| Commit message (Collapse) | Author | Age | Files | Lines |
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Actually, I don’t think they have needed it for a while.
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It is no longer used.
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This commit almost completely replaces the current mechanism
for detecting and handing common vars in list assignment, e.g.
($a,$b) = ($b,$a);
In general outline: it creates more false positives at compile-time
than before, but also no longer misses some false negatives. In
compensation, it considerably reduces the run-time cost of handling
potential and real commonality.
It does this firstly by splitting the OPpASSIGN_COMMON flag into 3
separate flags:
OPpASSIGN_COMMON_AGG
OPpASSIGN_COMMON_RC1
OPpASSIGN_COMMON_SCALAR
which indicate different classes of commonality that can be handled
in different ways at runtime.
Most importantly, it distinguishes between two basic cases. Firstly,
common scalars (OPpASSIGN_COMMON_SCALAR), e.g.
($x,....) = (....,$x,...)
where $x is modified and then sometime later its value is used again,
but that value has changed in the meantime. In this case, we need
replace such vars on the RHS with mortal copies before processing the
assign.
The second case is an aggregate on the LHS (OPpASSIGN_COMMON_AGG), e.g.
(...,@a) = (...., $a[0],...)
In this case, the issue is instead that when @a is cleared, it may free
items on the RHS (due to the stack not being ref counted). What is
required here is that rather than making of a copy of each RHS element and
storing it in the array as we progress, we make *all* the copies *before*
clearing the array, but mortalise them in case we die in the meantime.
We can further distinguish two scalar cases; sometimes it's possible
to confirm non-commonality at run-time merely by checking that all
the LHS scalars have a reference count of 1. If this is possible,
we set the OPpASSIGN_COMMON_RC1 flag rather than the
OPpASSIGN_COMMON_SCALAR flag.
The major improvement in the run-time performance in the
OPpASSIGN_COMMON_SCALAR case (or OPpASSIGN_COMMON_RC1 if rc>1 scalars are
detected), is to use a mark-and-sweep scan of the two lists using the
SVf_BREAK flag, to determine which elements are common, and only make
mortal copies of those elements. This has a very big effect on run-time
performance; for example in the classic
($a,$b) = ($b,$a);
it would formerly make temp copies of both $a and $b; now it only
copies $a.
In more detail, the mark and sweep mechanism in pp_aassign works by
looping through each LHS and RHS SV pair in parallel. It temporarily marks
each LHS SV with the SVf_BREAK flag, then makes a copy of each RHS element
only if it has the SVf_BREAK flag set. When the scan is finished, the flag
is unset on all LHS elements.
One major change in compile-time flagging is that package scalar vars are
now treated as if they could always be aliased. So we don't bother any
more to do the compile-time PL_generation checking on package vars (we
still do it on lexical vars). We also no longer make use of the run-time
PL_sawalias mechanism for detecting aliased package vars (and indeed the
next commit but one will remove that mechanism). This means that more list
assignment expressions which feature package vars will now need to
do a runtime mark-and-sweep (or where appropriate, RC1) test. In
compensation, we no longer need to test for aliasing and set PL_sawalias
in pp_gvsv and pp_gv, nor reset PL_sawalias in every pp_nextstate.
Part of the reasoning behind this is that it's nearly impossible to detect
all possible package var aliasing; for example PL_sawalias would fail to
detect XS code doing GvSV(gv) = sv.
Note that we now scan the two children of the OP_AASSIGN separately,
and in particular we mark lexicals with PL_generation only on the
LHS and test only on the RHS. So something like
($x,$y) = ($default, $default)
will no longer be regarded as having common vars.
In terms of performance, running Porting/perlbench.pl on the new
expr::aassign:: tests in t/perf/benchmarks show that the biggest slowdown
is around 13% more instruction reads and 20% more conditional branches in
this:
setup => 'my ($v1,$v2,$v3) = 1..3; ($x,$y,$z) = 1..3;',
code => '($x,$y,$z) = ($v1,$v2,$v3)',
where this is now a false positive due to the presence of package
variables.
The biggest speedup is 50% less instruction reads and conditional branches
in this:
setup => '@_ = 1..3; my ($x,$y,$z)',
code => '($x,$y,$z) = @_',
because formerly the presence of @_ pessimised things if the LHS wasn't
a my declaration (it's still pessimised, but the runtime's faster now).
