Add OP_MULTICONCAT op

Allow multiple OP_CONCAT, OP_CONST ops, plus optionally an OP_SASSIGN
or OP_STRINGIFY, to be combined into a single OP_MULTICONCAT op, which can
make things a *lot* faster: 4x or more.

In more detail: it will optimise into a single OP_MULTICONCAT, most
expressions of the form

    LHS RHS

where LHS is one of

    (empty)
    my $lexical =
    $lexical    =
    $lexical   .=
    expression  =
    expression .=

and RHS is one of

    (A . B . C . ...)            where A,B,C etc are expressions and/or
                                 string constants

    "aAbBc..."                   where a,A,b,B etc are expressions and/or
                                 string constants

    sprintf "..%s..%s..", A,B,.. where the format is a constant string
                                 containing only '%s' and '%%' elements,
                                 and A,B, etc are scalar expressions (so
                                 only a fixed, compile-time-known number of
                                 args: no arrays or list context function
                                 calls etc)

It doesn't optimise other forms, such as

    ($a . $b) . ($c. $d)

    ((($a .= $b) .= $c) .= $d);

(although sub-parts of those expressions might be converted to an
OP_MULTICONCAT). This is partly because it would be hard to maintain the
correct ordering of tie or overload calls.

The compiler uses heuristics to determine when to convert: in general,
expressions involving a single OP_CONCAT aren't converted, unless some
other saving can be made, for example if an OP_CONST can be eliminated, or
in the presence of 'my $x = .. ' which OP_MULTICONCAT can apply
OPpTARGET_MY to, but OP_CONST can't.

The multiconcat op is of type UNOP_AUX, with the op_aux structure directly
holding a pointer to a single constant char* string plus a list of segment
lengths. So for

    "a=$a b=$b\n";

the constant string is "a= b=\n", and the segment lengths are (2,3,1).
If the constant string has different non-utf8 and utf8 representations
(such as "\x80") then both variants are pre-computed and stored in the aux
struct, along with two sets of segment lengths.

For all the above LHS types, any SASSIGN op is optimised away. For a LHS
of '$lex=', '$lex.=' or 'my $lex=', the PADSV is optimised away too.

For example where $a and $b are lexical vars, this statement:

    my $c = "a=$a, b=$b\n";

formerly compiled to

    const[PV "a="] s
    padsv[$a:1,3] s
    concat[t4] sK/2
    const[PV ", b="] s
    concat[t5] sKS/2
    padsv[$b:1,3] s
    concat[t6] sKS/2
    const[PV "\n"] s
    concat[t7] sKS/2
    padsv[$c:2,3] sRM*/LVINTRO
    sassign vKS/2

and now compiles to:

    padsv[$a:1,3] s
    padsv[$b:1,3] s
    multiconcat("a=, b=\n",2,4,1)[$c:2,3] vK/LVINTRO,TARGMY,STRINGIFY

In terms of how much faster it is, this code:

    my $a = "the quick brown fox jumps over the lazy dog";
    my $b = "to be, or not to be; sorry, what was the question again?";

    for my $i (1..10_000_000) {
        my $c = "a=$a, b=$b\n";
    }

runs 2.7 times faster, and if you throw utf8 mixtures in it gets even
better. This loop runs 4 times faster:

    my $s;
    my $a = "ab\x{100}cde";
    my $b = "fghij";
    my $c = "\x{101}klmn";

    for my $i (1..10_000_000) {
        $s = "\x{100}wxyz";
        $s .= "foo=$a bar=$b baz=$c";
    }

The main ways in which OP_MULTICONCAT gains its speed are:

* any OP_CONSTs are eliminated, and the constant bits (already in the
  right encoding) are copied directly from the constant string attached to
  the op's aux structure.

* It optimises away any SASSIGN op, and possibly a PADSV op on the LHS, in
  all cases; OP_CONCAT only did this in very limited circumstances.

* Because it has a holistic view of the entire concatenation expression,
  it can do the whole thing in one efficient go, rather than creating and
  copying intermediate results. pp_multiconcat() goes to considerable
  efforts to avoid inefficiencies. For example it will only SvGROW() the
  target once, and to the exact size needed, no matter what mix of utf8
  and non-utf8 appear on the LHS and RHS.  It never allocates any
  temporary SVs except possibly in the case of tie or overloading.

* It does all its own appending and utf8 handling rather than calling
  out to functions like sv_catsv().

