Deal with switch case labels which share a block.

reference/opt/shaders/asm/frag/switch-label-shared-block.asm.frag

@@ -0,0 +1,33 @@
+#version 310 es
+precision mediump float;
+precision highp int;
+
+layout(location = 0) flat in mediump int vIndex;
+layout(location = 0) out float FragColor;
+
+void main()
+{
+    highp float _19;
+    switch (vIndex)
+    {
+        case 0:
+        case 2:
+        {
+            _19 = 1.0;
+            break;
+        }
+        case 1:
+        default:
+        {
+            _19 = 3.0;
+            break;
+        }
+        case 8:
+        {
+            _19 = 8.0;
+            break;
+        }
+    }
+    FragColor = _19;
+}
+

reference/shaders/asm/frag/switch-label-shared-block.asm.frag

@@ -0,0 +1,33 @@
+#version 310 es
+precision mediump float;
+precision highp int;
+
+layout(location = 0) flat in mediump int vIndex;
+layout(location = 0) out float FragColor;
+
+void main()
+{
+    highp float _19;
+    switch (vIndex)
+    {
+        case 0:
+        case 2:
+        {
+            _19 = 1.0;
+            break;
+        }
+        case 1:
+        default:
+        {
+            _19 = 3.0;
+            break;
+        }
+        case 8:
+        {
+            _19 = 8.0;
+            break;
+        }
+    }
+    FragColor = _19;
+}
+

shaders/asm/frag/switch-label-shared-block.asm.frag

@@ -0,0 +1,45 @@
+; SPIR-V
+; Version: 1.0
+; Generator: Khronos Glslang Reference Front End; 6
+; Bound: 28
+; Schema: 0
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %main "main" %vIndex %FragColor
+               OpExecutionMode %main OriginUpperLeft
+               OpSource ESSL 310
+               OpName %main "main"
+               OpName %vIndex "vIndex"
+               OpName %FragColor "FragColor"
+               OpDecorate %vIndex RelaxedPrecision
+               OpDecorate %vIndex Flat
+               OpDecorate %vIndex Location 0
+               OpDecorate %13 RelaxedPrecision
+               OpDecorate %FragColor RelaxedPrecision
+               OpDecorate %FragColor Location 0
+       %void = OpTypeVoid
+          %3 = OpTypeFunction %void
+      %float = OpTypeFloat 32
+    %float_8 = OpConstant %float 8
+        %int = OpTypeInt 32 1
+%_ptr_Input_int = OpTypePointer Input %int
+     %vIndex = OpVariable %_ptr_Input_int Input
+    %float_1 = OpConstant %float 1
+    %float_3 = OpConstant %float 3
+%_ptr_Output_float = OpTypePointer Output %float
+  %FragColor = OpVariable %_ptr_Output_float Output
+       %main = OpFunction %void None %3
+          %5 = OpLabel
+         %13 = OpLoad %int %vIndex
+               OpSelectionMerge %17 None
+               OpSwitch %13 %15 0 %14 2 %14 1 %15 8 %17
+         %15 = OpLabel
+               OpBranch %17
+         %14 = OpLabel
+               OpBranch %17
+         %17 = OpLabel
+         %27 = OpPhi %float %float_3 %15 %float_1 %14 %float_8 %5
+               OpStore %FragColor %27
+               OpReturn
+               OpFunctionEnd

spirv_glsl.cpp

@@ -9657,32 +9657,61 @@ void CompilerGLSL::emit_block_chain(SPIRBlock &block)
 		statement("switch (", to_expression(block.condition), ")");
 		begin_scope();
 
+		// Multiple case labels can branch to same block, so find all unique blocks.
+		bool emitted_default = false;
+		unordered_set<uint32_t> emitted_blocks;
+
 		for (auto &c : block.cases)
 		{
-			auto case_value =
-			    uint32_t_case ? convert_to_string(uint32_t(c.value)) : convert_to_string(int32_t(c.value));
-			statement("case ", case_value, ":");
+			if (emitted_blocks.count(c.block) != 0)
+				continue;
+
+			// Emit all case labels which branch to our target.
+			// FIXME: O(n^2), revisit if we hit shaders with 100++ case labels ...
+			for (auto &other_case : block.cases)
+			{
+				if (other_case.block == c.block)
+				{
+					auto case_value = uint32_t_case ? convert_to_string(uint32_t(other_case.value)) :
+					                                  convert_to_string(int32_t(other_case.value));
+					statement("case ", case_value, ":");
+				}
+			}
+
+			// Maybe we share with default block?
+			if (block.default_block == c.block)
+			{
+				statement("default:");
+				emitted_default = true;
+			}
+
+			// Complete the target.
+			emitted_blocks.insert(c.block);
+
 			begin_scope();
 			branch(block.self, c.block);
 			end_scope();
 		}
 
-		if (block.default_block != block.next_block)
+		if (!emitted_default)
 		{
-			statement("default:");
-			begin_scope();
-			if (is_break(block.default_block))
-				SPIRV_CROSS_THROW("Cannot break; out of a switch statement and out of a loop at the same time ...");
-			branch(block.self, block.default_block);
-			end_scope();
-		}
-		else if (flush_phi_required(block.self, block.next_block))
-		{
-			statement("default:");
-			begin_scope();
-			flush_phi(block.self, block.next_block);
-			statement("break;");
-			end_scope();
+			if (block.default_block != block.next_block)
+			{
+				statement("default:");
+				begin_scope();
+				if (is_break(block.default_block))
+					SPIRV_CROSS_THROW("Cannot break; out of a switch statement and out of a loop at the same time ...");
+				branch(block.self, block.default_block);
+				end_scope();
+			}
+			else if (flush_phi_required(block.self, block.next_block))
+			{
+				statement("default:");
+				begin_scope();
+				flush_phi(block.self, block.next_block);
+				statement("break;");
+				end_scope();
+			}
 		}
 
 		end_scope();