Debugging CTFE

Don nospam at
Thu Jun 19 03:11:55 PDT 2008

Matthias Walter wrote:
> Don Wrote:
>> Jarrett Billingsley wrote:
>>> "Matthias Walter" <Matthias.Walter at> wrote in message 
>>> news:g38na9$2gs7$1 at
>>>> Hello,
>>>> I have written some compile time executable functions with D 1.0 which 
>>>> work in runtime but hang (and allocate memory without end) at 
>>>> compile-time. Is there a way to debug this further? Can one print stuff 
>>>> out? (Don't know if writefln works at compile-time, as I'm using Tango) 
>>>> Can I somehow get a stack trace of the functions called?
>>> No, no, and no.  CTFE support in the current frontend has some rather 
>>> unworkable disadvantages.  For one, it's terribly buggy.You're better off 
>>> trying to convert it to templates in most cases.
>> I disagree. I've not had much trouble with CTFE. The really, really nice 
>> thing about CTFE is that you can write it as a runtime function, and 
>> make sure it works before using it a compile time.
>> The problem with CTFE is bugzilla issue #1382 (no memory release for 
>> CTFE functions)...
>> For two, CTFE is
>>> interpreting a garbage-collected language but is not itself 
>>> garbage-collected, meaning that memory-unconscious code evaluated at compile 
>>> time (i.e. a loop that appends data to the end of a string) will just leak 
>>> like hell and cause the compiler to easily use up several GB of memory. 
>>> That might be what's happening to your code.
>> That's very likely. The maximum size of code you can write with CTFE is 
>> pretty small.
>>>  Or it could be a bug in CTFE. 
>> Bugzilla issue #1382 is the killer. One of the most important bugs in 
>> bugzilla, I reckon.
> But in my case, the runtime-function needed some milliseconds, allocating not more than 1 MB data. the compiler then allocated 1 GB until I killed the process. I guess, there might be some other bugs. Unfortunatly, I was unable to make proofing code more easy to get the cause of the bug.

That'll be issue #1382. You can only do a few thousand memory 
allocations (including ~) before CTFE dies.

Basically, you can use CTFE for very small tasks, and for library 
development. Because of #1382, you can't use it large-scale stuff yet.

> Anyway, I now don't use functions for much stuff, but try to achieve my needs with templates, although template programming is sometimes a bit messier. But it works and debugging via pragmas is like printf-debugging in C and thus just a matter of time :) Training more functional thinking is also helpful for other things...
> best regards and thanks for the tips
> Matthias

More information about the Digitalmars-d-learn mailing list