perldebguts.pod

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    # Literals
    EXACT	sv	Match this string (preceded by length).
    EXACTF	sv	Match this string, folded (prec. by length).
    EXACTFL	sv	Match this string, folded in locale (w/len).

    # Do nothing
    NOTHING	no	Match empty string.
    # A variant of above which delimits a group, thus stops optimizations
    TAIL	no	Match empty string. Can jump here from outside.

    # STAR,PLUS	'?', and complex '*' and '+', are implemented as circular
    #		BRANCH structures using BACK.  Simple cases (one character
    #		per match) are implemented with STAR and PLUS for speed
    #		and to minimize recursive plunges.
    #
    STAR	node	Match this (simple) thing 0 or more times.
    PLUS	node	Match this (simple) thing 1 or more times.

    CURLY	sv 2	Match this simple thing {n,m} times.
    CURLYN	no 2	Match next-after-this simple thing 
    #			{n,m} times, set parens.
    CURLYM	no 2	Match this medium-complex thing {n,m} times.
    CURLYX	sv 2	Match this complex thing {n,m} times.

    # This terminator creates a loop structure for CURLYX
    WHILEM	no	Do curly processing and see if rest matches.

    # OPEN,CLOSE,GROUPP	...are numbered at compile time.
    OPEN	num 1	Mark this point in input as start of #n.
    CLOSE	num 1	Analogous to OPEN.

    REF		num 1	Match some already matched string
    REFF	num 1	Match already matched string, folded
    REFFL	num 1	Match already matched string, folded in loc.

    # grouping assertions
    IFMATCH	off 1 2	Succeeds if the following matches.
    UNLESSM	off 1 2	Fails if the following matches.
    SUSPEND	off 1 1	"Independent" sub-regex.
    IFTHEN	off 1 1	Switch, should be preceded by switcher .
    GROUPP	num 1	Whether the group matched.

    # Support for long regex
    LONGJMP	off 1 1	Jump far away.
    BRANCHJ	off 1 1	BRANCH with long offset.

    # The heavy worker
    EVAL	evl 1	Execute some Perl code.

    # Modifiers
    MINMOD	no	Next operator is not greedy.
    LOGICAL	no	Next opcode should set the flag only.

    # This is not used yet
    RENUM	off 1 1	Group with independently numbered parens.

    # This is not really a node, but an optimized away piece of a "long" node.
    # To simplify debugging output, we mark it as if it were a node
    OPTIMIZED	off	Placeholder for dump.

=for unprinted-credits
Next section M-J. Dominus ([e-mail]) 20010421

Following the optimizer information is a dump of the offset/length
table, here split across several lines:

  Offsets: [45]
  	1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
  	0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
  	11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
  	0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]  

The first line here indicates that the offset/length table contains 45
entries.  Each entry is a pair of integers, denoted by C<offset[length]>.
Entries are numbered starting with 1, so entry #1 here is C<1[4]> and
entry #12 is C<5[1]>.  C<1[4]> indicates that the node labeled C<1:>
(the C<1: ANYOF[bc]>) begins at character position 1 in the
pre-compiled form of the regex, and has a length of 4 characters.
C<5[1]> in position 12 
indicates that the node labeled C<12:>
(the C<< 12: EXACT <d> >>) begins at character position 5 in the
pre-compiled form of the regex, and has a length of 1 character.
C<12[1]> in position 14 
indicates that the node labeled C<14:>
(the C<< 14: CURLYX[0] {1,32767} >>) begins at character position 12 in the
pre-compiled form of the regex, and has a length of 1 character---that
is, it corresponds to the C<+> symbol in the precompiled regex.

C<0[0]> items indicate that there is no corresponding node.

=head2 Run-time output

First of all, when doing a match, one may get no run-time output even
if debugging is enabled.  This means that the regex engine was never
entered and that all of the job was therefore done by the optimizer.

If the regex engine was entered, the output may look like this:

  Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
    Setting an EVAL scope, savestack=3
     2 <ab> <cdefg__gh_>    |  1: ANYOF
     3 <abc> <defg__gh_>    | 11: EXACT <d>
     4 <abcd> <efg__gh_>    | 13: CURLYX {1,32767}
     4 <abcd> <efg__gh_>    | 26:   WHILEM
				0 out of 1..32767  cc=effff31c
     4 <abcd> <efg__gh_>    | 15:     OPEN1
     4 <abcd> <efg__gh_>    | 17:     EXACT <e>
     5 <abcde> <fg__gh_>    | 19:     STAR
			     EXACT <f> can match 1 times out of 32767...
    Setting an EVAL scope, savestack=3
     6 <bcdef> <g__gh__>    | 22:       EXACT <g>
     7 <bcdefg> <__gh__>    | 24:       CLOSE1
     7 <bcdefg> <__gh__>    | 26:       WHILEM
				    1 out of 1..32767  cc=effff31c
    Setting an EVAL scope, savestack=12
     7 <bcdefg> <__gh__>    | 15:         OPEN1
     7 <bcdefg> <__gh__>    | 17:         EXACT <e>
       restoring \1 to 4(4)..7
				    failed, try continuation...
     7 <bcdefg> <__gh__>    | 27:         NOTHING
     7 <bcdefg> <__gh__>    | 28:         EXACT <h>
				    failed...
				failed...

The most significant information in the output is about the particular I<node>
of the compiled regex that is currently being tested against the target string.
The format of these lines is

C<    >I<STRING-OFFSET> <I<PRE-STRING>> <I<POST-STRING>>   |I<ID>:  I<TYPE>

The I<TYPE> info is indented with respect to the backtracking level.
Other incidental information appears interspersed within.