Conversely, we pessimise the 'my' variant too now:
setup => '@_ = 1..3;',
code => 'my ($x,$y,$z) = @_',
this gives 5% more instruction reads and 11% more conditional branches now.
But see the next commit, which will cheat for that particular construct.
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The targlex optimisation (which makes the op write directly to the
lexical in $lexical = some op, skipping the assignment) does not take
typeglob assignment into account. Since this optimisation has been
enabled for some ops in 5.21.x, we actually have a regression. So
this commit disables the optimisation once more for ops that did not
have it on in 5.20. This is a temporary fix, until we find a better
overall fix. Other ops that still have the optimisation are buggy,
but no more buggy than in 5.20.
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// in the scope of lexical $_ gets the OPpTARGET_MY flag set. If it
is used as an operand to smartmatch, it gets converted to a qr op.
There is no need to turn off the flag, since it is harmless at run
time. But we need to allow the flag on this op type to avoid asser-
tion failures when it is freed.
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This case fails an assertion:
my($_);0=split
because a pushre op is not expected to have that flag. A pushre op
starts out as a match op (which may indeed have the flag set), as is
subsequently converted to a pushre op.
[The commit message was written by the committer.]
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ck_grep calls ck_fun, which sets the lower private bits to indicate
the number of operands. ck_grep usually undoes that, by clobbering
op_private completetly. If an error has occurred, it doesn’t, so we
may fail an assertion if the lower bits are not expected to be set on
mapstart and grepstart.
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and also implement the pp functions, though nothing compiles to
these ops yet.
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It always reads its argument out the outset and always returns its
target, so there is no reason its target cannot be a lexical. (The
OPpTARGET_MY optimisation makes $lexical = <some op> have the op
write directly to the lexical; the assignment gets optimised away.)
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We can only do it for <=> under ‘use integer’.
The non-integer <=> will push undef on to the stack. Enabling
the optimisation for it would cause \($lexical = $x <=> "nan") to
leave $lexical with its previous value and return a reference to
&PL_sv_undef.
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Previously it would read and replace the previous item on the stack:
$ ./perl -le 'print "CORE::undef", prototype'
;\[$@%&*]
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The target of entersub ops is only used by XSUBs to return things.
Pure-Perl subs don’t use the target. (And if a pure-Perl sub is
later replaced with an XS one, dXSTARG already has a workaround.)
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They only ever return scalars, but were not flagged that way.
This change allows \*foo{THING} and \(\$x=\$y) to use srefgen, a
faster version of refgen that handles only one item.
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This way there is no need to call scalar() on the op separately.
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This op is an optimisation for any series of one or more array or hash
lookups and dereferences, where the key/index is a simple constant or
package/lexical variable. If the first-level lookup is of a simple
array/hash variable or scalar ref, then that is included in the op too.
So all of the following are replaced with a single op:
$h{foo}
$a[$i]
$a[5][$k][$i]
$r->{$k}
local $a[0][$i]
exists $a[$i]{$k}
delete $h{foo}
while these aren't:
$a[0] already handled by OP_AELEMFAST
$a[$x+1] not a simple index
and these are partially replaced:
(expr)->[0]{$k} the bit following (expr) is replaced
$h{foo}[$x+1][0] the first and third lookups are each done with
a multideref op, while the $x+1 expression and
middle lookup are done by existing add, aelem etc
ops.
Up until now, aggregate dereferencing has been very heavyweight in ops; for
example, $r->[0]{$x} is compiled as:
gv[*r] s
rv2sv sKM/DREFAV,1
rv2av[t2] sKR/1
const[IV 0] s
aelem sKM/DREFHV,2
rv2hv sKR/1
gvsv[*x] s
helem vK/2
When executing this, in addition to the actual calls to av_fetch() and
hv_fetch(), there is a lot of overhead of pushing SVs on and off the
stack, and calling lots of little pp() functions from the runops loop
(each with its potential indirect branch miss).
