Added SkSL workaround for devices which cannot safely access gl_FragCoord

This is the root cause of https://github.com/flutter/flutter/issues/13216
I've got a GM that demonstrates the bug, but only in Viewer.

Bug: skia:7410
Change-Id: Iaa1f27b10166aa09e4dc5949e5a6ca1bd14c99ac
Reviewed-on: https://skia-review.googlesource.com/93920
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

2 files changed, 52 insertions, 2 deletions

diff --git a/src/sksl/SkSLGLSLCodeGenerator.cpp b/src/sksl/SkSLGLSLCodeGenerator.cpp
index 9e2336e22e..f3400c7f5a 100644
--- a/src/sksl/SkSLGLSLCodeGenerator.cpp
+++ b/src/sksl/SkSLGLSLCodeGenerator.cpp
@@ -597,6 +597,12 @@ void GLSLCodeGenerator::writeConstructor(const Constructor& c, Precedence parent
 }
 
 void GLSLCodeGenerator::writeFragCoord() {
+    if (!fProgram.fSettings.fCaps->canUseFragCoord()) {
+        this->write("vec4(sk_FragCoord_Workaround.xyz / sk_FragCoord_Workaround.w, "
+                    "1.0 / sk_FragCoord_Workaround.w)");
+        return;
+    }
+
     // We only declare "gl_FragCoord" when we're in the case where we want to use layout qualifiers
     // to reverse y. Otherwise it isn't necessary and whether the "in" qualifier appears in the
     // declaration varies in earlier GLSL specs. So it is simpler to omit it.
@@ -619,7 +625,7 @@ void GLSLCodeGenerator::writeFragCoord() {
             // The Adreno compiler seems to be very touchy about access to "gl_FragCoord".
             // Accessing glFragCoord.zw can cause a program to fail to link. Additionally,
             // depending on the surrounding code, accessing .xy with a uniform involved can
-            // do the same thing. Copying gl_FragCoord.xy into a temp float2beforehand
+            // do the same thing. Copying gl_FragCoord.xy into a temp float2 beforehand
             // (and only accessing .xy) seems to "fix" things.
             const char* precision = usesPrecisionModifiers() ? "highp " : "";
             fHeader.writeText("uniform ");
@@ -640,7 +646,6 @@ void GLSLCodeGenerator::writeFragCoord() {
     }
 }
 
-
 void GLSLCodeGenerator::writeVariableReference(const VariableReference& ref) {
     switch (ref.fVariable.fModifiers.fLayout.fBuiltin) {
         case SK_FRAGCOLOR_BUILTIN:
@@ -680,6 +685,10 @@ void GLSLCodeGenerator::writeIndexExpression(const IndexExpression& expr) {
     this->write("]");
 }
 
+bool is_sk_position(const FieldAccess& f) {
+    return "sk_Position" == f.fBase->fType.fields()[f.fFieldIndex].fName;
+}
+
 void GLSLCodeGenerator::writeFieldAccess(const FieldAccess& f) {
     if (f.fOwnerKind == FieldAccess::kDefault_OwnerKind) {
         this->writeExpression(*f.fBase, kPostfix_Precedence);
@@ -755,11 +764,22 @@ void GLSLCodeGenerator::writeBinaryExpression(const BinaryExpression& b,
     if (precedence >= parentPrecedence) {
         this->write("(");
     }
+    bool positionWorkaround = Compiler::IsAssignment(b.fOperator) &&
+                              Expression::kFieldAccess_Kind == b.fLeft->fKind &&
+                              is_sk_position((FieldAccess&) *b.fLeft) &&
+                              !strstr(b.fRight->description().c_str(), "sk_RTAdjust") &&
+                              !fProgram.fSettings.fCaps->canUseFragCoord();
+    if (positionWorkaround) {
+        this->write("sk_FragCoord_Workaround = (");
+    }
     this->writeExpression(*b.fLeft, precedence);
     this->write(" ");
     this->write(Compiler::OperatorName(b.fOperator));
     this->write(" ");
     this->writeExpression(*b.fRight, precedence);
+    if (positionWorkaround) {
+        this->write(")");
+    }
     if (precedence >= parentPrecedence) {
         this->write(")");
     }
@@ -1188,6 +1208,25 @@ void GLSLCodeGenerator::writeHeader() {
             this->writeExtension((Extension&) *e);
         }
     }
+    if (!fProgram.fSettings.fCaps->canUseFragCoord()) {
+        Layout layout;
+        switch (fProgram.fKind) {
+            case Program::kVertex_Kind: {
+                Modifiers modifiers(layout, Modifiers::kOut_Flag | Modifiers::kHighp_Flag);
+                this->writeModifiers(modifiers, true);
+                this->write("vec4 sk_FragCoord_Workaround;\n");
+                break;
+            }
+            case Program::kFragment_Kind: {
+                Modifiers modifiers(layout, Modifiers::kIn_Flag | Modifiers::kHighp_Flag);
+                this->writeModifiers(modifiers, true);
+                this->write("vec4 sk_FragCoord_Workaround;\n");
+                break;
+            }
+            default:
+                break;
+        }
+    }
 }
 
 void GLSLCodeGenerator::writeProgramElement(const ProgramElement& e) {
diff --git a/src/sksl/SkSLUtil.h b/src/sksl/SkSLUtil.h
index 7a39386269..787698f2c6 100644
--- a/src/sksl/SkSLUtil.h
+++ b/src/sksl/SkSLUtil.h
@@ -174,6 +174,10 @@ public:
     bool canUseFractForNegativeValues() const {
         return true;
     }
+
+    bool canUseFragCoord() const {
+        return true;
+    }
 };
 
 extern StandaloneShaderCaps standaloneCaps;
@@ -300,6 +304,13 @@ public:
         result->fCanUseAnyFunctionInShader = false;
         return result;
     }
+
+    static sk_sp<GrShaderCaps> CannotUseFragCoord() {
+        sk_sp<GrShaderCaps> result = sk_make_sp<GrShaderCaps>(GrContextOptions());
+        result->fVersionDeclString = "#version 400";
+        result->fCanUseFragCoord = false;
+        return result;
+    }
 };
 #endif