HLSL backend: work around FXC ignoring switch with only default case

In HLSL code, if a switch statement has only a default case, FXC will
effectively ignore the code in that case. In this change, we detect this
and work around it by emitting the code in the default block without the
switch.

Bug: tint:1188
Change-Id: I69b405cdb4c669fb093eb49aa138923419dcf8f8
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/68440
Kokoro: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@chromium.org>

test/statements/switch/common.wgsl

@@ -0,0 +1,20 @@
+[[stage(compute), workgroup_size(1)]]
+fn f() {
+    var i : i32;
+    var result : i32;
+    switch(i) {
+        case 0: {
+            result = 10;
+        }
+        case 1: {
+            result = 22;
+        }
+        case 2: {
+            result = 33;
+        }
+        default: {
+            result = 44;
+            break;
+        }
+    }
+}

test/statements/switch/common.wgsl.expected.hlsl

@@ -0,0 +1,24 @@
+[numthreads(1, 1, 1)]
+void f() {
+  int i = 0;
+  int result = 0;
+  switch(i) {
+    case 0: {
+      result = 10;
+      break;
+    }
+    case 1: {
+      result = 22;
+      break;
+    }
+    case 2: {
+      result = 33;
+      break;
+    }
+    default: {
+      result = 44;
+      break;
+    }
+  }
+  return;
+}

test/statements/switch/common.wgsl.expected.msl

@@ -0,0 +1,27 @@
+#include <metal_stdlib>
+
+using namespace metal;
+kernel void f() {
+  int i = 0;
+  int result = 0;
+  switch(i) {
+    case 0: {
+      result = 10;
+      break;
+    }
+    case 1: {
+      result = 22;
+      break;
+    }
+    case 2: {
+      result = 33;
+      break;
+    }
+    default: {
+      result = 44;
+      break;
+    }
+  }
+  return;
+}
+

test/statements/switch/common.wgsl.expected.spvasm

@@ -0,0 +1,43 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 20
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %f "f"
+               OpExecutionMode %f LocalSize 1 1 1
+               OpName %f "f"
+               OpName %i "i"
+               OpName %result "result"
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+        %int = OpTypeInt 32 1
+%_ptr_Function_int = OpTypePointer Function %int
+          %8 = OpConstantNull %int
+     %int_10 = OpConstant %int 10
+     %int_22 = OpConstant %int 22
+     %int_33 = OpConstant %int 33
+     %int_44 = OpConstant %int 44
+          %f = OpFunction %void None %1
+          %4 = OpLabel
+          %i = OpVariable %_ptr_Function_int Function %8
+     %result = OpVariable %_ptr_Function_int Function %8
+         %11 = OpLoad %int %i
+               OpSelectionMerge %10 None
+               OpSwitch %11 %12 0 %13 1 %14 2 %15
+         %13 = OpLabel
+               OpStore %result %int_10
+               OpBranch %10
+         %14 = OpLabel
+               OpStore %result %int_22
+               OpBranch %10
+         %15 = OpLabel
+               OpStore %result %int_33
+               OpBranch %10
+         %12 = OpLabel
+               OpStore %result %int_44
+               OpBranch %10
+         %10 = OpLabel
+               OpReturn
+               OpFunctionEnd

test/statements/switch/common.wgsl.expected.wgsl

@@ -0,0 +1,20 @@
+[[stage(compute), workgroup_size(1)]]
+fn f() {
+  var i : i32;
+  var result : i32;
+  switch(i) {
+    case 0: {
+      result = 10;
+    }
+    case 1: {
+      result = 22;
+    }
+    case 2: {
+      result = 33;
+    }
+    default: {
+      result = 44;
+      break;
+    }
+  }
+}

test/statements/switch/only_default_case.wgsl

@@ -0,0 +1,11 @@
+[[stage(compute), workgroup_size(1)]]
+fn f() {
+    var i : i32;
+    var result : i32;
+    switch(i) {
+        default: {
+            result = 44;
+            break;
+        }
+    }
+}

test/statements/switch/only_default_case.wgsl.expected.hlsl

@@ -0,0 +1,11 @@
+[numthreads(1, 1, 1)]
+void f() {
+  int i = 0;
+  int result = 0;
+  i;
+  do {
+    result = 44;
+    break;
+  } while (false);
+  return;
+}

test/statements/switch/only_default_case.wgsl.expected.msl

@@ -0,0 +1,15 @@
+#include <metal_stdlib>
+
+using namespace metal;
+kernel void f() {
+  int i = 0;
+  int result = 0;
+  switch(i) {
+    default: {
+      result = 44;
+      break;
+    }
+  }
+  return;
+}
+

test/statements/switch/only_default_case.wgsl.expected.spvasm

@@ -0,0 +1,31 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 14
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %f "f"
+               OpExecutionMode %f LocalSize 1 1 1
+               OpName %f "f"
+               OpName %i "i"
+               OpName %result "result"
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+        %int = OpTypeInt 32 1
+%_ptr_Function_int = OpTypePointer Function %int
+          %8 = OpConstantNull %int
+     %int_44 = OpConstant %int 44
+          %f = OpFunction %void None %1
+          %4 = OpLabel
+          %i = OpVariable %_ptr_Function_int Function %8
+     %result = OpVariable %_ptr_Function_int Function %8
+         %11 = OpLoad %int %i
+               OpSelectionMerge %10 None
+               OpSwitch %11 %12
+         %12 = OpLabel
+               OpStore %result %int_44
+               OpBranch %10
+         %10 = OpLabel
+               OpReturn
+               OpFunctionEnd

test/statements/switch/only_default_case.wgsl.expected.wgsl

@@ -0,0 +1,11 @@
+[[stage(compute), workgroup_size(1)]]
+fn f() {
+  var i : i32;
+  var result : i32;
+  switch(i) {
+    default: {
+      result = 44;
+      break;
+    }
+  }
+}