Enforce layout(location=...) on Metal out variables.

Previously, this would generate invalid code such as `[[user(locn-1)]]`.
We now generate a more-useful error at SkSL compilation time.

Change-Id: Ifbe335ec6d4abcbdfe89b892ba51063c94d22b11
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/339397
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>

gn/sksl_tests.gni

@@ -175,6 +175,7 @@ sksl_metal_tests = [
   "$_tests/sksl/metal/NumericGlobals.sksl",
   "$_tests/sksl/metal/OpaqueTypeInInterfaceBlock.sksl",
   "$_tests/sksl/metal/OpaqueTypeInStruct.sksl",
+  "$_tests/sksl/metal/OutVarsRequireLocation.sksl",
   "$_tests/sksl/metal/SamplerGlobals.sksl",
 ]
 

src/sksl/SkSLMetalCodeGenerator.cpp

@@ -1559,12 +1559,15 @@ void MetalCodeGenerator::writeOutputStruct() {
                 this->writeType(var.type());
                 this->write(" ");
                 this->writeName(var.name());
-                if (fProgram.fKind == Program::kVertex_Kind) {
-                    this->write("  [[user(locn" +
-                                to_string(var.modifiers().fLayout.fLocation) + ")]]");
+
+                int location = var.modifiers().fLayout.fLocation;
+                if (location < 0) {
+                    fErrors.error(var.fOffset,
+                                  "Metal out variables must have 'layout(location=...)'");
+                } else if (fProgram.fKind == Program::kVertex_Kind) {
+                    this->write(" [[user(locn" + to_string(location) + ")]]");
                 } else if (fProgram.fKind == Program::kFragment_Kind) {
-                    this->write(" [[color(" +
-                                to_string(var.modifiers().fLayout.fLocation) +")");
+                    this->write(" [[color(" + to_string(location) + ")");
                     int colorIndex = var.modifiers().fLayout.fIndex;
                     if (colorIndex) {
                         this->write(", index(" + to_string(colorIndex) + ")");

tests/sksl/metal/OutVarsRequireLocation.sksl

@@ -0,0 +1 @@
+out int noLocation;

tests/sksl/metal/golden/OutVarsRequireLocation.metal

@@ -0,0 +1,4 @@
+### Compilation failed:
+
+error: 1: Metal out variables must have 'layout(location=...)'
+1 error

tests/sksl/shared/InstanceID.vert

@@ -1,4 +1,4 @@
-out int id;
+layout(location=1) out int id;
 
 void main() {
     id = sk_InstanceID;

tests/sksl/shared/VertexID.vert

@@ -1,4 +1,4 @@
-out int id;
+layout(location=1) out int id;
 
 void main() {
     id = sk_VertexID;

tests/sksl/shared/golden/InstanceID.asm.vert

@@ -6,6 +6,7 @@ OpName %sk_InstanceID "sk_InstanceID"
 OpName %id "id"
 OpName %main "main"
 OpDecorate %sk_InstanceID BuiltIn InstanceIndex
+OpDecorate %id Location 1
 %int = OpTypeInt 32 1
 %_ptr_Input_int = OpTypePointer Input %int
 %sk_InstanceID = OpVariable %_ptr_Input_int Input

tests/sksl/shared/golden/InstanceID.glsl

@@ -1,5 +1,5 @@
 
-out int id;
+layout (location = 1) out int id;
 void main() {
     id = gl_InstanceID;
 }

tests/sksl/shared/golden/InstanceID.metal

@@ -5,7 +5,7 @@ struct Inputs {
 };
 struct Outputs {
     float4 sk_Position [[position]];
-    int id  [[user(locn-1)]];
+    int id [[user(locn1)]];
     float sk_PointSize [[point_size]];
 };
 

tests/sksl/shared/golden/VertexID.asm.vert

@@ -6,6 +6,7 @@ OpName %sk_VertexID "sk_VertexID"
 OpName %id "id"
 OpName %main "main"
 OpDecorate %sk_VertexID BuiltIn VertexIndex
+OpDecorate %id Location 1
 %int = OpTypeInt 32 1
 %_ptr_Input_int = OpTypePointer Input %int
 %sk_VertexID = OpVariable %_ptr_Input_int Input

tests/sksl/shared/golden/VertexID.glsl

@@ -1,5 +1,5 @@
 
-out int id;
+layout (location = 1) out int id;
 void main() {
     id = gl_VertexID;
 }

tests/sksl/shared/golden/VertexID.metal

@@ -5,7 +5,7 @@ struct Inputs {
 };
 struct Outputs {
     float4 sk_Position [[position]];
-    int id  [[user(locn-1)]];
+    int id [[user(locn1)]];
     float sk_PointSize [[point_size]];
 };