The multideref op avoids that by running all the code in a loop in a
switch statement. It makes use of the new UNOP_AUX type to hold an array
of
typedef union {
PADOFFSET pad_offset;
SV *sv;
IV iv;
UV uv;
} UNOP_AUX_item;
In something like $a[7][$i]{foo}, the GVs or pad offsets for @a and $i are
stored as items in the array, along with a pointer to a const SV holding
'foo', and the UV 7 is stored directly. Along with this, some UVs are used
to store a sequence of actions (several actions are squeezed into a single
UV).
Then the main body of pp_multideref is a big while loop round a switch,
which reads actions and values from the AUX array. The two big branches in
the switch are ones that are affectively unrolled (/DREFAV, rv2av, aelem)
and (/DREFHV, rv2hv, helem) triplets. The other branches are various entry
points that handle retrieving the different types of initial value; for
example 'my %h; $h{foo}' needs to get %h from the pad, while '(expr)->{foo}'
needs to pop expr off the stack.
Note that there is a slight complication with /DEREF; in the example above
of $r->[0]{$x}, the aelem op is actually
aelem sKM/DREFHV,2
which means that the aelem, after having retrieved a (possibly undef)
value from the array, is responsible for autovivifying it into a hash,
ready for the next op. Similarly, the rv2sv that retrieves $r from the
typeglob is responsible for autovivifying it into an AV. This action
of doing the next op's work for it complicates matters somewhat. Within
pp_multideref, the autovivification action is instead included as the
first step of the current action.
In terms of benchmarking with Porting/bench.pl, a simple lexical
$a[$i][$j] shows a reduction of approx 40% in numbers of instructions
executed, while $r->[0][0][0] uses 54% fewer. The speed-up for hash
accesses is relatively more modest, since the actual hash lookup (i.e.
hv_fetch()) is more expensive than an array lookup. A lexical $h{foo}
uses 10% fewer, while $r->{foo}{bar}{baz} uses 34% fewer instructions.
Overall,
bench.pl --tests='/expr::(array|hash)/' ...
gives:
PRE POST
------ ------
Ir 100.00 145.00
Dr 100.00 165.30
Dw 100.00 175.74
COND 100.00 132.02
IND 100.00 171.11
COND_m 100.00 127.65
IND_m 100.00 203.90
with cache misses unchanged at 100%.
In general, the more lookups done, the bigger the proportionate saving.
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It was done by adding new OP_METHOD_REDIR and OP_METHOD_REDIR_SUPER optypes.
Class name to redirect is saved into METHOP as a shared hash string.
Method name is changed (class name removed) an saved into op_meth_sv as
a shared string hash.
So there is no need now to scan for '::' and calculate class and method names
at runtime (in gv_fetchmethod_*) and searching cache HV without precomputed hash.
B::* modules are changed to support new op types.
method_redir is now printed by Concise like (for threaded perl)
$obj->AAA::meth
5 <.> method_redir[PACKAGE "AAA", PV "meth"] ->6
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In ck_method:
Scan for '/::. If found SUPER::, create OP_METHOD_SUPER op
with precomputed hash value for method name.
In B::*, added support for method_super
In pp_hot.c, pp_method_*:
S_method_common removed, code related to getting stash is
moved to S_opmethod_stash, other code is moved to
pp_method_* functions.
As a result, SUPER::func() calls speeded up by 50%.
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$ ./perl -Ilib -le 'use integer; my $a = "fake"; $a = -$a; print "[$a]"'
[--]
As of 1c2b3fd6f1, negation under ‘use integer’ can do string negation,
which modifies the return value before reading the argument. So, like
regular non-integer negation, it must forego this optimisation.
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I was wrong in 9e319cc4f. postfix ++/-- writes to its return value
before reading its argument. If we optimise away the scalar
assignment in
$a = $b++;
(that’s what OPpTARGET_MY does), then $a gets written to before $b is
read. If $a and $b are the same, we get the wrong answer. This bug
has been present under ‘use integer’ since 5.6.0. I accidentally
extended it to non-integer ++/-- in 9e319cc4f.
(It’s not likely that someone will write $a = $b++, but it could hap-
pen inadvertently in more complex code.)
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It doesn’t matter whether things in this table are ordered by opcode,
because the indices into it are stored in PL_op_private_bitdef_ix.
