Add OP_MULTIDEREF

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.

1 files changed, 53 insertions, 3 deletions

diff --git a/op.h b/op.h
index 61a382fb76..9e60beb64d 100644
--- a/op.h
+++ b/op.h
@@ -124,9 +124,10 @@ Deprecated.  Use C<GIMME_V> instead.
 				/*  On OP_SMARTMATCH, an implicit smartmatch */
 				/*  On OP_ANONHASH and OP_ANONLIST, create a
 				    reference to the new anon hash or array */
-				/*  On OP_HELEM and OP_HSLICE, localization will be followed
-				    by assignment, so do not wipe the target if it is special
-				    (e.g. a glob or a magic SV) */
+				/*  On OP_HELEM, OP_MULTIDEREF and OP_HSLICE,
+                                    localization will be followed by assignment,
+                                    so do not wipe the target if it is special
+                                    (e.g. a glob or a magic SV) */
 				/*  On OP_MATCH, OP_SUBST & OP_TRANS, the
 				    operand of a logical or conditional
 				    that was optimised away, so it should
@@ -177,6 +178,14 @@ typedef union  {
     UV        uv;
 } UNOP_AUX_item;
 
+#ifdef USE_ITHREADS
+#  define UNOP_AUX_item_sv(item) PAD_SVl((item)->pad_offset);
+#else
+#  define UNOP_AUX_item_sv(item) ((item)->sv);
+#endif
+
+
+
 
 struct op {
     BASEOP
@@ -988,6 +997,47 @@ Sets the sibling of o to sib
 #  define OP_CHECK_MUTEX_TERM		NOOP
 #endif
 
+
+/* Stuff for OP_MULTDEREF/pp_multideref. */
+
+/* actions */
+
+/* Load another word of actions/flag bits. Must be 0 */
+#define MDEREF_reload                       0
+
+#define MDEREF_AV_pop_rv2av_aelem           1
+#define MDEREF_AV_gvsv_vivify_rv2av_aelem   2
+#define MDEREF_AV_padsv_vivify_rv2av_aelem  3
+#define MDEREF_AV_vivify_rv2av_aelem        4
+#define MDEREF_AV_padav_aelem               5
+#define MDEREF_AV_gvav_aelem                6
+
+#define MDEREF_HV_pop_rv2hv_helem           8
+#define MDEREF_HV_gvsv_vivify_rv2hv_helem   9
+#define MDEREF_HV_padsv_vivify_rv2hv_helem 10
+#define MDEREF_HV_vivify_rv2hv_helem       11
+#define MDEREF_HV_padhv_helem              12
+#define MDEREF_HV_gvhv_helem               13
+
+#define MDEREF_ACTION_MASK                0xf
+
+/* key / index type */
+
+#define MDEREF_INDEX_none   0x00 /* run external ops to generate index */
+#define MDEREF_INDEX_const  0x10 /* index is const PV/UV */
+#define MDEREF_INDEX_padsv  0x20 /* index is lexical var */
+#define MDEREF_INDEX_gvsv   0x30 /* index is GV */
+
+#define MDEREF_INDEX_MASK   0x30
+
+/* bit flags */
+
+#define MDEREF_FLAG_last    0x40 /* the last [ah]elem; PL_op flags apply */
+
+#define MDEREF_MASK         0x7F
+#define MDEREF_SHIFT           7
+
+
 /*
  * Local variables:
  * c-indentation-style: bsd