+    // http://anglebug.com/4820
+    ANGLE_SKIP_TEST_IF(IsOSX() && IsOpenGL());
+
+    constexpr char kVS[] =
+        "attribute vec4 inputAttribute;\n"
+        "varying vec4 varColor;\n"
+        "void main()\n"
+        "{\n"
+        "    gl_Position = inputAttribute;\n"
+        "}\n";
+
+    constexpr char kFS[] =
+        "precision mediump float;\n"
+        "varying vec4 varColor;\n"
+        "void main()\n"
+        "{\n"
+        "    gl_FragColor = varColor;\n"
+        "}\n";
+
+    ANGLE_GL_PROGRAM(program, kVS, kFS);
+    drawQuad(program.get(), "inputAttribute", 0.5f);
+    ASSERT_GL_NO_ERROR();
+}
+
 // Test that a varying struct that's not statically used in the fragment shader works.
 // GLSL ES 3.00.6 section 4.3.10.
 TEST_P(GLSLTest_ES3, VaryingStructNotStaticallyUsedInFragmentShader)