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

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           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.


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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.


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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?


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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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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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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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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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--- Comment #8 from Don <clugdbug yahoo.com.au> 2011-09-02 00:21:14 PDT ---
(In reply to comment #7)

 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)


This was fixed in 2.054. There were several cases where reading or writing a
single array element could cause the entire array to be copied! These cases
have now been fixed, giving an order of magnitude improvement in memory use and
compilation time. (The original test case (concatenation) hasn't changed; it's
simply caused by absence of a compile-time gc).
This bug is now a far less serious problem than bug 6498.

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Jesse Phillips <Jesse.K.Phillips+D gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Jesse.K.Phillips+D gmail.co
                   |                            |m
   Target Milestone|---                         |2.059


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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|2.059                       |---


--- Comment #9 from Don <clugdbug yahoo.com.au> 2012-01-21 01:26:54 PST ---
Please don't set milestones without consultation (unless you plan to fix the
bug yourself). This bug is still open because it is HARD.
I've been slowly making progress on it for the last year. It's not going to be
fixed soon -- the remaining work to be done is still about the equivalent of 30
avarage bugs. However, 90% of the symptoms were fixed in 2.049.

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Leandro Lucarella <leandro.lucarella sociomantic.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |leandro.lucarella sociomant
                   |                            |ic.com


--- Comment #10 from Leandro Lucarella <leandro.lucarella sociomantic.com>
2012-01-23 02:41:48 PST ---
Is there any technical reason not to use the Bohem GC as a temporary workaround
until this can get properly fixed? I'm just curious.

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--- Comment #11 from Walter Bright <bugzilla digitalmars.com> 2012-01-23
11:23:06 PST ---
I made an experimental build of dmd that uses a gc. The compiler slowed down
quite a bit.

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--- Comment #12 from Leandro Lucarella <leandro.lucarella sociomantic.com>
2012-01-24 02:19:04 PST ---
(In reply to comment #11)

 I made an experimental build of dmd that uses a gc. The compiler slowed down
 quite a bit.


In which cases did you tried it? For files that allocates a lot of "CTFE
memory" it should be the other way around, as the memory consumption is so high
that the system is using most of the time moving things around between the
memory and the swap.

Do you have a patch that I can try (for D1)? Thanks.

As bad as it sounds, maybe a good tradeoff would be to add a command line
option (as obscure an undocumented as you want) to activate the GC for cases
where not using it is not really an option. Being that it seems that this bug
is really hard, I think it might deserve looking for a workaround to be able to
use the compiler in this extreme cases in the short term.

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--- Comment #13 from Walter Bright <bugzilla digitalmars.com> 2012-01-24
02:29:36 PST ---
I tried it by building the library and running its unittests, and running the
test suite. It was considerably slower.

The GC used was the old C++ version of the D runtime GC.

You can build it by switching the GCOBJS macro in win32.mak.

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--- Comment #14 from Leandro Lucarella <leandro.lucarella sociomantic.com>
2012-01-24 02:37:46 PST ---
(In reply to comment #13)

 I tried it by building the library and running its unittests, and running the
 test suite. It was considerably slower.
 
 The GC used was the old C++ version of the D runtime GC.
 
 You can build it by switching the GCOBJS macro in win32.mak.


Oh, I was talking about the Bohem GC, the one tried by Christian Kamm, which is
a pretty good state of the art collector AFAIK. I think LDC used it (I don't
know if it still does) with pretty good results (see comment 3).

Maybe Christian can give us some more information about it :)

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--- Comment #15 from Don <clugdbug yahoo.com.au> 2012-01-24 06:21:47 PST ---
An important thing to realize about this bug is that it is *not* the primary
cause of slow performance and high memory consumption in CTFE.
Fixing this bug would make very little difference, except in cases involving
concatenation.

I think it's had a lot of votes because people think it's the key CTFE
performance issue, but actually the bad guy is bug 6498. Which is easier to
fix.

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