=head1 NAME
perloptree - The Perl op tree
=head1 DESCRIPTION
Various material about the internal Perl compilation representation
during parsing and optimization, before the actual execution
begins. The B<"B" op tree>.
The well-known L<perlguts.pod> focuses more on the internal
representation of the variables, but not so on the structure, the
sequence and the optimization of the basic operations, the ops.
And we have L<perlhack.pod>, which shows e.g. ways to hack into
the op tree structure within the debugger. It focuses on getting
people to start patching and hacking on the CORE, not
understanding or writing compiler backends or optimizations,
which the op tree mainly is used for. Btw., this here is merely
prose around the source comments. But who reads source?
=head1 Brief Summary
The brief summary is very well described in the
L<"Compiled-code"/perlguts
at the top of F<op.c>.
When Perl parses the source code (via Yacc C<perly.y>), the so-called
op tree, a tree of basic perl OP structs pointing to simple
C<pp_>I<opname> functions, is generated bottom-up. Those C<pp_>
functions - "PP Code" (for "Push / Pop Code") - have the same uniform
API as the XS functions, all arguments and return values are
transported on the stack. For example, an C<OP_CONST> op points to
the C<pp_const()> function and to an C<SV> containing the constant
value. When C<pp_const()> is executed, its job is to push that C<SV>
onto the stack.
OPs are created by the C<newFOO()> functions, which are called
from the parser (in F<perly.y>) as the code is parsed. For
example the Perl code C<$a + $b * $c> would cause the equivalent
of the following to be called (oversimplifying a bit):
newBINOP(OP_ADD, flags,
newSVREF($a),
newBINOP(OP_MULTIPLY, flags, newSVREF($b), newSVREF($c))
)
See also L<perlhack.pod
The simplest type of an op structure is C<OP>, a L</BASEOP>: this
has no children. Unary operators, L</UNOP>s, have one child, and
this is pointed to by the C<op_first> field. Binary operators
(L</BINOP>s) have not only an C<op_first> field but also an
C<op_last> field. The most complex type of op is a L</LISTOP>,
which has any number of children. In this case, the first child
is pointed to by C<op_first> and the last child by
C<op_last>. The children in between can be found by iteratively
following the C<op_sibling> pointer from the first child to the
last.
There are also two other op types: a L</"PMOP"> holds a regular
expression, and has no children, and a L</"LOOP"> may or may not
have children. If the C<op_sibling> field is non-zero, it behaves
like a C<LISTOP>. To complicate matters, if an C<UNOP> is
actually a null op after optimization (see L</"Compile pass 2:
context propagation"> below) it will still have children in
accordance with its former type.
The beautiful thing about the op tree representation is that it
is a strict 1:1 mapping to the actual source code, which is
proven by the L<B::Deparse> module, which generates readable
source for the current op tree. Well, almost.
=head1 The Compiler
Perl's compiler is essentially a 3-pass compiler with interleaved
phases:
1. A bottom-up pass
2. A top-down pass
3. An execution-order pass
=head2 Compile pass 1: check routines and constant folding
The bottom-up pass is represented by all the C<"newOP"> routines
and the C<ck_> routines. The bottom-upness is actually driven by
F<yacc>. So at the point that a C<ck_> routine fires, we have no
idea what the context is, either upward in the syntax tree, or
either forward or backward in the execution order. The bottom-up
parser builds that part of the execution order it knows about,
but if you follow the "next" links around, you'll find it's
actually a closed loop through the top level node.
So when creating the ops in the first step, still bottom-up, for
each op a check function (C<ck_ ()>) is called, which which
theroretically may destructively modify the whole tree, but
because it knows almost nothing, it mostly just nullifies the
current op. Or it might set the L</op_next> pointer. See
L</"Check Functions"> for more.
Also, the subsequent constant folding routine C<fold_constants()>
may fold certain arithmetic op sequences. See L</"Constant Folding">
for more.
=head2 Compile pass 2: context propagation
The context determines the type of the return value. When a
context for a part of compile tree is known, it is propagated
down through the tree. At this time the context can have 5 values
(instead of 2 for runtime context): C<void>, C<boolean>,
C<scalar>, C<list>, and C<lvalue>. In contrast with the pass 1
this pass is processed from top to bottom: a node's context
determines the context for its children.
Whenever the bottom-up parser gets to a node that supplies
context to its components, it invokes that portion of the
top-down pass that applies to that part of the subtree (and marks
the top node as processed, so if a node further up supplies
context, it doesn't have to take the plunge again). As a
particular subcase of this, as the new node is built, it takes
all the closed execution loops of its subcomponents and links
them into a new closed loop for the higher level node. But it's
still not the real execution order.
I<Todo: Sample where this context flag is stored>
Additional context-dependent optimizations are performed at this
time. Since at this moment the compile tree contains back-references
(via "thread" pointers), nodes cannot be C<free()>d now. To allow
optimized-away nodes at this stage, such nodes are C<null()>ified
instead of C<free()>'ing (i.e. their type is changed to C<OP_NULL>).
=head2 Compile pass 3: peephole optimization
The actual execution order is not known till we get a grammar
reduction to a top-level unit like a subroutine or file that will
be called by "name" rather than via a "next" pointer. At that
point, we can call into peep() to do that code's portion of the
3rd pass. It has to be recursive, but it's recursive on basic
blocks, not on tree nodes.
So finally, when the full parse tree is generated, the "peephole
optimizer" C<peep()> is running. This pass is neither top-down
or bottom-up, but in the execution order (with additional
complications for conditionals).
This examines each op in the tree and attempts to determine "local"
optimizations by "thinking ahead" one or two ops and seeing if
multiple operations can be combined into one (by nullifying and
re-ordering the next pointers).
It also checks for lexical issues such as the effect of C<use
strict> on bareword constants. Note that since the last walk the
early sibling pointers for recursive (bottom-up) meta-inspection
are useless, the final exec order is guaranteed by the next and
flags fields.
