Add workaround for switch() containing only a default case.

The Mac Radeon GLSL driver crashes when given a switch statement that
only contains a default case and returns a value. Adding a case works
around the crash, and doesn't affect the meaning of the switch.

Change-Id: Iabbd267e0e31e8df7d3b7e747a7204d50931d0be
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/450977
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

gn/sksl_tests.gni

@@ -398,6 +398,7 @@ sksl_shared_tests = [
   "/sksl/shared/StructMaxDepth.sksl",
   "/sksl/shared/Switch.sksl",
   "/sksl/shared/SwitchContainingDeadCode.sksl",
+  "/sksl/shared/SwitchDefaultOnly.sksl",
   "/sksl/shared/SwitchWithFallthrough.sksl",
   "/sksl/shared/SwizzleBoolConstants.sksl",
   "/sksl/shared/SwizzleByConstantIndex.sksl",

resources/sksl/shared/SwitchDefaultOnly.sksl

@@ -0,0 +1,7 @@
+uniform half4 colorGreen, colorRed;
+
+half4 main(float2 coords) {
+    switch (int(colorGreen.g)) {
+        default: return colorGreen;
+    }
+}

src/sksl/codegen/SkSLGLSLCodeGenerator.cpp

@@ -1358,6 +1358,12 @@ void GLSLCodeGenerator::writeSwitchStatement(const SwitchStatement& s) {
     this->writeExpression(*s.value(), Precedence::kTopLevel);
     this->writeLine(") {");
     fIndentation++;
+    // If a switch contains only a `default` case and nothing else, this confuses some drivers and
+    // can lead to a crash. Adding a real case before the default seems to work around the bug,
+    // and doesn't change the meaning of the switch.
+    if (s.cases().size() == 1 && !s.cases().front()->as<SwitchCase>().value()) {
+        this->writeLine("case 0:");
+    }
     for (const std::unique_ptr<Statement>& stmt : s.cases()) {
         const SwitchCase& c = stmt->as<SwitchCase>();
         if (c.value()) {

tests/SkSLTest.cpp

@@ -324,6 +324,7 @@ SKSL_TEST_ES3(SkSLStaticSwitch,                "shared/StaticSwitch.sksl")
 SKSL_TEST(SkSLStructArrayFollowedByScalar,     "shared/StructArrayFollowedByScalar.sksl")
 SKSL_TEST(SkSLStructsInFunctions,              "shared/StructsInFunctions.sksl")
 SKSL_TEST_ES3(SkSLSwitch,                      "shared/Switch.sksl")
+SKSL_TEST_ES3(SkSLSwitchDefaultOnly,           "shared/SwitchDefaultOnly.sksl")
 SKSL_TEST(SkSLSwizzleBoolConstants,            "shared/SwizzleBoolConstants.sksl")
 SKSL_TEST(SkSLSwizzleByConstantIndex,          "shared/SwizzleByConstantIndex.sksl")
 SKSL_TEST(SkSLSwizzleConstants,                "shared/SwizzleConstants.sksl")

tests/sksl/shared/ReturnsValueOnEveryPathES3.glsl

@@ -64,6 +64,7 @@ bool switch_with_all_returns_b() {
 }
 bool switch_only_default_b() {
     switch (int(unknownInput)) {
+        case 0:
         default:
             return true;
     }

