Fix swizzle-of-swizzle lvalues in ByteCodeGenerator

Bug: skia:10785
Change-Id: I01708af63d7e2ffc160022074ea9ff2b3c69eab5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/320638
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

src/sksl/SkSLByteCodeGenerator.cpp

@@ -1557,15 +1557,24 @@ public:
         // because the stack doesn't let us retain that address between stores. Dynamic locations
         // are rare though, and swizzled writes to those are even rarer, so we just live with this.
         for (int i = count; i-- > 0;) {
-            ByteCodeGenerator::Location location = fGenerator.getLocation(*fSwizzle.fBase);
+            // If we have a swizzle-of-swizzle lvalue, we need to flatten that down to the final
+            // component index. (getLocation can't handle this case).
+            const Expression* expr = &fSwizzle;
+            int component = i;
+            do {
+                component = expr->as<Swizzle>().fComponents[component];
+                expr = expr->as<Swizzle>().fBase.get();
+            } while (expr->is<Swizzle>());
+
+            ByteCodeGenerator::Location location = fGenerator.getLocation(*expr);
             if (!location.isOnStack()) {
                 fGenerator.write(location.selectStore(ByteCodeInstruction::kStore,
                                                       ByteCodeInstruction::kStoreGlobal),
                                  1);
-                fGenerator.write8(location.fSlot + fSwizzle.fComponents[i]);
+                fGenerator.write8(location.fSlot + component);
             } else {
                 fGenerator.write(ByteCodeInstruction::kPushImmediate);
-                fGenerator.write32(fSwizzle.fComponents[i]);
+                fGenerator.write32(component);
                 fGenerator.write(ByteCodeInstruction::kAddI, 1);
                 fGenerator.write(location.selectStore(ByteCodeInstruction::kStoreExtended,
                                                       ByteCodeInstruction::kStoreExtendedGlobal),

tests/SkSLInterpreterTest.cpp

@@ -815,6 +815,20 @@ DEF_TEST(SkSLInterpreterOutParams, r) {
          1, 2, 3, 4, 3, 3, 1, 5);
 }
 
+DEF_TEST(SkSLInterpreterSwizzleSingleLvalue, r) {
+    // Add in your SkSL here.
+    test(r,
+         "void main(inout half4 color) { color.xywz = half4(1,2,3,4); }",
+         0, 0, 0, 0, 1, 2, 4, 3);
+}
+
+DEF_TEST(SkSLInterpreterSwizzleDoubleLvalue, r) {
+    // Add in your SkSL here.
+    test(r,
+         "void main(inout half4 color) { color.xywz.yxzw = half4(1,2,3,4); }",
+         0, 0, 0, 0, 2, 1, 4, 3);
+}
+
 DEF_TEST(SkSLInterpreterMathFunctions, r) {
     float value[4], expected[4];