Improve code generation for functions

Cesium.CodeGen.Tests/CodeGenMethodTests.cs

@@ -342,14 +342,23 @@ public Task ImplicitReturnAllowedForMain() => DoTest(@"int main()
     int unused = 0;
 }");
 
-    [Fact, NoVerify]
-    public void ImplicitReturnDisallowedNonMain() => DoesNotCompile(@"int foo()
+    [Fact]
+    public void ImplicitReturnAllowedNonMain() => DoTest(@"int foo()
 {
     int unused;
-}", "Function foo has no return statement.");
+}");
+
+    [Fact, NoVerify]
+    public void ExpressionReturnDisallowedInVoidFunction() => DoesNotCompile(@"void foo()
+{
+    return 4;
+}", "Function foo has return type void, and thus cannot have expression in return.");
+
+    [Fact]
+    public Task ReturnInsideInfiniteForTest() => DoTest("void test() { for(;;) { return; } }");
 
     [Fact]
-    public Task ReturnInsideInfiniteForTest() => DoTest("int test() { for(;;) { return; } }");
+    public Task ReturnValueInsideInfiniteForTest() => DoTest("int test() { for(;;) { return 5; } }");
 
     [Fact]
     public Task ConsumeUnusedResult() => DoTest(@"

Cesium.CodeGen.Tests/verified/CodeGenIfTests.IfElseTest.verified.txt

@@ -16,6 +16,8 @@
   IL_0014: ldc.i4.0
   IL_0015: ret
   IL_0016: nop
+  IL_0017: ldc.i4.0
+  IL_0018: ret
 
 System.Int32 <Module>::<SyntheticEntrypoint>()
   Locals:

Cesium.CodeGen.Tests/verified/CodeGenIfTests.IfElseWithNestedDeclarationTest.verified.txt

@@ -15,6 +15,8 @@
   IL_0012: ldc.i4.0
   IL_0013: ret
   IL_0014: nop
+  IL_0015: ldc.i4.0
+  IL_0016: ret
 
 System.Int32 <Module>::<SyntheticEntrypoint>()
   Locals:

Cesium.CodeGen.Tests/verified/CodeGenMethodTests.ImplicitReturnAllowedNonMain.verified.txt

@@ -0,0 +1,3 @@
+System.Int32 <Module>::foo()
+  IL_0000: ldc.i4.0
+  IL_0001: ret

Cesium.CodeGen.Tests/verified/CodeGenMethodTests.ReturnInsideInfiniteForTest.verified.txt

@@ -1,4 +1,4 @@
-System.Int32 <Module>::test()
+System.Void <Module>::test()
   IL_0000: nop
   IL_0001: ret
   IL_0002: nop

Cesium.CodeGen.Tests/verified/CodeGenMethodTests.ReturnValueInsideInfiniteForTest.verified.txt

@@ -0,0 +1,9 @@
+System.Int32 <Module>::test()
+  IL_0000: nop
+  IL_0001: ldc.i4.5
+  IL_0002: ret
+  IL_0003: nop
+  IL_0004: br IL_0000
+  IL_0009: nop
+  IL_000a: ldc.i4.0
+  IL_000b: ret

Cesium.CodeGen.Tests/verified/CodeGenMethodTests.VariablesInsideNestedIf.verified.txt

@@ -32,6 +32,8 @@
   IL_002b: nop
   IL_002c: br IL_0006
   IL_0031: nop
+  IL_0032: ldc.i4.0
+  IL_0033: ret
 
 System.Int32 <Module>::<SyntheticEntrypoint>()
   Locals:

Cesium.CodeGen/Ir/BlockItems/FunctionDefinition.cs

@@ -280,5 +280,11 @@ private void EmitCode(FunctionScope scope)
         );
 
         BlockItemEmitting.EmitCode(scope, transformed);
+        var isVoid = scope.FunctionInfo.ReturnType.Equals(CTypeSystem.Void);
+        if (!isVoid && scope.Method.Body.Instructions.Last().OpCode != OpCodes.Ret)
+        {
+            scope.Method.Body.Instructions.Add(Instruction.Create(OpCodes.Ldc_I4_0));
+            scope.Method.Body.Instructions.Add(Instruction.Create(OpCodes.Ret));
+        }
     }
 }

Cesium.CodeGen/Ir/ControlFlow/ControlFlowChecker.cs

@@ -314,19 +314,22 @@ bool isMain
         var isVoidFn = returnType.Equals(CTypeSystem.Void);
         var isReturnRequired = !isVoidFn && !isMain;
 
+        if (isVoidFn)
+        {
+            var hasExpressionReturn = (ReturnStatement?)dset.Vertices.FirstOrDefault(_ => _.BlockItem is ReturnStatement { Expression: { } })?.BlockItem;
+            if (hasExpressionReturn is not null)
+            {
+                throw new CompilationException($"Function {scope.Method.Name} has return type void, and thus cannot have expression in return.");
+            }
+        }
+
         foreach (var preTerminator in preTerminators)
         {
             if (preTerminator.BlockItem is ReturnStatement) continue;
             if (preTerminator.Reached == false) continue;
 
-            if (isReturnRequired)
-            {
-                // bad error message
-                throw new CompilationException($"Function {scope.Method.Name} has no return statement.");
-            }
-
             // inserting fake return
-            var retn = new ReturnStatement(isMain ? new ConstantLiteralExpression(new IntegerConstant(0)) : null);
+            var retn = new ReturnStatement(!isVoidFn ? new ConstantLiteralExpression(new IntegerConstant(0)) : null);
 
             if (preTerminator.BlockItem is {} original)
             {

Cesium.IntegrationTests/functions.c

@@ -23,6 +23,11 @@ void forward_declaration_void_2()
     // Do nothing here.
 }
 
+int missing_return()
+{
+    // Do nothing here.
+}
+
 int main(void)
 {
     return foo();

Cesium.IntegrationTests/if.c

@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int test()
+{
+    int a = 0;
+    if (1)
+        return 3;
+    else
+        a = 2;
+}
+
+int main(int argc, char *argv[])
+{
+    if (test() != 3) return -1;
+
+    return 42;
+}