=head1 basic vs exec order
The highly recursive Yacc parser generates the initial op tree in
B<basic> order. To save memory and run-time the final execution
order of the ops in sequential order is not copied around, just
the next pointers are rehooked in C<Perl_linklist()> to the
so-called B<exec> order. So the exec walk through the
linked-list of ops is not too cache-friendly.
In detail C<Perl_linklist()> traverses the op tree, and sets
op-next pointers to give the execution order for that op
tree. op-sibling pointers are rarely unneeded after that.
Walkers can run in "basic" or "exec" order. "basic" is useful
for the memory layout, it contains the history, "exec" is more
useful to understand the logic and program flow. The
L</B::Bytecode> section has an extensive example about the order.
=head1 OP Structure and Inheritance
The basic C<struct op> looks basically like
C<{ OP* op_next, OP* op_sibling, OP* op_ppaddr, ..., int op_flags, int op_private } OP;>
See L</BASEOP> below.
Each op is defined in size, arguments, return values, class and
more in the F<opcode.pl> table. (See L</"OP Class Declarations in
opcode.pl"> below.)
The class of an OP determines its size and the number of
children. But the number and type of arguments is not so easy to
declare as in C. F<opcode.pl> tries to declare some XS-prototype
like arguments, but in lisp we would say most ops are "special"
functions, context-dependent, with special parsing and precedence rules.
classes and inheritance:
@B::OP::ISA = 'B::OBJECT';
@B::UNOP::ISA = 'B::OP';
@B::BINOP::ISA = 'B::UNOP';
@B::LOGOP::ISA = 'B::UNOP';
@B::LISTOP::ISA = 'B::BINOP';
@B::SVOP::ISA = 'B::OP';
@B::PADOP::ISA = 'B::OP';
@B::PVOP::ISA = 'B::OP';
@B::LOOP::ISA = 'B::LISTOP';
@B::PMOP::ISA = 'B::LISTOP';
@B::COP::ISA = 'B::OP';
@B::SPECIAL::ISA = 'B::OBJECT';
@B::optype = qw(OP UNOP BINOP LOGOP LISTOP PMOP SVOP PADOP PVOP LOOP COP);
I<TODO: ascii graph from perlguts>
contains all the gory details. Let's check it out:
=head2 OP Class Declarations in opcode.pl
The full list of op declarations is defined as C<DATA> in
F<opcode.pl>. It defines the class, the name, some flags, and
the argument types, the so-called "operands". C<make regen> (via
F<regen.pl>) recreates out of this DATA table the files
F<opcode.h>, F<opnames.h>, F<pp_proto.h> and F<pp.sym>.
The class signifiers in F<opcode.pl> are:
baseop - 0 unop - 1 binop - 2
logop - | listop - @ pmop - /
padop/svop - $ padop -
baseop/unop - % loopexop - } filestatop - -
pvop/svop - " cop - ;
Other options within F<opcode.pl> are:
needs stack mark - m
needs constant folding - f
produces a scalar - s
produces an integer - i
needs a target - t
target can be in a pad - T
has a corresponding integer version - I
has side effects - d
uses $_ if no argument given - u
Values for the operands are:
scalar - S list - L array - A
hash - H sub (CV) - C file - F
socket - Fs filetest - F- reference - R
"?" denotes an optional operand.
=head2 BASEOP
All op classes have a single character signifier for easier
definition in F<opcode.pl>. The BASEOP class signifier is B<0>,
for no children.
Below are the BASEOP fields, which reflect the object C<B::OP>,
since Perl 5.10. These are shared for all op classes. The parts
after C<op_type> and before C<op_flags> changed during history.
=over
=item op_next
Pointer to next op to execute after this one.
Top level pre-grafted op points to first op, but this is replaced
when op is grafted in, when this op will point to the real next
op, and the new parent takes over role of remembering the
starting op. I<Now, who wrote this prose? Anyway, that is why it
is called guts.>
=item op_sibling
Pointer to connect the children's list.
The first child is L</op_first>, the last is L</op_last>, and the
children in between are interconnected by op_sibling. This is at
run-time only used for L</LISTOP>s.
So why is it in the BASEOP struct carried around for every op?
Because of the complicated Yacc parsing and later optimization
order as explained in L<"Compile pass 1: check routines and
constant folding"> the L</op_next> pointers are not enough, so
op_sibling's are required. The final and fast execution order by
just following the op_next chain is expensive to calculate.
See
for a 20% space-reduction patch to get rid of it at run-time.
=item op_ppaddr
Pointer to current ppcode's function.
The so called "opcode".
=item op_madprop
Pointer to the MADPROP struct. Only with -DMAD, and since
5.10. See L</MAD> (Misc Attribute Decoration) below.
=item op_targ
PADOFFSET to "unnamed" op targets/GVs/constants, wasting no
SV. Has for some op's also a different meaning.
=item op_type
The type of the operation.
Since 5.10 we have the next five fields added, which replace
C<U16 op_seq>.
=item op_opt
"optimized"
Whether or not the op has been optimised by the peephole optimiser.
See the comments in C<S_clear_yystack()> in F<perly.c> for more
details on the following three flags. They are just for freeing
temporary ops on the stack. But we might have statically
allocated op in the data segment, esp. with the perl compiler's
L<B::C> module. Then we are not allowed to free those static
ops. For a short time, from 5.9.0 until 5.9.4, until the B::C
module was removed from CORE, we had another field here for this
reason: B<op_static>. On 1 it didn't free the static op. Before
5.9.0 the L</op_seq> field was used with the magic value B<-1> to
indicate a static op, not to be freed. Note: Trying to free a
static struct is considered harmful.
=item op_latefree
Tell C<op_free()> to clear this op (and free any kids) but not
yet deallocate the struct. This means that the op may be safely
C<op_free()>d multiple times.
On static ops you just set this to B<1> and after the first
C<op_free()> the C<op_latefreed> is automatically set and further
C<op_free()> called are just ignored.
=item op_latefreed
If 1, an C<op_latefree> op has been C<op_free()>d.
=item op_attached
This op (sub)tree has been attached to the CV C<PL_compcv> so it
doesn't need to be free'd.
