Give the ts compiler special handling for returns in then branches

bril-ts/builder.ts

@@ -67,7 +67,7 @@ export class Builder {
       return this.buildEffect("call", args, [func], undefined);
     }
   }
-  
+
   /**
    * Build a constant instruction. As above, the destination name is optional.
    */
@@ -124,6 +124,32 @@ export class Builder {
     this.curFunction.instrs.push(instr);
   }
 
+  /**
+   * Checks whether the last emitted instruction in the current function is the specified op code.
+   * Useful for checking for terminating instructions.
+   */
+  isLastEmittedEffectOp(op: bril.EffectOpCode): boolean {
+    if (!this.curFunction) {
+      return false
+    }
+
+    if (!this.curFunction.instrs) {
+      return false
+    }
+
+    if (this.curFunction.instrs.length === 0) {
+      return false
+    }
+
+    const last_instr : bril.Instruction | bril.Label = this.curFunction.instrs[this.curFunction.instrs.length - 1];
+
+    if ('label' in last_instr) {
+      return false
+    }
+
+    return last_instr.op === op
+  }
+
   /**
    * Generate an unused variable name.
    */

test/ts/factorial.out

@@ -16,9 +16,7 @@
 .then.0:
   v4: int = const 1;
   ret v4;
-  jmp .endif.0;
 .else.0:
-.endif.0:
   v5: int = id x;
   v6: int = id x;
   v7: int = const 1;

test/ts/simplified.out

@@ -0,0 +1,13 @@
+@main {
+  v0: int = const 1;
+  call @erronious v0;
+  v1: int = const 0;
+}
+@erronious(x: int): int {
+  v1: bool = const true;
+  br v1 .then.0 .else.0;
+.then.0:
+  v2: int = id x;
+  ret v2;
+.else.0:
+}

test/ts/simplified.ts

@@ -0,0 +1,8 @@
+erronious(1n);
+
+// This snippet checks that the ts compiler doesn't try to create an unreachable else branch
+function erronious(x: bigint): bigint {
+    if (true) {
+        return x;
+    }
+}

ts2bril.ts

@@ -145,20 +145,20 @@ function emitBril(prog: ts.Node, checker: ts.TypeChecker): bril.Program {
 
         // Check if effect statement, i.e., a call that is not a subexpression
         if (call.parent.kind === ts.SyntaxKind.ExpressionStatement) {
-          builder.buildCall(callText, 
+          builder.buildCall(callText,
             values.map(v => v.dest));
           return builder.buildInt(0);  // Expressions must produce values
         } else {
           let decl = call.parent as ts.VariableDeclaration;
           let type = brilType(decl, checker);
           let name = (decl.name != undefined) ? decl.name.getText() : undefined;
           return builder.buildCall(
-            callText, 
-            values.map(v => v.dest), 
-            type, 
+            callText,
+            values.map(v => v.dest),
+            type,
             name,
           );
-        } 
+        }
       }
     default:
       throw `unsupported expression kind: ${expr.getText()}`;
@@ -215,12 +215,18 @@ function emitBril(prog: ts.Node, checker: ts.TypeChecker): bril.Program {
         // Statement chunks.
         builder.buildLabel(thenLab);
         emit(if_.thenStatement);
-        builder.buildEffect("jmp", [], undefined, [endLab]);
+        const then_branch_terminated = builder.isLastEmittedEffectOp("ret");
+        if (!then_branch_terminated) {
+          builder.buildEffect("jmp", [], undefined, [endLab]);
+        }
         builder.buildLabel(elseLab);
         if (if_.elseStatement) {
           emit(if_.elseStatement);
         }
-        builder.buildLabel(endLab);
+        // The else branch otherwise just falls through without needing a target label
+        if (!then_branch_terminated) {
+          builder.buildLabel(endLab);
+        }
 
         break;
       }
@@ -258,7 +264,7 @@ function emitBril(prog: ts.Node, checker: ts.TypeChecker): bril.Program {
         break;
       }
 
-      case ts.SyntaxKind.FunctionDeclaration: 
+      case ts.SyntaxKind.FunctionDeclaration:
         let funcDef = node as ts.FunctionDeclaration;
         if (funcDef.name === undefined) {
           throw `no anonymous functions!`;
@@ -298,7 +304,7 @@ function emitBril(prog: ts.Node, checker: ts.TypeChecker): bril.Program {
 
       case ts.SyntaxKind.ImportDeclaration:
         break;
-  
+
       default:
         throw `unhandled TypeScript AST node kind ${ts.SyntaxKind[node.kind]}`;
     }