Bug: wrong jump threading optimization in out-of-line functions

[cardillan]

Jump Threading optimizer is supposed to replace an unconditional jump to the function return instruction with the return instruction itself. There's a but that results in replacing conditional jump with the return instruction too. The bug only manifests in a non-inlined function that has several exit points, for example:

noinline def median(x1, x2, x3, x4, x5)
    y1 = min(x1, x2);
    y2 = max(x1, x2);
    y4 = min(x4, x5);
    y5 = max(x4, x5);

    if y4 < y1 then
        swap(out y1, out y4);
        swap(out y2, out y5);
    end;

    if x3 > y2 then
        y2 < y4 ? min(x3, y4) : min(y2, y5);
    else
        x3 > y4 ? min(x3, y5) : min(y2, y4);
    end;
end;

inline def swap(in out a, in out b)
    t = a; a = b; b = t;
end;

print(median(1,2,3,4,5));

This prints 2 instead of 3.

The bug probably doesn't manifest itself very often: the function needs to be non-inlined and sufficiently complex for the bug to occur.

As a workaround, setting Jump Threading to basic fixes the issue:

#set jump-threading=basic;

It might be better to keep it set until the fix is ready.