=item op_spare
Three spare bits in this bitfield above. At least they survived 5.10.
Those last two fields have been in all perls:
=item op_flags
Flags common to all operations.
See C<OPf_*> in F<op.h>, or more verbose in L<B::Flags> or F<dump.c>
=item op_private
Flags peculiar to a particular operation (BUT, by default, set to
the number of children until the operation is privatized by a
check routine, which may or may not check number of children).
This flag is normally used to hold op specific context hints,
such as C<HINT_INTEGER>. This flag is directly attached to each
relevant op in the subtree of the context. Note that there's no
general context or class pointer for each op, a typical
functional language usually holds this in the ops arguments. So
we are limited to max 32 lexical pragma hints or less. See
L</Lexical Pragmas>.
=back
The exact op.h L</BASEOP> history for the parts after C<op_type> and
before C<op_flags> is:
<=5.8: U16 op_seq;
5.9.4: unsigned op_opt:1; unsigned op_static:1; unsigned op_spare:5;
>=5.10: unsigned op_opt:1; unsigned op_latefree:1; unsigned op_latefreed:1;
unsigned op_attached:1; unsigned op_spare:3;
The L</BASEOP> class signifier is B<0>, for no children.
The full list of all BASEOP's is:
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /0$/' opcode.pl
null null operation ck_null 0
stub stub ck_null 0
pushmark pushmark ck_null s0
wantarray wantarray ck_null is0
padsv private variable ck_null ds0
padav private array ck_null d0
padhv private hash ck_null d0
padany private value ck_null d0
sassign scalar assignment ck_sassign s0
unstack iteration finalizer ck_null s0
enter block entry ck_null 0
iter foreach loop iterator ck_null 0
break break ck_null 0
continue continue ck_null 0
fork fork ck_null ist0
wait wait ck_null isT0
getppid getppid ck_null isT0
time time ck_null isT0
tms times ck_null 0
ghostent gethostent ck_null 0
gnetent getnetent ck_null 0
gprotoent getprotoent ck_null 0
gservent getservent ck_null 0
ehostent endhostent ck_null is0
enetent endnetent ck_null is0
eprotoent endprotoent ck_null is0
eservent endservent ck_null is0
gpwent getpwent ck_null 0
spwent setpwent ck_null is0
epwent endpwent ck_null is0
ggrent getgrent ck_null 0
sgrent setgrent ck_null is0
egrent endgrent ck_null is0
getlogin getlogin ck_null st0
custom unknown custom operator ck_null 0
=head3 null
null ops are skipped during the runloop, and are created by the peephole optimizer.
=head2 UNOP
The unary op class signifier is B<1>, for one child, pointed to
by C<op_first>.
struct unop {
BASEOP
OP * op_first;
}
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /1$/' opcode.pl
rv2gv ref-to-glob cast ck_rvconst ds1
rv2sv scalar dereference ck_rvconst ds1
av2arylen array length ck_null is1
rv2cv subroutine dereference ck_rvconst d1
refgen reference constructor ck_spair m1 L
srefgen single ref constructor ck_null fs1 S
regcmaybe regexp internal guard ck_fun s1 S
regcreset regexp internal reset ck_fun s1 S
preinc preincrement (++) ck_lfun dIs1 S
i_preinc integer preincrement (++) ck_lfun dis1 S
predec predecrement (--) ck_lfun dIs1 S
i_predec integer predecrement (--) ck_lfun dis1 S
postinc postincrement (++) ck_lfun dIst1 S
i_postinc integer postincrement (++) ck_lfun disT1 S
postdec postdecrement (--) ck_lfun dIst1 S
i_postdec integer postdecrement (--) ck_lfun disT1 S
negate negation (-) ck_null Ifst1 S
i_negate integer negation (-) ck_null ifsT1 S
not not ck_null ifs1 S
complement 1's complement (~) ck_bitop fst1 S
rv2av array dereference ck_rvconst dt1
rv2hv hash dereference ck_rvconst dt1
flip range (or flip) ck_null 1 S S
flop range (or flop) ck_null 1
method method lookup ck_method d1
entersub subroutine entry ck_subr dmt1 L
leavesub subroutine exit ck_null 1
leavesublv lvalue subroutine return ck_null 1
leavegiven leave given block ck_null 1
leavewhen leave when block ck_null 1
leavewrite write exit ck_null 1
dofile do "file" ck_fun d1 S
leaveeval eval "string" exit ck_null 1 S
=head2 BINOP
The BINOP class signifier is B<2>, for two children, pointed to by
C<op_first> and C<op_last>.
struct binop {
BASEOP
OP * op_first;
OP * op_last;
}
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /2$/' opcode.pl
gelem glob elem ck_null d2 S S
aassign list assignment ck_null t2 L L
pow exponentiation (**) ck_null fsT2 S S
multiply multiplication (*) ck_null IfsT2 S S
i_multiply integer multiplication (*) ck_null ifsT2 S S
divide division (/) ck_null IfsT2 S S
i_divide integer division (/) ck_null ifsT2 S S
modulo modulus (%) ck_null IifsT2 S S
i_modulo integer modulus (%) ck_null ifsT2 S S
repeat repeat (x) ck_repeat mt2 L S
add addition (+) ck_null IfsT2 S S
i_add integer addition (+) ck_null ifsT2 S S
subtract subtraction (-) ck_null IfsT2 S S
i_subtract integer subtraction (-) ck_null ifsT2 S S
concat concatenation (.) or string ck_concat fsT2 S S
left_shift left bitshift (<<) ck_bitop fsT2 S S
right_shift right bitshift (>>) ck_bitop fsT2 S S
lt numeric lt (<) ck_null Iifs2 S S
i_lt integer lt (<) ck_null ifs2 S S
gt numeric gt (>) ck_null Iifs2 S S
i_gt integer gt (>) ck_null ifs2 S S
le numeric le (<=) ck_null Iifs2 S S
i_le integer le (<=) ck_null ifs2 S S
ge numeric ge (>=) ck_null Iifs2 S S
i_ge integer ge (>=) ck_null ifs2 S S
eq numeric eq (==) ck_null Iifs2 S S
i_eq integer eq (==) ck_null ifs2 S S
ne numeric ne (!=) ck_null Iifs2 S S
i_ne integer ne (!=) ck_null ifs2 S S
ncmp numeric comparison (<=>)ck_null Iifst2 S S
i_ncmp integer comparison (<=>)ck_null ifst2 S S
slt string lt ck_null ifs2 S S
sgt string gt ck_null ifs2 S S
sle string le ck_null ifs2 S S
sge string ge ck_null ifs2 S S
seq string eq ck_null ifs2 S S
sne string ne ck_null ifs2 S S
scmp string comparison (cmp) ck_null ifst2 S S
bit_and bitwise and (&) ck_bitop fst2 S S
bit_xor bitwise xor (^) ck_bitop fst2 S S
bit_or bitwise or (|) ck_bitop fst2 S S
smartmatch smart match ck_smartmatch s2
aelem array element ck_null s2 A S
helem hash element ck_null s2 H S
lslice list slice ck_null 2 H L L
xor logical xor ck_null fs2 S S
leaveloop loop exit ck_null 2
=head2 LOGOP
The LOGOP class signifier is B<|>.
