Add passthrough shader snippet.

This gives us a building block that mimics the effect of evaluating a
missing child effect.

Change-Id: Ic091f0692febc3370debcd4742751b175e98fde9
Bug: skia:13508
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/559076
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

src/core/SkBuiltInCodeSnippetID.h

@@ -18,6 +18,7 @@ enum class SkBuiltInCodeSnippetID : int32_t {
     kError,
 
     // SkShader code snippets
+    kPassthroughShader,
     kSolidColorShader,
     kLinearGradientShader4,
     kLinearGradientShader8,

src/core/SkKeyHelpers.cpp

@@ -33,6 +33,23 @@ constexpr SkPMColor4f kErrorColor = { 1, 0, 0, 1 };
 
 //--------------------------------------------------------------------------------------------------
 
+void PassthroughShaderBlock::BeginBlock(const SkKeyContext& keyContext,
+                                        SkPaintParamsKeyBuilder* builder,
+                                        SkPipelineDataGatherer* gatherer) {
+#ifdef SK_GRAPHITE_ENABLED
+    if (builder->backend() == SkBackend::kGraphite) {
+        builder->beginBlock(SkBuiltInCodeSnippetID::kPassthroughShader);
+        return;
+    }
+#endif // SK_GRAPHITE_ENABLED
+
+    if (builder->backend() == SkBackend::kSkVM || builder->backend() == SkBackend::kGanesh) {
+        // TODO: add implementation of other backends
+    }
+}
+
+//--------------------------------------------------------------------------------------------------
+
 #ifdef SK_GRAPHITE_ENABLED
 
 namespace {
@@ -71,7 +88,6 @@ void SolidColorShaderBlock::BeginBlock(const SkKeyContext& keyContext,
     if (builder->backend() == SkBackend::kSkVM || builder->backend() == SkBackend::kGanesh) {
         // TODO: add implementation of other backends
     }
-
 }
 
 //--------------------------------------------------------------------------------------------------

src/core/SkKeyHelpers.h

@@ -32,6 +32,14 @@ namespace skgpu::graphite { class TextureProxy; }
 
 // The KeyHelpers can be used to manually construct an SkPaintParamsKey
 
+struct PassthroughShaderBlock {
+
+    static void BeginBlock(const SkKeyContext&,
+                           SkPaintParamsKeyBuilder*,
+                           SkPipelineDataGatherer*);
+
+};
+
 struct SolidColorShaderBlock {
 
     static void BeginBlock(const SkKeyContext&,

src/core/SkShaderCodeDictionary.cpp

@@ -718,6 +718,9 @@ static constexpr char kMatrixColorFilterName[] = "sk_matrix_colorfilter";
 //--------------------------------------------------------------------------------------------------
 static constexpr char kErrorName[] = "sk_error";
 
+//--------------------------------------------------------------------------------------------------
+static constexpr char kPassthroughName[] = "sk_passthrough";
+
 //--------------------------------------------------------------------------------------------------
 // This method generates the glue code for the case where the SkBlendMode-based blending is
 // handled with fixed function blending.
@@ -969,6 +972,17 @@ SkShaderCodeDictionary::SkShaderCodeDictionary() {
             kNoChildren,
             { }      // no data payload
     };
+    fBuiltInCodeSnippets[(int) SkBuiltInCodeSnippetID::kPassthroughShader] = {
+            "Passthrough",
+            { },     // no uniforms
+            SnippetRequirementFlags::kPriorStageOutput,
+            { },     // no samplers
+            kPassthroughName,
+            GenerateDefaultExpression,
+            GenerateDefaultPreamble,
+            kNoChildren,
+            { }      // no data payload
+    };
     fBuiltInCodeSnippets[(int) SkBuiltInCodeSnippetID::kSolidColorShader] = {
             "SolidColor",
             SkSpan(kSolidShaderUniforms),

src/sksl/sksl_graphite_frag.sksl

@@ -4,6 +4,10 @@ half4 sk_error() {
     return half4(1.0, 0.0, 1.0, 1.0);
 }
 
+half4 sk_passthrough(half4 color) {
+    return color;
+}
+
 half4 sk_solid_shader(float4 colorParam) {
     return half4(colorParam);
 }