GLSL: fix Position built-in variable.

Position maps to gl_Position in vertex shaders, and gl_FragCoord in
fragment shaders.

Bug: tint:1296
Change-Id: I22069af2c82bb03521f2721d16c29ba98c85f3f2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/69280
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/writer/glsl/generator_impl.cc

@@ -1758,10 +1758,20 @@ sem::Type* GeneratorImpl::builtin_type(ast::Builtin builtin) {
   }
 }
 
-const char* GeneratorImpl::builtin_to_string(ast::Builtin builtin) const {
+const char* GeneratorImpl::builtin_to_string(ast::Builtin builtin,
+                                             ast::PipelineStage stage) {
   switch (builtin) {
     case ast::Builtin::kPosition:
-      return "gl_Position";
+      switch (stage) {
+        case ast::PipelineStage::kVertex:
+          return "gl_Position";
+        case ast::PipelineStage::kFragment:
+          return "gl_FragCoord";
+        default:
+          TINT_ICE(Writer, builder_.Diagnostics())
+              << "position builtin unexpected in this pipeline stage";
+          return "";
+      }
     case ast::Builtin::kVertexIndex:
       return "gl_VertexID";
     case ast::Builtin::kInstanceIndex:
@@ -1959,10 +1969,10 @@ bool GeneratorImpl::EmitEntryPointFunction(const ast::Function* func) {
             return false;
           }
           out << "(";
-          out << builtin_to_string(builtin->builtin);
+          out << builtin_to_string(builtin->builtin, func->PipelineStage());
           out << ")";
         } else {
-          out << builtin_to_string(builtin->builtin);
+          out << builtin_to_string(builtin->builtin, func->PipelineStage());
         }
       } else {
         out << name;
@@ -1992,7 +2002,7 @@ bool GeneratorImpl::EmitEntryPointFunction(const ast::Function* func) {
       out << "  ";
       if (auto* builtin = ast::GetDecoration<ast::BuiltinDecoration>(
               member->Declaration()->decorations)) {
-        out << builtin_to_string(builtin->builtin);
+        out << builtin_to_string(builtin->builtin, func->PipelineStage());
       } else {
         out << name;
       }

src/writer/glsl/generator_impl.h

@@ -347,8 +347,9 @@ class GeneratorImpl : public TextGenerator {
   std::string generate_builtin_name(const sem::Intrinsic* intrinsic);
   /// Converts a builtin to a gl_ string
   /// @param builtin the builtin to convert
+  /// @param stage pipeline stage in which this builtin is used
   /// @returns the string name of the builtin or blank on error
-  const char* builtin_to_string(ast::Builtin builtin) const;
+  const char* builtin_to_string(ast::Builtin builtin, ast::PipelineStage stage);
   /// Converts a builtin to a sem::Type appropriate for GLSL.
   /// @param builtin the builtin to convert
   /// @returns the appropriate semantic type or null on error.

src/writer/glsl/generator_impl_function_test.cc

@@ -208,7 +208,7 @@ tint_symbol_2 frag_main(tint_symbol_1 tint_symbol) {
 }
 void main() {
   tint_symbol_1 inputs;
-  inputs.coord = gl_Position;
+  inputs.coord = gl_FragCoord;
   tint_symbol_2 outputs;
   outputs = frag_main(inputs);
   gl_FragDepth = outputs.value;
@@ -320,7 +320,7 @@ void main() {
   tint_symbol_2 inputs;
   inputs.col1 = col1;
   inputs.col2 = col2;
-  inputs.pos = gl_Position;
+  inputs.pos = gl_FragCoord;
   frag_main(inputs);
 }
 

src/writer/glsl/generator_impl_test.cc

@@ -59,7 +59,7 @@ TEST_P(GlslBuiltinConversionTest, Emit) {
   auto params = GetParam();
   GeneratorImpl& gen = Build();
 
-  EXPECT_EQ(gen.builtin_to_string(params.builtin),
+  EXPECT_EQ(gen.builtin_to_string(params.builtin, ast::PipelineStage::kVertex),
             std::string(params.attribute_name));
 }
 INSTANTIATE_TEST_SUITE_P(