A LOGOP has the same structure as a L</BINOP>, two children, just the
second field has another name C<op_other> instead of C<op_last>.
But as you see on the list below, the two arguments as above are optional and
not strictly required.
struct logop {
BASEOP
OP * op_first;
OP * op_other;
};
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /\|$/' opcode.pl
regcomp regexp compilation ck_null s| S
substcont substitution iterator ck_null dis|
grepwhile grep iterator ck_null dt|
mapwhile map iterator ck_null dt|
range flipflop ck_null | S S
and logical and (&&) ck_null |
or logical or (||) ck_null |
dor defined or (//) ck_null |
cond_expr conditional expression ck_null d|
andassign logical and assignment (&&=) ck_null s|
orassign logical or assignment (||=) ck_null s|
dorassign defined or assignment (//=) ck_null s|
entergiven given() ck_null d|
enterwhen when() ck_null d|
entertry eval {block} ck_null |
once once ck_null |
=head3 and
Checks for falseness on the first argument on the stack.
If false, returns immediately, keeping the false value on the stack.
If true pops the stack, and returns the op at C<op_other>.
Note: B<and> is also used for a simple B<if> without B<else>/B<elsif>.
The general B<if> is done with L<cond_expr>.
=head3 cond_expr
Checks for trueness on the first argument on the stack.
If true returns the op at C<op_other>, if false C<op_next>.
Note: A simple B<if> without else is done by L<and>.
=head2 LISTOP
The LISTOP class signifier is B<@>.
struct listop {
BASEOP
OP * op_first;
OP * op_last;
};
This is most complex type, it may have any number of children. The
first child is pointed to by C<op_first> and the last child by
C<op_last>. The children in between can be found by iteratively
following the C<op_sibling> pointer from the first child to the last.
At all 99 ops from 366 are LISTOP's. This is the least
restrictive format, that's why.
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /\@$/' opcode.pl
bless bless ck_fun s@ S S?
glob glob ck_glob t@ S?
stringify string ck_fun fsT@ S
atan2 atan2 ck_fun fsT@ S S
substr substr ck_substr st@ S S S? S?
vec vec ck_fun ist@ S S S
index index ck_index isT@ S S S?
rindex rindex ck_index isT@ S S S?
sprintf sprintf ck_fun fmst@ S L
formline formline ck_fun ms@ S L
crypt crypt ck_fun fsT@ S S
aslice array slice ck_null m@ A L
hslice hash slice ck_null m@ H L
unpack unpack ck_unpack @ S S?
pack pack ck_fun mst@ S L
split split ck_split t@ S S S
join join or string ck_join mst@ S L
list list ck_null m@ L
anonlist anonymous list ([]) ck_fun ms@ L
anonhash anonymous hash ({}) ck_fun ms@ L
splice splice ck_fun m@ A S? S? L
... and so on, until
syscall syscall ck_fun imst@ S L
=head2 PMOP
The PMOP "pattern matching" class signifier is B</> for matching.
It inherits from the L</LISTOP>.
The internal struct changed completely with 5.10, as the
underlying engine. Starting with 5.11 the PMOP can even hold
native L<"REGEX"/perlguts
have to use the C<PM> macros to stay compatible.Below is the current C<struct pmop>. You will not like it.
struct pmop {
BASEOP
OP * op_first;
OP * op_last;
IV op_pmoffset;
REGEXP * op_pmregexp; /* compiled expression */
U32 op_pmflags;
union {
OP * op_pmreplroot; /* For OP_SUBST */
PADOFFSET op_pmtargetoff; /* For OP_PUSHRE */
GV * op_pmtargetgv;
} op_pmreplrootu;
union {
OP * op_pmreplstart; /* Only used in OP_SUBST */
char * op_pmstashpv; /* Only used in OP_MATCH, with PMf_ONCE set */
HV * op_pmstash;
} op_pmstashstartu;
};
Before we had no union, but a C<op_pmnext>, which never worked.
Maybe because of the typo in the comment.
The old struct (up to 5.8.x) was as simple as:
struct pmop {
BASEOP
OP * op_first;
OP * op_last;
U32 op_children;
OP * op_pmreplroot;
OP * op_pmreplstart;
PMOP * op_pmnext; /* list of all scanpats */
REGEXP * op_pmregexp; /* compiled expression */
U16 op_pmflags;
U16 op_pmpermflags;
U8 op_pmdynflags;
}
So C<op_pmnext>, C<op_pmpermflags> and C<op_pmdynflags> are gone.
The C<op_pmflags> are not the whole deal, there's also C<op_pmregexp.extflags>
- interestingly called C<B::PMOP::reflags> in B - for the new features.
This is btw. the only inconsistency in the B mapping.
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /\/$/' opcode.pl
pushre push regexp ck_null d/
match pattern match (m//) ck_match d/
qr pattern quote (qr//) ck_match s/
subst substitution (s///) ck_match dis/ S
=head2 SVOP
The SVOP class is very special, and can even change dynamically.
