re-re-land of skslc now automatically turns on derivatives support

src/sksl/SkSLGLSLCodeGenerator.cpp

Index: src/sksl/SkSLGLSLCodeGenerator.cpp
diff --git a/src/sksl/SkSLGLSLCodeGenerator.cpp b/src/sksl/SkSLGLSLCodeGenerator.cpp
index 45644e8cd10032c12a824826a0073d4505d703af..8a26f6a71362ae825dd6432bc621f1196b2053ed 100644
--- a/src/sksl/SkSLGLSLCodeGenerator.cpp
+++ b/src/sksl/SkSLGLSLCodeGenerator.cpp
@@ -150,7 +150,7 @@ void GLSLCodeGenerator::writeMinAbsHack(Expression& absExpr, Expression& otherEx
 }
 
 void GLSLCodeGenerator::writeFunctionCall(const FunctionCall& c) {
-    if (!fCaps.fCanUseMinAndAbsTogether && c.fFunction.fName == "min") {
+    if (!fCaps.fCanUseMinAndAbsTogether && c.fFunction.fName == "min" && c.fFunction.fBuiltin) {
         ASSERT(c.fArguments.size() == 2);
         if (is_abs(*c.fArguments[0])) {
             this->writeMinAbsHack(*c.fArguments[0], *c.fArguments[1]);
@@ -164,7 +164,8 @@ void GLSLCodeGenerator::writeFunctionCall(const FunctionCall& c) {
         }
     }
     if (fCaps.fMustForceNegatedAtanParamToFloat && c.fFunction.fName == "atan" && 
-        c.fArguments.size() == 2 && c.fArguments[1]->fKind == Expression::kPrefix_Kind) {
+        c.fFunction.fBuiltin && c.fArguments.size() == 2 && 
+        c.fArguments[1]->fKind == Expression::kPrefix_Kind) {
         const PrefixExpression& p = (PrefixExpression&) *c.fArguments[1];
         if (p.fOperator == Token::MINUS) {
             this->write("atan(");
@@ -175,6 +176,12 @@ void GLSLCodeGenerator::writeFunctionCall(const FunctionCall& c) {
             return;
         }
     }
+    if (!fFoundDerivatives && fCaps.fShaderDerivativeExtensionString != "" && 
+        (c.fFunction.fName == "dFdx" || c.fFunction.fName == "dFdy") && c.fFunction.fBuiltin) {
+        ASSERT(fCaps.fShaderDerivativeSupport);
+        fHeader << "#extension " << fCaps.fShaderDerivativeExtensionString << " : require\n";
+        fFoundDerivatives = true;
+    }
     this->write(c.fFunction.fName + "(");
     const char* separator = "";
     for (const auto& arg : c.fArguments) {
@@ -578,7 +585,7 @@ void GLSLCodeGenerator::writeReturnStatement(const ReturnStatement& r) {
 
 void GLSLCodeGenerator::generateCode(const Program& program, std::ostream& out) {
     ASSERT(fOut == nullptr);
-    fOut = &out;
+    fOut = &fHeader;
     fProgramKind = program.fKind;
     this->write("#version " + to_string(fCaps.fVersion));
     if (fCaps.fStandard == GLCaps::kGLES_Standard && fCaps.fVersion >= 300) {
@@ -592,6 +599,8 @@ void GLSLCodeGenerator::generateCode(const Program& program, std::ostream& out)
             this->writeExtension((Extension&) *e);
         }
     }
+    std::stringstream body;
+    fOut = &body;
     if (fCaps.fStandard == GLCaps::kGLES_Standard) {
         this->write("precision ");
         switch (program.fDefaultPrecision) {
@@ -649,6 +658,9 @@ void GLSLCodeGenerator::generateCode(const Program& program, std::ostream& out)
         }
     }
     fOut = nullptr;
+
+    out << fHeader.str();
+    out << body.str();
 }
 
 }