tests/sksl/shared/SwitchDefaultOnly.asm.frag

@@ -0,0 +1,70 @@
+OpCapability Shader
+%1 = OpExtInstImport "GLSL.std.450"
+OpMemoryModel Logical GLSL450
+OpEntryPoint Fragment %_entrypoint_v "_entrypoint" %sk_FragColor %sk_Clockwise
+OpExecutionMode %_entrypoint_v OriginUpperLeft
+OpName %sk_FragColor "sk_FragColor"
+OpName %sk_Clockwise "sk_Clockwise"
+OpName %_UniformBuffer "_UniformBuffer"
+OpMemberName %_UniformBuffer 0 "colorGreen"
+OpMemberName %_UniformBuffer 1 "colorRed"
+OpName %_entrypoint_v "_entrypoint_v"
+OpName %main "main"
+OpDecorate %sk_FragColor RelaxedPrecision
+OpDecorate %sk_FragColor Location 0
+OpDecorate %sk_FragColor Index 0
+OpDecorate %sk_Clockwise BuiltIn FrontFacing
+OpMemberDecorate %_UniformBuffer 0 Offset 0
+OpMemberDecorate %_UniformBuffer 0 RelaxedPrecision
+OpMemberDecorate %_UniformBuffer 1 Offset 16
+OpMemberDecorate %_UniformBuffer 1 RelaxedPrecision
+OpDecorate %_UniformBuffer Block
+OpDecorate %10 Binding 0
+OpDecorate %10 DescriptorSet 0
+OpDecorate %30 RelaxedPrecision
+OpDecorate %31 RelaxedPrecision
+OpDecorate %36 RelaxedPrecision
+%float = OpTypeFloat 32
+%v4float = OpTypeVector %float 4
+%_ptr_Output_v4float = OpTypePointer Output %v4float
+%sk_FragColor = OpVariable %_ptr_Output_v4float Output
+%bool = OpTypeBool
+%_ptr_Input_bool = OpTypePointer Input %bool
+%sk_Clockwise = OpVariable %_ptr_Input_bool Input
+%_UniformBuffer = OpTypeStruct %v4float %v4float
+%_ptr_Uniform__UniformBuffer = OpTypePointer Uniform %_UniformBuffer
+%10 = OpVariable %_ptr_Uniform__UniformBuffer Uniform
+%void = OpTypeVoid
+%15 = OpTypeFunction %void
+%v2float = OpTypeVector %float 2
+%float_0 = OpConstant %float 0
+%19 = OpConstantComposite %v2float %float_0 %float_0
+%_ptr_Function_v2float = OpTypePointer Function %v2float
+%23 = OpTypeFunction %v4float %_ptr_Function_v2float
+%_ptr_Uniform_v4float = OpTypePointer Uniform %v4float
+%int = OpTypeInt 32 1
+%int_0 = OpConstant %int 0
+%_entrypoint_v = OpFunction %void None %15
+%16 = OpLabel
+%20 = OpVariable %_ptr_Function_v2float Function
+OpStore %20 %19
+%22 = OpFunctionCall %v4float %main %20
+OpStore %sk_FragColor %22
+OpReturn
+OpFunctionEnd
+%main = OpFunction %v4float None %23
+%24 = OpFunctionParameter %_ptr_Function_v2float
+%25 = OpLabel
+%26 = OpAccessChain %_ptr_Uniform_v4float %10 %int_0
+%30 = OpLoad %v4float %26
+%31 = OpCompositeExtract %float %30 1
+%32 = OpConvertFToS %int %31
+OpSelectionMerge %33 None
+OpSwitch %32 %34
+%34 = OpLabel
+%35 = OpAccessChain %_ptr_Uniform_v4float %10 %int_0
+%36 = OpLoad %v4float %35
+OpReturnValue %36
+%33 = OpLabel
+OpUnreachable
+OpFunctionEnd

tests/sksl/shared/SwitchDefaultOnly.glsl

@@ -0,0 +1,11 @@
+
+out vec4 sk_FragColor;
+uniform vec4 colorGreen;
+uniform vec4 colorRed;
+vec4 main() {
+    switch (int(colorGreen.y)) {
+        case 0:
+        default:
+            return colorGreen;
+    }
+}

tests/sksl/shared/SwitchDefaultOnly.metal

@@ -0,0 +1,22 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+using namespace metal;
+struct Uniforms {
+    float4 colorGreen;
+    float4 colorRed;
+};
+struct Inputs {
+};
+struct Outputs {
+    float4 sk_FragColor [[color(0)]];
+};
+fragment Outputs fragmentMain(Inputs _in [[stage_in]], constant Uniforms& _uniforms [[buffer(0)]], bool _frontFacing [[front_facing]], float4 _fragCoord [[position]]) {
+    Outputs _out;
+    (void)_out;
+    switch (int(_uniforms.colorGreen.y)) {
+        default:
+            _out.sk_FragColor = _uniforms.colorGreen;
+            return _out;
+    }
+    return _out;
+}