If we have multiple ops with exactly the same private flags, we don’t
need multiple entries in PL_op_private_bitdefs.
One practical advantage is that patches are less likely to conflict,
which will make rebasing easier. (I hope.)
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
values and each can both return scalars that are referenced elsewhere,
causing list assignment to behave erratically if temporary copies
are not made.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
kill only ever returns a target unused elsewhere, so it does not
necessitate temp copies in list assignment.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
When exec fails, it only ever returns a target unused elsewhere, so it
does not necessitate temp copies in list assignment.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
‘write’ only ever returns a read-only true or false, so temp copies
are not necessary for its sake.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
tied returns the same scalar that the tied variables uses to hold a
reference to the object (so weaken(tied(...)) works). tie uses the
very scalar that TIESCALAR (or TIEWHATEVER) returns and attaches it to
the tied variable by magic. That returned scalar could be referenced
elsewhere. That means
($a, $b) = ($c, tied $d)
could have common vars on either side, if the tie constructor for $d
happened to return $a or $b. (Normally it would have to be an XSUB or
an lvalue sub for its return value not to have been copied.)
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
exit usually doesn’t return. When it fails, it returns a read-only
undef, so we don’t need temp copies for its sake.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
These operators never return, so they shouldn’t necessitate
temp copies.
(This could probably apply to dump, too, but I don’t fully under-
stand dump.)
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
die never returns, so it shouldn’t necessitate temp copies.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
?: always returns one of its arguments. Since aassign_common_vars,
which does the danger check, also checks the kids of the cond_expr op,
it is not necessary for cond_expr to be flagged this way.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
grep returns its arguments, while map returns the results of its first
expression. Since aassign_common_vars, which does the danger check,
will also check the kids of the mapstart/grepstart ops, it is not nec-
essary for grep and map themselves to be flagged this way.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
pp_sort returns its arguments. aassign_common_vars will check its kid
ops for danger as well, so it’s not necessary for sort itself to be
flagged this way. This will allow cases like ($a,$b) = sort($c,$d) to
forego the temp copy.
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Commit d5ec29879 in 2006 started storing all the hints in COPs. Some
VMS-specific hints have nonetheless still been copied from PL_hints to
cop->op_private, though that is no longer necessary.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
While pp_postinc may return a lexical scalar, that only happens when
the OPpTARGET_MY optimisation is happening; and aassign_common_vars in
op.c checks specifically for that.
Otherwise, it only returns a mortal or target, so it is not
OA_DANGEROUS.
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I don’t know why this was not done to begin with. The previous two
commits fixed the only outstanding problems I am aware of.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
pp_subst and pp_substcont only push new mortal scalars or read-only
values on to the stack. Hence they are not OA_DANGEROUS.
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OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:
($a, $b) = ($b, $a);
In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.
pp_match pushes new mortals on to the stack in list context (or a
read-only boolean; read-only values don’t matter), so they can’t
occur elsewhere. Hence it is not OA_DANGEROUS.
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Many operators have a special SV allocated in the pad which is used
for return values (the target). If we make that pad offset point to
a lexical variable, then we can optimise, say, $lexical = "$foo" into
just "$foo", where $lexical is stringify’s ‘target’. And pp_stringify
doesn’t need to know any better. We already do that for many ops.
This is safe to extend to split. split only uses its target in this
code at the end:
GETTARGET;
PUSHi(iters);
so there is no danger of modifying its argument before reading it.
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I need split to be able to take both flags. Renumbering OUR_INTRO
sets off a chain reaction requiring that various other flags be renum-
bered. These are the affected ops:
gvsv rv2sv rv2av rv2hv enteriter split rv2gv padsv aelem helem entersub padav padhv lvavref lvref refassign pushmark rv2cv
----------------------------------------------------------------------------------------------------------------------------------
0 inargs
1 strct strct strct strct strct strct
2 slicw silcw reversed noinit targ slicw slicw elem elem targ
3 lvsub lvsub def lvsub lvsub lvsub amper lvsub lvsub iter iter amper
4 our our our our our our fake state defer defer dbg state state state state state state dbg
5-6 dref bool dref dref dref dref dref bool type type const(6)
7 intro intro intro intro intro implim intro intro intro intro intro intro intro intro intro intro intro nopar
If we use 6 for OUR_INTRO and shift 5-6 down to 4-5, that frees up 4
(aka OPpTARGET_MY) for use on split.
op.c:scalarvoid was testing the OPpOUR_INTRO flag on ops that don’t
take it, so it needed adjustment.