* It's very good at handling the LHS appearing on the RHS; for example in

    $x = "abcd";
    $x = "-$x-$x-";

  It will do roughly the equivalent of the following (where targ is $x);

    SvPV_force(targ);
    SvGROW(targ, 11);
    p = SvPVX(targ);
    Move(p,   p+1,  4, char);
    Copy("-", p,    1, char);
    Copy("-", p+5,  1, char);
    Copy(p+1, p+6,  4, char);
    Copy("-", p+10, 1, char);
    SvCUR(targ) = 11;
    p[11] = '\0';

  Formerly, pp_concat would have used multiple PADTMPs or temporary SVs to
  handle situations like that.

The code is quite big; both S_maybe_multiconcat() and pp_multiconcat()
(the main compile-time and runtime parts of the implementation) are over
700 lines each. It turns out that when you combine multiple ops, the
number of edge cases grows exponentially ;-)

1 files changed, 646 insertions, 5 deletions

diff --git a/t/opbasic/concat.t b/t/opbasic/concat.t
index 7802fc98ce..55965c1702 100644
--- a/t/opbasic/concat.t
+++ b/t/opbasic/concat.t
@@ -5,12 +5,13 @@ BEGIN {
     @INC = '../lib';
 }
 
-# ok() functions from other sources (e.g., t/test.pl) may use concatenation,
-# but that is what is being tested in this file.  Hence, we place this file
-# in the directory where do not use t/test.pl, and we write an ok() function
-# specially written to avoid any concatenation.
+# ok()/is() functions from other sources (e.g., t/test.pl) may use
+# concatenation, but that is what is being tested in this file.  Hence, we
+# place this file in the directory where do not use t/test.pl, and we
+# write functions specially written to avoid any concatenation.
 
 my $test = 1;
+
 sub ok {
     my($ok, $name) = @_;
 
@@ -22,7 +23,23 @@ sub ok {
     return $ok;
 }
 
-print "1..31\n";
+sub is {
+    my($got, $expected, $name) = @_;
+
+    my $ok = $got eq $expected;
+
+    printf "%sok %d - %s\n", ($ok ? "" : "not "), $test, $name;
+
+    if (!$ok) {
+        printf "# Failed test at line %d\n", (caller)[2];
+        printf "# got:      %s\n#expected: %s\n", $got, $expected;
+    }
+
+    $test++;
+    return $ok;
+}
+
+print "1..251\n";
 
 ($a, $b, $c) = qw(foo bar);
 
@@ -132,6 +149,7 @@ sub beq { use bytes; $_[0] eq $_[1]; }
     my $up = "\x{100}\xB6";
     my $x1 = $p;
     my $y1 = $u;
+    my ($x2, $x3, $x4, $y2);
 
     use bytes;
     ok(beq($p.$u, $p.$b), "perl #26905, left eq bytes");
@@ -144,11 +162,15 @@ sub beq { use bytes; $_[0] eq $_[1]; }
     $y1 .= $p;
     $y2 = $u . $p;
 
+    $x3 = $p; $x3 .= $u . $u;
+    $x4 = $p . $u . $u;
+
     no bytes;
     ok(beq($x1, $x2), "perl #26905, left,  .= vs = . in bytes");
     ok(beq($y1, $y2), "perl #26905, right, .= vs = . in bytes");
     ok(($x1 eq $x2),  "perl #26905, left,  .= vs = . in chars");
     ok(($y1 eq $y2),  "perl #26905, right, .= vs = . in chars");
+    ok(($x3 eq $x4),  "perl #26905, twin,  .= vs = . in chars");
 }
 
 {
@@ -164,8 +186,627 @@ sub beq { use bytes; $_[0] eq $_[1]; }
     ok($x eq "ab-append-", "Appending to something initialized using constant folding");
 }
 