Whole SV's are costly and are now just used as GV or RV.
The SVOP has no special signifier, as there are different subclasses.
See L</"SVOP_OR_PADOP">, L</"PVOP_OR_SVOP"> and L</"FILESTATOP">.
A SVOP holds a SV and is in case of an FILESTATOP the GV for the
filehandle argument, and in case of C<trans> (a L</PVOP>) with utf8 a
reference to a swash (i.e., an RV pointing to an HV).
struct svop {
BASEOP
SV * op_sv;
};
Most old SVOP's were changed to L</PADOP>'s when threading was introduced, to
privatize the global SV area to thread-local scratchpads.
=head3 SVOP_OR_PADOP
The op C<aelemfast> is either a L<PADOP> with threading and a simple L<SVOP> without.
This is thanksfully known at compile-time.
aelemfast constant array element ck_null s$ A S
=head3 PVOP_OR_SVOP
The only op here is C<trans>, where the class is dynamically defined,
dependent on the utf8 settings in the L</op_private> hints.
case OA_PVOP_OR_SVOP:
return (o->op_private & (OPpTRANS_TO_UTF|OPpTRANS_FROM_UTF))
? OPc_SVOP : OPc_PVOP;
trans transliteration (tr///) ck_null is" S
Character translations (C<tr///>) are usually a L<PVOP>, keeping a pointer
to a table of shorts used to look up translations. Under utf8,
however, a simple table isn't practical; instead, the OP is an L</SVOP>,
and the SV is a reference to a B<swash>, i.e. a RV pointing to an HV.
=head2 PADOP
The PADOP class signifier is B<$> for temp. scalars.
A new C<PADOP> creates a new temporary scratchpad, an PADLIST array.
C<padop->op_padix = pad_alloc(type, SVs_PADTMP);>
C<SVs_PADTMP> are targets/GVs/constants with undef names.
A C<PADLIST> scratchpad is a special context stack, a array-of-array data structure
attached to a CV (i.e. a sub), to store lexical variables and opcode temporary and
per-thread values. See L<perlguts/Scratchpads>.
Only my/our variable (C<SVs_PADMY>/C<SVs_PADOUR>) slots get valid names.
The rest are op targets/GVs/constants which are statically allocated
or resolved at compile time. These don't have names by which they
can be looked up from Perl code at run time through eval "" like
my/our variables can be. Since they can't be looked up by "name"
but only by their index allocated at compile time (which is usually
in C<op_targ>), wasting a name SV for them doesn't make sense.
struct padop {
BASEOP
PADOFFSET op_padix;
};
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /\$$/' opcode.pl
const constant item ck_svconst s$
gvsv scalar variable ck_null ds$
gv glob value ck_null ds$
anoncode anonymous subroutine ck_anoncode $
rcatline append I/O operator ck_null t$
aelemfast constant array element ck_null s$ A S
method_named method with known name ck_null d$
hintseval eval hints ck_svconst s$
=head2 PVOP
This is a simple unary op, holding a string.
The only PVOP is C<trans> op for L<tr///>.
See above at L</PVOP_OR_SVOP> for the dynamic nature of trans with utf8.
The PVOP class signifier is C<"> for strings.
struct pvop {
BASEOP
char * op_pv;
};
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /\"$/' opcode.pl
trans transliteration (tr///) ck_match is" S
=head2 LOOP
The LOOP class signifier is B<{>.
It inherits from the L</LISTOP>.
struct loop {
BASEOP
OP * op_first;
OP * op_last;
OP * op_redoop;
OP * op_nextop;
OP * op_lastop;
};
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /\{$/' opcode.pl
enteriter foreach loop entry ck_null d{
enterloop loop entry ck_null d{
=head2 COP
The C<struct cop>, the "Control OP", changed recently a lot, as the L</BASEOP>.
Remember from perlguts what a COP is? Got you. A COP is nowhere described.
I would have naively called it "Context OP", but not "Control OP". So why?
We have a global C<PL_curcop> and then we have threads. So it cannot be global
anymore. A COP can be said as helper context for debugging and error information
to store away file and line information. But since perl is a file-based
compiler, not block-based, also file based pragmata and hints are stored in the
COP. So we have for every source file a seperate COP. COP's are mostly not
really block level contexts, just file and line information. The block level
contexts are not controlled via COP's, but global C<Cx> structs.
F<cop.h> says:
Control ops (cops) are one of the two ops OP_NEXTSTATE and OP_DBSTATE
that (loosely speaking) are separate statements. They hold
information for lexical state and error reporting. At run time, C<PL_curcop> is set
to point to the most recently executed cop, and thus can be used to determine
our file-level current state.
But we need block context, eval context, subroutine context, loop context, and
even format context. All these are seperate structs defined in F<cop.h>.
So the COPs are not really that important, as the actual C<Cx> context structs
are. Just the C<CopSTASH> is, the current package symbol table hash ("stash").Another famous COP is C<PL_compiling>, which sets the temporary compilation
environment.
struct cop {
BASEOP
line_t cop_line; /* line
char * cop_label; /* label for this construct */
char * cop_stashpv; /* package line was compiled in */ char * cop_file; /* file name the following line
HV * cop_stash; /* package line was compiled in */ GV * cop_filegv; /* file the following line
U32 cop_hints; /* hints bits from pragmata */
U32 cop_seq; /* parse sequence number */
/* Beware. mg.c and warnings.pl assume the type of this is STRLEN *: */
STRLEN * cop_warnings; /* lexical warnings bitmask */
/* compile time state of %^H. See the comment in op.c for how this is
used to recreate a hash to return from caller. */
struct refcounted_he * cop_hints_hash;
};
The COP class signifier is B<;> and there are only two:
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /;$/' opcode.pl
nextstate next statement ck_null s;
dbstate debug next statement ck_null s;
C<NEXTSTATE> is replaced by C<DBSTATE> when you call perl with -d, the
debugger. You can even patch the C<NEXTSTATE> ops at runtime to
C<DBSTATE> as done in the module C<Enbugger>.