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Many operators have a special SV allocated in the pad which is used
for return values (the target). If we make that pad offset point to
a lexical variable, then we can optimise, say, $lexical = "$foo" into
just "$foo", where $lexical is stringify’s ‘target’. And pp_stringify
doesn’t need to know any better. We already do that for many ops.
This can be extended to x. Despite what the comment in op_private
says, list return values do not matter here, because the OPpTARGET_MY
optimisation only happens when the operator is in scalar context. And
the scalar code paths use TARG and push TARG on to the stack.
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Many operators have a special SV allocated in the pad which is used
for return values (the target). If we make that pad offset point to
a lexical variable, then we can optimise, say, $lexical = "$foo" into
just "$foo", where $lexical is stringify’s ‘target’. And pp_stringify
doesn’t need to know any better. We already do that for many ops.
This can be extended to vec(). Despite what the comment in op_pri-
vate says, lvalue usage does not matter here, because the OPpTARGET_MY
optimisation only happens when the operator is in rvalue context. And
the rvalue code paths use TARG and push TARG on to the stack.
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Current codebase wires Perl_pp_mapstart to Perl_unimplemented_op (by
regen/opcode.pl) [1], but then avoids runtime panics by Perl_ck_grep
changing all OP_MAPSTART nodes to use PL_ppaddr[OP_GREPSTART] [2].
This is all too clever by half, so this patch undoes the trickery, and
treats these 2 OPS like 93bad3fd5548 did for OP_AELEMFAST and \1_LEX.
I cant glean a reason for this historical arrangement:
Looking at regen/opcode.pl blamelog..
65bca31a68 added Perl_unimplemented_op() used by 3 'unreachable' ops,
and replaced a 'panic: mapstart' diag with a common one, so the trick
goes further back.
c78ff9799bf moved a minimal/DIEing pp_mapstart implementation to
mathoms.c from pp_ctl.c. Perl_ck_grep also did the GREPSTART patching
back then.
f54cb97a39f did minor tweaks to a full pp_mapstart implementation. I
couldnt find the commit between it and c78ff that changed pp_mapstart
to a DIEing one, or I fat-fingered it, or I got distracted.
looking at ck-grep(), the code doing [2] is from 22c35a8c23 in 1998.
So anyway, I tried the following, it worked, it seems the historical
reason is no longer relevant.
[1] change regen/opcode.pl generated mapping
-#define Perl_pp_mapstart Perl_unimplemented_op
+#define Perl_pp_mapstart Perl_pp_grepstart
this sets PL_ppaddr[OP_MAPSTART] = PL_ppaddr[OP_GREPSTART] during
init, which makes the optype trickery in ck_grep[2] unneeded.
[2] Drop re-type-ing of MAPSTARTs as GREPSTARTs by Perl_ck_grep(OP* o)
Given 1, mapstart & grepstart share code, so just leave optype alone.
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srefgen is faster than refgen, since it doesn’t have to loop through
the arguments (there is only one) and there is no pushmark to execute.
OA_RETSCALAR causes scalar context to be applied to anoncode ops, but
it always returns one item anyway, so that causes no problems.
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I accidentally broke ($_[0],$_[1])=($_[1],$_[0]) in be9de18, which was
only supposed to be a refactoring. Since it now happens later in the
compilation phase when optimisations like aelemfast have happened, the
search for common vars needs to take aelemfast into account.
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instead of stringify(join(...)).
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Due to the exigencies of the implementation, "$_->$*" ends up with a
join op (join $", $$_), which is unnecessary. This gave me the idea
of folding it where possible (instead of trying to tackle it in
toke.c), which would also make explicit joins benefit, too.
If the arguments are a simple scalar or constant followed by a
single-item list, then the join can become a stringify, and the sepa-
rator can simply disappear.
Further (and this is unrelated to "$_->$*"), if all of join’s argu-
ments are constant, the whole thing can be folded to a const op.
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