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digitalmars.D.bugs - [Issue 1382] New: memory allocated for arrays in CTFE functions during compilation is not released

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1382

           Summary: memory allocated for arrays in CTFE functions during
                    compilation is not released
           Product: D
           Version: 1.019
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: kamm-removethis incasoftware.de


This problem is encountered when large arrays are manipulated during CTFE.
Symtoms are dmd allocating a lot of memory, eating up all swap and finally
terminating with an out of memory error.

The core of the problem can be seen in this code
--
char[] make_empty_string(int n)
{
  char[] result;

  for(int i = 0; i < n; ++i)
    result ~= " ";
  return result;
}

const char[] largestring = make_empty_string(100000);
void main() {}
---

This snippet will require 5 gb of memory to compile instead of the mere 100 kb
the final string will require. It is caused by the intermediate strings stored
in result during the iteration never being discarded. The problem is not
limited to concatenation, modifications of a single element of the array will
also cause the whole array to be duplicated. 

While this particular piece of code can be rewritten to consume less memory,
that's not generally possible. An example where reduction is not possible is
splitting a string into substrings.

It does come up in practice: Someone wanted to generate a function to get the
Unicode general catergory for a dchar from the textfiles from the Unicode
Character Database and ran into this issue. I wanted to parse the D BNF to
generate a parser at compile time and had dmd exit with an out of memory error.

It seems CTFE needs a compile time garbage collector.


-- 
Jul 27 2007
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1382


clugdbug yahoo.com.au changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |blocker




------- Comment #1 from clugdbug yahoo.com.au  2008-04-29 05:30 -------
I'm raising the severity of this to blocker, since it makes CTFE
metaprogramming libraries unusable in practice. Fortunately development of such
libraries is still possible, although both BCS and I are experiencing some
horrific compilation times, even after inserting workarounds where possible. We
are having to reduce the complexity of our test code.


-- 
Apr 29 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
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------- Comment #2 from kamm-removethis incasoftware.de  2008-07-22 00:52
-------
Has DMD stopped using boehm-gc? If I try this very example on LLVMDC (which
does use boehm-gc), memory usage never exceeds a certain level (< 1 MB).

So maybe re-enabling the garbage collector for DMD will fix all CTFE related
memory issues?


-- 
Jul 21 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
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------- Comment #3 from kamm-removethis incasoftware.de  2008-12-02 12:31
-------
We've had some success with reenabling boehm-gc:
http://www.dsource.org/projects/ldc/ticket/49 .

"Another test with USE_BOEHM_GC=0, REDIRECT_MALLOC=GC_malloc and IGNORE_FREE
seemed to yield good results, with no segfaults and collecting CTFE memory
properly."


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Dec 02 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au




--- Comment #4 from Don <clugdbug yahoo.com.au>  2009-08-03 05:18:25 PDT ---
I don't think Boehm gc is the answer. Note that this is very closely related to
bug#1330. I think the CTFE implementation of arrays needs (a) reference
semantics and (b) reference counting. Here's an example of a terrible case,
which allocates several Gb of RAM:

int junk(int n)
{
  int[] result = new int[10000];

  for(int i = 0; i < n; ++i) {
    result[0]= i;
  }
  return 0;
}

const int bad = junk(100000);
void main() {}

This particular case could be solved by adding a reference-based system for
storing array values, instead of doing copy-on-write -- and that's required for
bug #1330 anyway.
Once that's in place, the array values could be allocated in a controlled
manner (eg, retain a list of all allocated CTFE arrays). A dedicated precise GC
can then be simple and fast, since it only needs to check for array references
in the current function, and they can only be in the local variables which are
arrays or structs.

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Aug 03 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|blocker                     |critical




--- Comment #5 from Don <clugdbug yahoo.com.au>  2009-08-31 23:59:27 PDT ---
Reducing severity back to critical, since the voting system takes care of the
importance.

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bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs eml.cc


--- Comment #6 from bearophile_hugs eml.cc 2010-06-04 15:19:11 PDT ---
A partially artificial test case. Faster and better versions are quite
possible, but I expect dmd to be able to run this quickly.


import std.stdio: writeln;

ubyte[1 << NPOW] setBits(int NPOW)() {
    nothrow pure uint setBits8(uint n) {
        uint result;
        foreach (i; 0 .. 8)
            if (n & (1 << i))
                result++;
        return result;
    }

    nothrow pure uint setBits16(uint n) {
        enum uint FIRST_UBYTE =  0b0000_0000_1111_1111;
        enum uint SECOND_UBYTE = 0b1111_1111_0000_0000;
        return setBits8(n & FIRST_UBYTE) + setBits8((n & SECOND_UBYTE) >> 8);
    }

    typeof(return) result;
    foreach (i; 1 .. result.length)
        result[i] = cast(typeof(result[0]))setBits16(i);
    return result;
}

enum nbits = setBits!16(); // currently 12 is about the max

void main() {
    writeln(nbits);
}

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Jun 04 2010
prev sibling parent d-bugmail puremagic.com writes:
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Rob Jacques <sandford jhu.edu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sandford jhu.edu


--- Comment #7 from Rob Jacques <sandford jhu.edu> 2010-12-06 12:09:04 PST ---
I just came across this bug while  working on improving std.variant: the
combination of templates + ctfe + unittests resulted in out of memory errors.
I've also traced down another issue (I don't know if it should be filed
separately or not):

It appears that _any_ access of an array variable allocates ram, resulting in
drastically slower compile times (+55 seconds) and excess memory usage (30+ mb
in this case using DMD 2.050)

string ctfeTest() {
    char[] result;
    result.length = ushort.max;
    char c;
    for(size_t i = 0; i < result.length; i++) {} // Allocates 
    for(size_t i = 0; i < ushort.max; i++) {}    // Doesn't allocate

    for(size_t i = 0; i < ushort.max; i++) {     // Allocates 
        c = result[i];
    }
    for(size_t i = 0; i < ushort.max; i++) {     // Doesn't allocate
        c = cast(ubyte)('A' + i%26);
    }
    return cast(string)result;
}

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