For a short time there used to be three. C<SETSTATE> was
added 1999 (pre Perl 5.6.0) to track linenumbers correctly
in optimized blocks, disabled 1999 with change 4309 for Perl
5.6.0, and removed with 5edb5b2abb at Perl 5.10.1.
=head2 BASEOP_OR_UNOP
BASEOP_OR_UNOP has the class signifier B<%>. As the name says, it may
be a L</BASEOP> or L</UNOP>, it may have an optional L</op_first> field.
The list of B<%> ops is quite large, it has 84 ops.
Some of them are e.g.
$ perl -F"/\cI+/" -ane 'print if $F[3] =~ /%$/' opcode.pl
...
quotemeta quotemeta ck_fun fstu% S?
aeach each on array ck_each % A
akeys keys on array ck_each t% A
avalues values on array ck_each t% A
each each ck_each % H
values values ck_each t% H
keys keys ck_each t% H
delete delete ck_delete % S
exists exists ck_exists is% S
pop pop ck_shift s% A?
shift shift ck_shift s% A?
caller caller ck_fun t% S?
reset symbol reset ck_fun is% S?
exit exit ck_exit ds% S?
...
=head2 FILESTATOP
A FILESTATOP may be a L</UNOP>, L</PADOP>, L</BASEOP> or L</SVOP>.
It has the class signifier B<->.
The file stat OPs are created via UNI(OP_foo) in toke.c but use theC<OPf_REF> flag to distinguish between OP types instead of the usual
C<OPf_SPECIAL> flag. As usual, if C<OPf_KIDS> is set, then we return
C<OPc_UNOP> so that C<walkoptree> can find our children. If C<OPf_KIDS> is not
set then we check C<OPf_REF>. Without C<OPf_REF> set (no argument to the
operator) it's an OP; with C<OPf_REF> set it's an SVOP (and the field C<op_sv> is the
GV for the filehandle argument).
case OA_FILESTATOP:
return ((o->op_flags & OPf_KIDS) ? OPc_UNOP :
(o->op_flags & OPf_REF) ? OPc_PADOP : OPc_BASEOP);
(o->op_flags & OPf_REF) ? OPc_SVOP : OPc_BASEOP);
lstat lstat ck_ftst u- F
stat stat ck_ftst u- F
ftrread -R ck_ftst isu- F-+
ftrwrite -W ck_ftst isu- F-+
ftrexec -X ck_ftst isu- F-+
fteread -r ck_ftst isu- F-+
ftewrite -w ck_ftst isu- F-+
fteexec -x ck_ftst isu- F-+
ftis -e ck_ftst isu- F-
ftsize -s ck_ftst istu- F-
ftmtime -M ck_ftst stu- F-
ftatime -A ck_ftst stu- F-
ftctime -C ck_ftst stu- F-
ftrowned -O ck_ftst isu- F-
fteowned -o ck_ftst isu- F-
ftzero -z ck_ftst isu- F-
ftsock -S ck_ftst isu- F-
ftchr -c ck_ftst isu- F-
ftblk -b ck_ftst isu- F-
ftfile -f ck_ftst isu- F-
ftdir -d ck_ftst isu- F-
ftpipe -p ck_ftst isu- F-
ftsuid -u ck_ftst isu- F-
ftsgid -g ck_ftst isu- F-
ftsvtx -k ck_ftst isu- F-
ftlink -l ck_ftst isu- F-
fttty -t ck_ftst is- F-
fttext -T ck_ftst isu- F-
ftbinary -B ck_ftst isu- F-
=head2 LOOPEXOP
A LOOPEXOP is almost a L<BASEOP_OR_UNOP>. It may be a L</UNOP> if stacked or
L</BASEOP> if special or L</PVOP> else.
C<next>, C<last>, C<redo>, C<dump> and C<goto> use C<OPf_SPECIAL> to indicate that alabel was omitted (in which case it's a L</BASEOP>) or else a term was
seen. In this last case, all except goto are definitely L</PVOP> but
goto is either a PVOP (with an ordinary constant label), an L</UNOP>
with C<OPf_STACKED> (with a non-constant non-sub) or an L</UNOP> for
C<OP_REFGEN> (with C<goto &sub>) in which case C<OPf_STACKED> also seems to
get set.
...
=head2 OP Definition Example
Let's take a simple example for a opcode definition in F<opcode.pl>:
left_shift left bitshift (<<) ck_bitop fsT2 S S
The op C<left_shift> has a check function C<ck_bitop> (normally most ops
have no check function, just C<ck_null>), and the options C<fsT2>.
The last two C<S S> describe the type of the two required operands:
SV or scalar. This is similar to XS protoypes.
The last C<2> in the options C<fsT2> denotes the class BINOP, with
two args on the stack.
Every binop takes two args and this produces one scalar, see the C<s> flag.
The other remaining flags are C<f> and C<T>.
C<f> tells the compiler in the first pass to call C<fold_constants()>
on this op. See L</"Compile pass 1: check routines and constant folding">
If both args are constant, the result is constant also and the op will
be nullified.
Now let's inspect the simple definition of this op in F<pp.c>.
C<pp_left_shift> is the C<op_ppaddr>, the function pointer, for every
left_shift op.
PP(pp_left_shift)
{
dVAR; dSP; dATARGET; tryAMAGICbin(lshift,opASSIGN);
{
const IV shift = POPi;
if (PL_op->op_private & HINT_INTEGER) {
const IV i = TOPi;
SETi(i << shift);
}
else {
const UV u = TOPu;
SETu(u << shift);
}
RETURN;
}
}
The first IV arg is pop'ed from the stack, the second arg is left on the stack (C<TOPi>/C<TOPu>),
because it is used as the return value. (I<Todo: explain the opASSIGN magic check.>)
One IV or UV is produced, dependent on C<HINT_INTEGER>, set by the C<use integer> pragma.So it has a special signed/unsigned integer behaviour, which is not defined in the opcode
declaration, because the API is indifferent on this, and it is also independent on the
argument type. The result, if IV or UV, is entirely context dependent at compile-time
( C<use integer at BEGIN> ) or run-time ( C<$^H |= 1> ), and only stored in the op.What is left is the C<T> flag, "target can be a pad". This is a useful optimization technique.