+# non-POK consts
+
+{
+    my $a = "a";
+    my $b;
+    $b = $a . $a . 1;
+    ok($b eq "aa1", "aa1");
+    $b = 2 . $a . $a;
+    ok($b eq "2aa", "2aa");
+}
+
 # [perl #124160]
 package o { use overload "." => sub { $_[0] }, fallback => 1 }
 $o = bless [], "o";
 ok(ref(CORE::state $y = "a $o b") eq 'o',
   'state $y = "foo $bar baz" does not stringify; only concats');
+
+
+# multiconcat: utf8 dest with non-utf8 args should grow dest sufficiently.
+# This is mainly for valgrind or ASAN to detect problems with.
+
+{
+    my $s = "\x{100}";
+    my $t = "\x80" x 1024;
+    $s .= "-$t-";
+    ok length($s) == 1027, "utf8 dest with non-utf8 args";
+}
+
+# target on RHS
+
+{
+    my $a = "abc";
+    $a .= $a;
+    ok($a eq 'abcabc', 'append self');
+
+    $a = "abc";
+    $a = $a . $a;
+    ok($a eq 'abcabc', 'double self');
+
+    $a = "abc";
+    $a .= $a . $a;
+    ok($a eq 'abcabcabc', 'append double self');
+
+    $a = "abc";
+    $a = "$a-$a";
+    ok($a eq 'abc-abc', 'double self with const');
+
+    $a = "abc";
+    $a .= "$a-$a";
+    ok($a eq 'abcabc-abc', 'append double self with const');
+
+    $a = "abc";
+    $a .= $a . $a . $a;
+    ok($a eq 'abcabcabcabc', 'append triple self');
+
+    $a = "abc";
+    $a = "$a-$a=$a";
+    ok($a eq 'abc-abc=abc', 'triple self with const');
+
+    $a = "abc";
+    $a .= "$a-$a=$a";
+    ok($a eq 'abcabc-abc=abc', 'append triple self with const');
+}
+
+# test the sorts of optree which may (or may not) get optimised into
+# a single MULTICONCAT op. It's based on a loop in t/perf/opcount.t,
+# but here the loop is unwound as we would need to use concat to
+# generate the expected results to compare with the actual results,
+# which would rather defeat the object.
+
+{
+    my ($a1, $a2, $a3) = qw(1 2 3);
+    our $pkg;
+    my $lex;
+
+    is("-", '-', '"-"');
+    is("-", '-', '"-"');
+    is("-", '-', '"-"');
+    is("-", '-', '"-"');
+    is($a1, '1', '$a1');
+    is("-".$a1, '-1', '"-".$a1');
+    is($a1."-", '1-', '$a1."-"');
+    is("-".$a1."-", '-1-', '"-".$a1."-"');
+    is("$a1", '1', '"$a1"');
+    is("-$a1", '-1', '"-$a1"');
+    is("$a1-", '1-', '"$a1-"');
+    is("-$a1-", '-1-', '"-$a1-"');
+    is($a1.$a2, '12', '$a1.$a2');
+    is($a1."-".$a2, '1-2', '$a1."-".$a2');
+    is("-".$a1."-".$a2, '-1-2', '"-".$a1."-".$a2');
+    is($a1."-".$a2."-", '1-2-', '$a1."-".$a2."-"');
+    is("-".$a1."-".$a2."-", '-1-2-', '"-".$a1."-".$a2."-"');
+    is("$a1$a2", '12', '"$a1$a2"');
+    is("$a1-$a2", '1-2', '"$a1-$a2"');
+    is("-$a1-$a2", '-1-2', '"-$a1-$a2"');
+    is("$a1-$a2-", '1-2-', '"$a1-$a2-"');
+    is("-$a1-$a2-", '-1-2-', '"-$a1-$a2-"');
+    is($a1.$a2.$a3, '123', '$a1.$a2.$a3');
+    is($a1."-".$a2."-".$a3, '1-2-3', '$a1."-".$a2."-".$a3');
+    is("-".$a1."-".$a2."-".$a3, '-1-2-3', '"-".$a1."-".$a2."-".$a3');
+    is($a1."-".$a2."-".$a3."-", '1-2-3-', '$a1."-".$a2."-".$a3."-"');
+    is("-".$a1."-".$a2."-".$a3."-", '-1-2-3-', '"-".$a1."-".$a2."-".$a3."-"');
+    is("$a1$a2$a3", '123', '"$a1$a2$a3"');
+    is("$a1-$a2-$a3", '1-2-3', '"$a1-$a2-$a3"');
+    is("-$a1-$a2-$a3", '-1-2-3', '"-$a1-$a2-$a3"');
