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, 29 insertions, 1 deletions

diff --git a/ext/B/t/concise-xs.t b/ext/B/t/concise-xs.t
index 365951d0bc..289f909cc9 100644
--- a/ext/B/t/concise-xs.t
+++ b/ext/B/t/concise-xs.t
@@ -159,6 +159,7 @@ my $testpkgs = {
 	constant => [qw/ ASSIGN CVf_LVALUE
 		     CVf_METHOD LIST_CONTEXT OP_CONST OP_LIST OP_RV2SV
 		     OP_AELEM OP_CUSTOM OP_NEXTSTATE OP_DBSTATE
+		     OP_HELEM OP_RV2AV OP_RV2HV
 		     OP_STRINGIFY OPf_KIDS OPf_MOD OPf_REF OPf_SPECIAL
 		     OPf_PARENS
 		     OPf_STACKED OPf_WANT OPf_WANT_LIST OPf_WANT_SCALAR
@@ -169,6 +170,8 @@ my $testpkgs = {
 		     OPpSORT_REVERSE OPpREVERSE_INPLACE OPpTARGET_MY
 		     OPpTRANS_COMPLEMENT OPpTRANS_DELETE OPpTRANS_SQUASH
 		     OPpREPEAT_DOLIST
+                     OPpMULTIDEREF_EXISTS
+                     OPpMULTIDEREF_DELETE
 		     PMf_CONTINUE PMf_EVAL PMf_EXTENDED PMf_EXTENDED_MORE
                      PMf_FOLD PMf_GLOBAL
 		     PMf_KEEP PMf_NONDESTRUCT
@@ -176,7 +179,32 @@ my $testpkgs = {
 		     POSTFIX SVf_FAKE SVf_IOK SVf_NOK SVf_POK SVf_ROK
 		     SVpad_STATE
 		     SVpad_OUR SVs_RMG SVs_SMG SWAP_CHILDREN OPpPAD_STATE
-		     OPpCONST_ARYBASE RXf_SKIPWHITE SVpad_TYPED/,
+		     OPpCONST_ARYBASE RXf_SKIPWHITE SVpad_TYPED
+
+                    MDEREF_reload
+                    MDEREF_AV_pop_rv2av_aelem
+                    MDEREF_AV_gvsv_vivify_rv2av_aelem
+                    MDEREF_AV_padsv_vivify_rv2av_aelem
+                    MDEREF_AV_vivify_rv2av_aelem
+                    MDEREF_AV_padav_aelem
+                    MDEREF_AV_gvav_aelem
+                    MDEREF_HV_pop_rv2hv_helem
+                    MDEREF_HV_gvsv_vivify_rv2hv_helem
+                    MDEREF_HV_padsv_vivify_rv2hv_helem
+                    MDEREF_HV_vivify_rv2hv_helem
+                    MDEREF_HV_padhv_helem
+                    MDEREF_HV_gvhv_helem
+                    MDEREF_ACTION_MASK
+                    MDEREF_INDEX_none
+                    MDEREF_INDEX_const
+                    MDEREF_INDEX_padsv
+                    MDEREF_INDEX_gvsv
+                    MDEREF_INDEX_MASK
+                    MDEREF_FLAG_last
+                    MDEREF_MASK
+                    MDEREF_SHIFT
+                     /,
+
 		     $] >= 5.015 ? qw(
 		     OP_GLOB PMf_SKIPWHITE RXf_PMf_CHARSET RXf_PMf_KEEPCOPY
 		     OPpEVAL_BYTES OPpSUBSTR_REPL_FIRST) : (),