Schrodinger's CTFE

[Andrei Alexandrescu]

On 7/15/20 6:58 PM, Steven Schveighoffer wrote:
> On 7/15/20 6:44 PM, Andrei Alexandrescu wrote:
>> On 7/15/20 4:33 PM, Steven Schveighoffer wrote:
>>>
>>> int foo(bool x)()
>>> {
>>>     if(x && (__ctfe || !__ctfe)) return 1;
>>>     return 2;
>>> }
>>>
>>> I think this is a way to fix some of those static foreach loops with 
>>> returns which can have these reachability problems. Only, it's really 
>>> verbose and ugly. Maybe we still should have that other variable?
>>
>> Would a function work?
>>
>> @property
>> bool __schro() pure nothrow @nogc { return __ctfe || !__ctfe; }
>>
> 
> I actually tried that, and it does prevent the "Unreachable statement" 
> problem, but the runtime code contains a call to that function where it 
> is used, which means that it's not as effective in all cases.
> 
> With a straight usage of the expression, you get (without -inline or -O):
> 
> pure nothrow @nogc @safe int onlineapp.foo!(false).foo():
>          push    RBP
>          mov    RBP,RSP
>          mov    EAX,2
>          pop    RBP
>          ret
>          add    [RAX],AL
> 
> pure nothrow @nogc @safe int onlineapp.foo!(true).foo():
>          push    RBP
>          mov    RBP,RSP
>          mov    EAX,1
>          pop    RBP
>          ret
>          add    [RAX],AL
> 
> With the schro function, you get the same result for foo!false (as the 
> short circuiting of the if statement prunes out the check of __ctfe), 
> but you get this for foo!true:
> 
> pure nothrow @nogc int onlineapp.foo!(true).foo():
>          push    RBP
>          mov    RBP,RSP
>          call      pure nothrow @property @nogc bool 
> onlineapp.schro()@PLT32
>          test    AL,AL
>          je    L14
>          mov    EAX,1
>          pop    RBP
>          ret
> L14:        mov    EAX,2
>          pop    RBP
>          ret
>          add    [RAX],AL
> 
> With -inline, it results as the same thing.
> 
> Note that if you put it in a function, you can just return true, you 
> don't need the __ctfe || !__ctfe.
> 
> -Steve

I got good news for you then: https://github.com/dlang/dmd/pull/11236