Compile time function execution...

[Dave]

Andrei Alexandrescu (See Website For Email) wrote:
> Walter Bright wrote:
>> janderson wrote:
>>> Walter Bright wrote:
>>>> Right now, the compiler will fail if the compile time execution 
>>>> results in infinite recursion or an infinite loop. I'll have to 
>>>> figure out some way to eventually deal with this.
>>>
>>> Maybe you could allow the user to specify stack size and maximum 
>>> iteration per loop/recursion function to the compiler as flags (with 
>>> some defaults).   This way the user can up the size if they really 
>>> need it.  This would make it a platform thing.  That way a D compiler 
>>> could still be made for less powerful systems.
>>
>> Whether you tell it to fail at a smaller limit, or it fails by itself 
>> at a smaller limit, doesn't make any difference as to whether it runs 
>> on a less powerful system or not <g>.
>>
>> The C standard has these "minimum translation limits" for all kinds of 
>> things - number of lines, chars in a string, expression nesting level, 
>> etc. It's all kind of bogus, hearkening back to primitive compilers 
>> that actually used fixed array sizes internally (Brand X, 
>> who-shall-not-be-named, was notorious for exceeding internal table 
>> limits, much to the delight of Zortech's sales staff). The right way 
>> to build a compiler is it either runs out of stack or memory, and 
>> that's the only limit.
>>
>> If your system is too primitive to run the compiler, you use a cross 
>> compiler running on a more powerful machine.
>>
>> I have thought of just putting a timer in the interpreter - if it runs 
>> for more than a minute, assume things have gone terribly awry and quit 
>> with a message.
> 
> That could be achieved with a watchdog process without changing the 
> compiler, and it's more flexible.
> 
> I think you just let the compiler go and crunch at it. Since you 
> esentially have partial evaluation anyway, the execution process can be 
> seen as extended to compile time. If you have a non-terminating program, 
> that non-termination can be naturally manifest itself during 
> compilation=partial evaluation.
> 

Completely agree, otherwise it contradicts your "right way to build a compiler" statement except 
with time as the metric instead of memory. Imagine the frustration of someone who has legitimate 
code and the compiler always craps out half-way through a looong makefile, or worse only sometimes 
craps out depending on machine load.

I think the best solution is to list out any compile-time execution with the '-v' switch. That way 
if someone runs into this, they can throw -v and find out where it's happening.

> 
> Andrei