=head1 Debugging Perl memory usage

Perl is a profligate wastrel when it comes to memory use.  There
is a saying that to estimate memory usage of Perl, assume a reasonable
algorithm for memory allocation, multiply that estimate by 10, and
while you still may miss the mark, at least you won't be quite so
astonished.  This is not absolutely true, but may provide a good
grasp of what happens.

Assume that an integer cannot take less than 20 bytes of memory, a
float cannot take less than 24 bytes, a string cannot take less
than 32 bytes (all these examples assume 32-bit architectures, the
result are quite a bit worse on 64-bit architectures).  If a variable
is accessed in two of three different ways (which require an integer,
a float, or a string), the memory footprint may increase yet another
20 bytes.  A sloppy malloc(3) implementation can inflate these
numbers dramatically.

On the opposite end of the scale, a declaration like

  sub foo;

may take up to 500 bytes of memory, depending on which release of Perl
you're running.

Anecdotal estimates of source-to-compiled code bloat suggest an
eightfold increase.  This means that the compiled form of reasonable
(normally commented, properly indented etc.) code will take
about eight times more space in memory than the code took
on disk.

The B<-DL> command-line switch is obsolete since circa Perl 5.6.0
(it was available only if Perl was built with C<-DDEBUGGING>).
The switch was used to track Perl's memory allocations and possible
memory leaks.  These days the use of malloc debugging tools like
F<Purify> or F<valgrind> is suggested instead.  See also
L<perlhack/PERL_MEM_LOG>.

One way to find out how much memory is being used by Perl data
structures is to install the Devel::Size module from CPAN: it gives
you the minimum number of bytes required to store a particular data
structure.  Please be mindful of the difference between the size()
and total_size().

If Perl has been compiled using Perl's malloc you can analyze Perl
memory usage by setting the $ENV{PERL_DEBUG_MSTATS}.

=head2 Using C<$ENV{PERL_DEBUG_MSTATS}>

If your perl is using Perl's malloc() and was compiled with the
necessary switches (this is the default), then it will print memory
usage statistics after compiling your code when C<< $ENV{PERL_DEBUG_MSTATS}
> 1 >>, and before termination of the program when C<<
$ENV{PERL_DEBUG_MSTATS} >= 1 >>.  The report format is similar to
the following example:

  $ PERL_DEBUG_MSTATS=2 perl -e "require Carp"
  Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
     14216 free:   130   117    28     7     9   0   2     2   1 0 0
		437    61    36     0     5
     60924 used:   125   137   161    55     7   8   6    16   2 0 1
		 74   109   304    84    20
  Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
  Memory allocation statistics after execution:   (buckets 4(4)..8188(8192)
     30888 free:   245    78    85    13     6   2   1     3   2 0 1
		315   162    39    42    11
    175816 used:   265   176  1112   111    26  22  11    27   2 1 1
		196   178  1066   798    39
  Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.

It is possible to ask for such a statistic at arbitrary points in
your execution using the mstat() function out of the standard
Devel::Peek module.

Here is some explanation of that format:

=over 4

=item C<buckets SMALLEST(APPROX)..GREATEST(APPROX)>

Perl's malloc() uses bucketed allocations.  Every request is rounded
up to the closest bucket size available, and a bucket is taken from
the pool of buckets of that size.

The line above describes the limits of buckets currently in use.
Each bucket has two sizes: memory footprint and the maximal size
of user data that can fit into this bucket.  Suppose in the above
example that the smallest bucket were size 4.  The biggest bucket
would have usable size 8188, and the memory footprint would be 8192.

In a Perl built for debugging, some buckets may have negative usable
size.  This means that these buckets cannot (and will not) be used.
For larger buckets, the memory footprint may be one page greater
than a power of 2.  If so, case the corresponding power of two is
printed in the C<APPROX> field above.

=item Free/Used

The 1 or 2 rows of numbers following that correspond to the number
of buckets of each size between C<SMALLEST> and C<GREATEST>.  In
the first row, the sizes (memory footprints) of buckets are powers
of two--or possibly one page greater.  In the second row, if present,
the memory footprints of the buckets are between the memory footprints
of two buckets "above".

For example, suppose under the previous example, the memory footprints
were

     free:    8     16    32    64    128  256 512 1024 2048 4096 8192
	   4     12    24    48    80

With non-C<DEBUGGING> perl, the buckets starting from C<128> have
a 4-byte overhead, and thus an 8192-long bucket may take up to
8188-byte allocations.

=item C<Total sbrk(): SBRKed/SBRKs:CONTINUOUS>

The first two fields give the total amount of memory perl sbrk(2)ed
(ess-broken? :-) and number of sbrk(2)s used.  The third number is
what perl thinks about continuity of returned chunks.  So long as
this number is positive, malloc() will assume that it is probable
that sbrk(2) will provide continuous memory.

Memory allocated by external libraries is not counted.

=item C<pad: 0>

The amount of sbrk(2)ed memory needed to keep buckets aligned.

=item C<heads: 2192>

Although memory overhead of bigger buckets is kept inside the bucket, for
smaller buckets, it is kept in separate areas.  This field gives the
total size of these areas.

=item C<chain: 0>

malloc() may want to subdivide a bigger bucket into smaller buckets.
If only a part of the deceased bucket is left unsubdivided, the rest
is kept as an element of a linked list.  This field gives the total
size of these chunks.

=item C<tail: 6144>

To minimize the number of sbrk(2)s, malloc() asks for more memory.  This
field gives the size of the yet unused part, which is sbrk(2)ed, but
never touched.

=back

=head1 SEE ALSO

L<perldebug>,
L<perlguts>,
L<perlrun>
L<re>,
and
L<Devel::DProf>.