This is checked in the macro C<dATARGET>
C<SV *targ = (PL_op->op_flags & OPf_STACKED ? sp[-1] : PAD_SV(PL_op->op_targ));>
C<OPf_STACKED> means "Some arg is arriving on the stack." (see F<op.h>)
So this reads, if the op contains C<OPf_STACKED>, the magic C<targ> ("target argument")
is simply on the stack, but if not, the C<op_targ> points to a SV on a private scratchpad.
"target can be a pad", voila.
For reference see L<perlguts/"Putting a C value on Perl stack">.
=head2 Check Functions
They are defined in F<op.c> and not in F<pp.c>, because they belong tightly to the
ops and newOP definition, and not to the actual pp_ opcode. That's why
the actual F<op.c> file is bigger than F<pp.c> where the real gore for each op begins.
The name of each op's check function is defined in F<opcodes.pl>, as shown above.
The C<ck_null> check function is the most common.
$ perl -F"/\cI+/" -ane 'print $F[2],"\n" if $F[2] =~ /ck_null/' opcode.pl|wc -l
128
But we do have a lot of those check functions.
$ perl -F"/\cI+/" -ane 'print $F[2],"\n" if $F[2] =~ /ck_/' opcode.pl|sort -u|wc -l
43
B<When are they called, how do they look like, what do they do.>
The macro CHECKOP(type,o) used to call the ck_ function has a little bit of
common logic.
((PL_op_mask && PL_op_mask[type]) \
? ( op_free((OP*)o), \
Perl_croak(aTHX_ "'%s' trapped by operation mask", PL_op_desc[type]), \
(OP*)0 ) \
: CALL_FPTR(PL_check[type])(aTHX_ (OP*)o))
So when a global B<PL_op_mask> is fitting to the type the OP is nullified at once.
If not, the type specific check function with the help of F<opcodes.pl> generating
the C<PL_check> array in F<opnames.h> is called.
=head2 Constant Folding
In theory pretty easy. If all op's arguments in a sequence are constant and the
op is sideffect free ("purely functional"), replace the op sequence with an
constant op as result.
We do it like this: We define the C<f> flag in F<opcodes.pl>, which tells the
compiler in the first pass to call C<fold_constants()> on this op. See
L<"Compile pass 1: check routines and constant folding"> above. If all args are
constant, the result is constant also and the op sequence will be replaced by
the constant.
But take care, every C<f> op must be sideeffect free.
E.g. our C<newUNOP()> calls at the end:
return fold_constants((OP *) unop);
OA_FOLDCONST ...
=head2 Lexical Pragmas
To implement user lexical pragmas, there needs to be a way at run time to get
the compile time state of `%^H` for that block. Storing `%^H` in every
block (or even COP) would be very expensive, so a different approach is
taken. The (running) state of C<%^H> is serialised into a tree of HE-like
structs. Stores into C<%^H> are chained onto the current leaf as a struct
refcounted_he * with the key and the value. Deletes from C<%^H> are saved
with a value of C<PL_sv_placeholder>. The state of C<%^H> at any point can be
turned back into a regular HV by walking back up the tree from that point's
leaf, ignoring any key you've already seen (placeholder or not), storing
the rest into the HV structure, then removing the placeholders. Hence
memory is only used to store the C<%^H> deltas from the enclosing COP, rather
than the entire C<%^H> on each COP.
To cause actions on C<%^H> to write out the serialisation records, it has
magic type 'H'. This magic (itself) does nothing, but its presence causes
the values to gain magic type 'h', which has entries for set and clear.
C<Perl_magic_sethint> updates C<PL_compiling.cop_hints_hash> with a store
record, with deletes written by C<Perl_magic_clearhint>. C<SAVEHINTS>
saves the current C<PL_compiling.cop_hints_hash> on the save stack, so that
it will be correctly restored when any inner compiling scope is exited.
=head1 Hooks
=head2 Special execution blocks BEGIN, CHECK, UNITCHECK, INIT, END
Perl keeps special arrays of subroutines that are executed at the
beginning and at the end of a running Perl program and its program
units. These subroutines correspond to the special code blocks:
C<BEGIN>, C<CHECK>, C<UNITCHECK>, C<INIT> and C<END>. (See basics at
L<perlmod/basics>.)
Such arrays belong to Perl's internals that you're not supposed to
see. Entries in these arrays get consumed by the interpreter as it
enters distinct compilation phases, triggered by statements like
C<require>, C<use>, C<do>, C<eval>, etc. To play as safest as
possible, the only allowed operations are to add entries to the start
and to the end of these arrays.
BEGIN, UNITCHECK and INIT are FIFO (first-in, first-out) blocks while
CHECK and END are LIFO (last-in, first-out).
L<Devel::Hook> allows adding code the start or end of these
blocks. L<Manip::END> even tries to remove certain entries.
=head3 The BEGIN block
A special array of code at C<PL_beginav>, that is executed before
C<main_start>, the first op, which is defined be called C<ENTER>.
E.g. C<use module;> adds its require and importer code into the BEGINblock.
=head3 The CHECK block
The B compiler starting block at C<PL_checkav>. This hooks int the
check function which is executed for every op created in bottom-up,
basic order.
=head3 The UNITCHECK block
A new block since Perl 5.10 at C<PL_unitcheckav> runs right after the
CHECK block, to seperate possible B compilation hooks from other
checks.
=head3 The INIT block
At C<PL_initav>.
=head3 The END block
At C<PL_endav>.
L<Manip::END> started to mess around with this block.
The array contains an C<undef> for each block that has been
encountered. It's not really an C<undef> though, it's a kind of raw
coderef that's not wrapped in a scalar ref. This leads to funky error
messages like C<Bizarre copy of CODE in sassign> when you try to assign
one of these values to another variable. See L<Manip::END> how to
manipulate these values array.
=head2 B and O module. The perl compiler.
