compile-time function evaluation consuming too much memory

[Thomas Kuehne]

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Christian Kamm schrieb am 2007-06-28:
> I have been experimenting with using ctfe and static foreach to generate
> code from strings. While for small strings all is fine, larger strings make
> DMD's memory usage go over the top. It came up when splitting a string into
> an array of strings - something I'd think is relatively common. Simple
> example that illustrates the problem:
>
> char[] make_empty_string(int n)
> {
>   char[] result;
>
>   for(int i = 0; i < n; ++i)
>     result ~= " ";
>   return result;
> }
>
> const char[] largestring1 = make_empty_string(10000);
> const char[] largestring2 = make_empty_string(10000);
> const char[] largestring3 = make_empty_string(10000);
> ...
>
> Every call to make_empty_string(10000) will consume 50 MB of memory which it
> seems will not be released until dmd terminates. That happens because every
> intermediate value in 'result' is stored.
>
> My question is if
> - this is expected and actually intentional as ctfe merely extends constant
> folding or if 
> - this is a bug and dmd could just free the old memory on array
> appends/assigns/... (would this ever be a problem?) or if
> - D needs a compile-time garbage collector. ;)

The memory performance of ctfe can definatly be improved, however your
sample code too. Yes I know the above was just a sample code to
demonstrate the issue but quite frequently that issue can be evaded by
using another algoritm:

# char[] make_empty_string(int n){
#    char[] result;
#
#    if(n){
#       result = " ";
#       while(result.length * 2 <= n){
#          result ~= result;
#       }
#       if(result.length < n){
#          result ~= result[0 .. n - $];
#       }
#    }
#
#    return result;
# }

Thomas


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