Unreachability

[Paul Sandoz]

Hi,

Gary encountered an issue when transforming say:

    @CodeReflection
    public static float f(float input) {
        if (input >= 0f) {
            return 1f;
        } else {
            return 0f;
        }
        // Unreachable
    }

Compiler produced model:

func @"f" @loc="10:5:..." (%0 : float)float -> {
    %1 : Var<float> = var %0 @"input" @loc="10:5";
    java.if @loc="13:9"
        ()boolean -> {
            %2 : float = var.load %1 @loc="13:13";
            %3 : float = constant @"0.0" @loc="13:22";
            %4 : boolean = ge %2 %3 @loc="13:13";
            yield %4 @loc="13:9";
        }
        ()void -> {
            %5 : float = constant @"1.0" @loc="14:20";
            return %5 @loc="14:13";
        }
        ()void -> {
            %6 : float = constant @"0.0" @loc="16:20";
            return %6 @loc="16:13";
        };
    return @loc="10:5";
};

Lowered model:

func @"f" @loc="10:5:..." (%0 : float)float -> {
    %1 : Var<float> = var %0 @"input" @loc="10:5";
    %2 : float = var.load %1 @loc="13:13";
    %3 : float = constant @"0.0" @loc="13:22";
    %4 : boolean = ge %2 %3 @loc="13:13";
    cbranch %4 ^block_1 ^block_2;
  
  ^block_1:
    %5 : float = constant @"1.0" @loc="14:20";
    return %5 @loc="14:13";
  
  ^block_2:
    %6 : float = constant @"0.0" @loc="16:20";
    return %6 @loc="16:13";
  
  ^block_3:
    return @loc="10:5";
};


The compiler produced model is incorrect placing a return operation when it should be an unreachable operation.

There are some reminders in ReflectMethods:

        CoreOp.FuncOp scanMethod() {
            scan(body);
            // @@@ Check if unreachable
            appendTerminating(CoreOp::_return);
            CoreOp.FuncOp func = CoreOp.func(name.toString(), stack.body);
            func.setLocation(generateLocation(currentNode, true));
            return func;
        }

We need to determine if the last statement yields control or not. (It’s easy to see if we lower, as ^block_3 is never referenced.)

In this case we can know because there is no return value in scope, but generally we cannot rely upon that.

The compiler already checks this, I believe in Flow.AliveAnalyzer. However, it does not add additional information to the attributed tree - it does not need to.

Can we reuse the method Flow.aliveAfter? Although I don’t know how to get access to an instance of Env<AttrContext>. Do we get it from TypeEnvs?

Paul.