Malcom Beattie's B modules hooked into the early op tree stages to
represent the internal ops as perl objects and added the perl compiler
backends. See L<B> and L<perlcompile.pod>.
The three main compiler backends are still B<Bytecode>, B<C> and B<CC>.
I<Todo: Describe B's object representation a little bit deeper, its
CHECK hook, its internal transformers for Bytecode (asm and vars) and
C (the sections).>
=head2 MAD
MAD stands for "Misc Attributed Data".
Larry Wall worked on a new MAD compiler backend outside of the B
approach, dumping the internal op tree representation as B<XML> or
B<YAML>, not as tree of perl B objects.
The idea is that all the information needed to recreate the original source is
stored in the op tree. To do this the tokens for the ops are associated with ops,
these madprops are a list of key-value pairs, where the key is a character as
listed at the end of F<op.h>, the value normally is a string, but it might also be
a op, as in the case of a optimized op ('O'). Special for the whitespace key '_'
(whitespace before) and '#' (whitespace after), which indicate the whitespace or
comment before/after the previous key-value pair.
Also when things normally compiled out, like a BEGIN block, which normally do
not results in any ops, instead create a NULLOP with madprops used to recreate
the object.
I<Is there any documentation on this?>
Why this awful XML and not the rich tree of perl objects?
Well there's an advantage.
The MAD XML can be seen as some kind of XML Storable/Freeze of the B
op tree, and can be therefore converted outside of the CHECK block,
which means you can easier debug the conversion (= compilation)
process. To debug the CHECK block in the B backends you have to
use the L<B::Debugger> B<Od> or B<Od_o> modules, which defer the CHECK to INIT.
Perl 5 source to the kurila dialect.
To convert a file 'source.pm' from Perl 5.10 to Kurila you need to do:
kurilapath=/usr/src/perl/kurila-1.9
bleadpath=/usr/src/perl/blead
cd $kurilapath
madfrom='perl-5.10' madto='kurila-1.9' \
madconvert="/usr/bin/perl $kurilapath/mad/p5kurila.pl" \
madpath="$bleadpath/mad" \
mad/convert /path/to/source.pm
related to the op tree at all, could also have been used for that.
=head2 Pluggable runops
The compile tree is executed by one of two existing runops functions, in F<run.c>
or in F<dump.c>. C<Perl_runops_debug> is used with C<DEBUGGING> and the faster
C<Perl_runops_standard> is used otherwise (See below in L</"Walkers">). For fine
control over the execution of the compile tree it is possible to provide your
own runops function.
It's probably best to copy one of the existing runops functions and
change it to suit your needs. Then, in the C<BOOT> section of your XS
file, add the line:
PL_runops = my_runops;
This function should be as efficient as possible to keep your programs
running as fast as possible. See L<Jit> for an even faster just-in-time
compilation runloop.
=head3 Walkers or runops
The standard op tree B<walker> or B<runops> is as simple as this fast
C<Perl_runops_standard()> in (F<run.c>). It starts with C<main_start> and walks
the C<op_next> chain until the end. No need to check other fields, strictly
linear through the tree.
int
Perl_runops_standard(pTHX)
{
dVAR;
while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
PERL_ASYNC_CHECK(); /* until 5.13.2 */
}
TAINT_NOT;
return 0;
}
To inspect the op tree within a perl program, you can also hook C<PL_runops> (see
above at L</"Pluggable runops">) to your own perl walker (see e.g. L<B::Utils>
for various useful walkers), but you cannot modify the tree from within the B
accessors, only via XS. Or via L<B::Generate> as explained in Simon Cozen's
I<Todo: Show the other runloops, and esp. the B:Utils ones.>
I<Todo: Describe the dumper, the debugging and more extended walkers.>
=head1 Internal and external modifications
See the short description of the internal optimizer in the "Brief Summary".
I<Todo: Describe the exported variables and functions which can be
hooked, besides simply adding code to the blocks.>
Via L</"Pluggable runops"> you can provide your own walker function, as it
is done in most B modules. Best see L<B::Utils>.
You may also create custom ops at runtime (well, strictly speaking at
compile-time) via L<B::Generate>.
=head1 Modules
The most important op tree module is L<B::Concise> by Stephen McCamant.
L<B::Utils> provides abstract-enough op tree grep's and walkers with
callbacks from the perl level.
L<Devel::Hook> allows adding perl hooks into the BEGIN, CHECK,
UNITCHECK, INIT blocks.
L<Devel::TypeCheck> tries to verify possible static typing for
expressions and variables, a pretty hard problem for compilers,
esp. with such dynamic and untyped variables as Perl 5.
Reini Urban maintains the interactive op tree debugger L<B::Debugger>,
the Compiler suite (B::C, B::CC, B::Bytecode), L<B::Generate> and
is working on L<Jit>.
=head1 Various Articles
The best source of information is the source. It is very well documented.
Simon Cozens has posted the course material to NetThink's
training course. This is the currently best available description on
that subject.
"Hacking the Optree for Fun..." at
Simon Cozens.
Joshua ben Jore wrote a 50 minute presentation on "Perl 5
focusing on the op tree for SPUG, the Seattle Perl User's Group.
Eric Wilhelm wrote a brief tour through the perl compiler backends for
the impatient refactorerer. The perl_guts_tour as mp3
This text was created in this wiki article:
The svn version should be more actual.
=head1 Conclusion
So this is about 30% of the basic op tree information so far. Not speaking about
the guts. Simon Cozens and Scott Walters have more 30%, in the source are more
10% to copy&paste, and in the compilers and run-time information is the rest. I
hope with the help of some hackers we'll get it done, so that some people will
begin poking around in the B backends. And write the wonderful new dump/undump
functionality (which actually worked in the early years on Solaris) to
save-image and load-image at runtime as in LISP, analyse and optimize the
output, output PIR (parrot code), emit LLVM or another JIT optimized code or
even write assemblers. I have a simple one at home. :)
Written 2008 on the perl5 wiki with socialtext and pod in parallel
by Reini Urban, CPAN ID rurban.