Added SkSL DSL SampleChild function

Change-Id: Icd94540bbf0afcc57cfd93f3b576249e26e25724
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369776
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>

src/sksl/SkSLCFGGenerator.cpp

@@ -381,6 +381,9 @@ void CFGGenerator::addExpression(CFG& cfg, std::unique_ptr<Expression>* e, bool
             }
             break;
         }
+        case Expression::Kind::kCodeString:
+            SkDEBUGFAIL("shouldn't be able to receive kCodeString here");
+            break;
         case Expression::Kind::kConstructor: {
             Constructor& c = e->get()->as<Constructor>();
             for (auto& arg : c.arguments()) {

src/sksl/SkSLDehydrator.cpp

@@ -273,6 +273,9 @@ void Dehydrator::write(const Expression* e) {
                 this->writeU8(b.value());
                 break;
             }
+            case Expression::Kind::kCodeString:
+                SkDEBUGFAIL("shouldn't be able to receive kCodeString here");
+                break;
             case Expression::Kind::kConstructor: {
                 const Constructor& c = e->as<Constructor>();
                 this->writeCommand(Rehydrator::kConstructor_Command);

src/sksl/dsl/DSLExpression.h

@@ -109,6 +109,8 @@ private:
 
     std::unique_ptr<SkSL::Expression> fExpression;
 
+    friend DSLExpression SampleChild(int index, DSLExpression coords);
+
     template<class... Cases>
     friend DSLStatement Switch(DSLExpression value, Cases... cases);
 

src/sksl/dsl/priv/DSLFPs.cpp

@@ -8,6 +8,7 @@
 #include "src/sksl/dsl/priv/DSLFPs.h"
 
 #include "src/sksl/dsl/priv/DSLWriter.h"
+#include "src/sksl/ir/SkSLCodeStringExpression.h"
 
 #if !defined(SKSL_STANDALONE) && SK_SUPPORT_GPU
 
@@ -24,6 +25,15 @@ void EndFragmentProcessor() {
     DSLWriter::EndFragmentProcessor();
 }
 
+DSLExpression SampleChild(int index, DSLExpression coords) {
+    std::unique_ptr<SkSL::Expression> coordsExpr = coords.release();
+    SkString code = DSLWriter::CurrentProcessor()->invokeChild(index, *DSLWriter::CurrentEmitArgs(),
+                                                              coordsExpr ? coordsExpr->description()
+                                                                         : "");
+    return DSLExpression(std::make_unique<SkSL::CodeStringExpression>(code.c_str(),
+                                                         DSLWriter::Context().fTypes.fHalf4.get()));
+}
+
 GrGLSLUniformHandler::UniformHandle VarUniformHandle(const DSLVar& var) {
     return DSLWriter::VarUniformHandle(var);
 }

src/sksl/dsl/priv/DSLFPs.h

@@ -23,6 +23,8 @@ void StartFragmentProcessor(GrGLSLFragmentProcessor* processor,
 
 void EndFragmentProcessor();
 
+DSLExpression SampleChild(int index, DSLExpression coords = DSLExpression());
+
 GrGLSLUniformHandler::UniformHandle VarUniformHandle(const DSLVar& var);
 
 } // namespace dsl

src/sksl/ir/SkSLCodeStringExpression.h

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2020 Google LLC.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SKSL_CODESTRINGEXPRESSION
+#define SKSL_CODESTRINGEXPRESSION
+
+#include "src/sksl/ir/SkSLExpression.h"
+
+namespace SkSL {
+
+/**
+ * Represents a literal string of SkSL code. This is only valid within SkSL DSL code.
+ * TODO(skia:11330): This class is intended as a temporary measure to support a couple of spots
+ * within Skia that are currently generating raw strings of code. These will eventually transition
+ * to producing Expressions, allowing this class to be deleted.
+ */
+class CodeStringExpression final : public Expression {
+public:
+    static constexpr Kind kExpressionKind = Kind::kCodeString;
+
+    CodeStringExpression(String code, const Type* type)
+        : INHERITED(/*offset=*/-1, kExpressionKind, type)
+        , fCode(std::move(code)) {}
+
+    bool hasProperty(Property property) const override {
+        return false;
+    }
+
+    std::unique_ptr<Expression> clone() const override {
+        return std::make_unique<CodeStringExpression>(fCode, &this->type());
+    }
+
+    String description() const override {
+        return fCode;
+    }
+
+private:
+    String fCode;
+
+    using INHERITED = Expression;
+};
+
+}  // namespace SkSL
+
+#endif

src/sksl/ir/SkSLExpression.h

@@ -29,6 +29,7 @@ public:
     enum class Kind {
         kBinary = (int) Statement::Kind::kLast + 1,
         kBoolLiteral,
+        kCodeString,
         kConstructor,
         kDefined,
         kExternalFunctionCall,