+    is("$a1-$a2-$a3-", '1-2-3-', '"$a1-$a2-$a3-"');
+    is("-$a1-$a2-$a3-", '-1-2-3-', '"-$a1-$a2-$a3-"');
+    $pkg  = "-";
+    is($pkg, '-', '$pkg  = "-"');
+    $pkg  = "-";
+    is($pkg, '-', '$pkg  = "-"');
+    $pkg  = "-";
+    is($pkg, '-', '$pkg  = "-"');
+    $pkg  = "-";
+    is($pkg, '-', '$pkg  = "-"');
+    $pkg  = $a1;
+    is($pkg, '1', '$pkg  = $a1');
+    $pkg  = "-".$a1;
+    is($pkg, '-1', '$pkg  = "-".$a1');
+    $pkg  = $a1."-";
+    is($pkg, '1-', '$pkg  = $a1."-"');
+    $pkg  = "-".$a1."-";
+    is($pkg, '-1-', '$pkg  = "-".$a1."-"');
+    $pkg  = "$a1";
+    is($pkg, '1', '$pkg  = "$a1"');
+    $pkg  = "-$a1";
+    is($pkg, '-1', '$pkg  = "-$a1"');
+    $pkg  = "$a1-";
+    is($pkg, '1-', '$pkg  = "$a1-"');
+    $pkg  = "-$a1-";
+    is($pkg, '-1-', '$pkg  = "-$a1-"');
+    $pkg  = $a1.$a2;
+    is($pkg, '12', '$pkg  = $a1.$a2');
+    $pkg  = $a1."-".$a2;
+    is($pkg, '1-2', '$pkg  = $a1."-".$a2');
+    $pkg  = "-".$a1."-".$a2;
+    is($pkg, '-1-2', '$pkg  = "-".$a1."-".$a2');
+    $pkg  = $a1."-".$a2."-";
+    is($pkg, '1-2-', '$pkg  = $a1."-".$a2."-"');
+    $pkg  = "-".$a1."-".$a2."-";
+    is($pkg, '-1-2-', '$pkg  = "-".$a1."-".$a2."-"');
+    $pkg  = "$a1$a2";
+    is($pkg, '12', '$pkg  = "$a1$a2"');
+    $pkg  = "$a1-$a2";
+    is($pkg, '1-2', '$pkg  = "$a1-$a2"');
+    $pkg  = "-$a1-$a2";
+    is($pkg, '-1-2', '$pkg  = "-$a1-$a2"');
+    $pkg  = "$a1-$a2-";
+    is($pkg, '1-2-', '$pkg  = "$a1-$a2-"');
+    $pkg  = "-$a1-$a2-";
+    is($pkg, '-1-2-', '$pkg  = "-$a1-$a2-"');
+    $pkg  = $a1.$a2.$a3;
+    is($pkg, '123', '$pkg  = $a1.$a2.$a3');
+    $pkg  = $a1."-".$a2."-".$a3;
+    is($pkg, '1-2-3', '$pkg  = $a1."-".$a2."-".$a3');
+    $pkg  = "-".$a1."-".$a2."-".$a3;
+    is($pkg, '-1-2-3', '$pkg  = "-".$a1."-".$a2."-".$a3');
+    $pkg  = $a1."-".$a2."-".$a3."-";
+    is($pkg, '1-2-3-', '$pkg  = $a1."-".$a2."-".$a3."-"');
+    $pkg  = "-".$a1."-".$a2."-".$a3."-";
+    is($pkg, '-1-2-3-', '$pkg  = "-".$a1."-".$a2."-".$a3."-"');
+    $pkg  = "$a1$a2$a3";
+    is($pkg, '123', '$pkg  = "$a1$a2$a3"');
+    $pkg  = "$a1-$a2-$a3";
+    is($pkg, '1-2-3', '$pkg  = "$a1-$a2-$a3"');
+    $pkg  = "-$a1-$a2-$a3";
+    is($pkg, '-1-2-3', '$pkg  = "-$a1-$a2-$a3"');
+    $pkg  = "$a1-$a2-$a3-";
+    is($pkg, '1-2-3-', '$pkg  = "$a1-$a2-$a3-"');
+    $pkg  = "-$a1-$a2-$a3-";
+    is($pkg, '-1-2-3-', '$pkg  = "-$a1-$a2-$a3-"');
+    $pkg = 'P';
+    $pkg .= "-";
+    is($pkg, 'P-', '$pkg .= "-"');
+    $pkg = 'P';
+    $pkg .= "-";
+    is($pkg, 'P-', '$pkg .= "-"');
+    $pkg = 'P';
+    $pkg .= "-";
+    is($pkg, 'P-', '$pkg .= "-"');
+    $pkg = 'P';
+    $pkg .= "-";
+    is($pkg, 'P-', '$pkg .= "-"');
+    $pkg = 'P';
+    $pkg .= $a1;
+    is($pkg, 'P1', '$pkg .= $a1');
+    $pkg = 'P';
+    $pkg .= "-".$a1;
+    is($pkg, 'P-1', '$pkg .= "-".$a1');
+    $pkg = 'P';
+    $pkg .= $a1."-";
+    is($pkg, 'P1-', '$pkg .= $a1."-"');
+    $pkg = 'P';
+    $pkg .= "-".$a1."-";
+    is($pkg, 'P-1-', '$pkg .= "-".$a1."-"');
+    $pkg = 'P';
+    $pkg .= "$a1";
+    is($pkg, 'P1', '$pkg .= "$a1"');
+    $pkg = 'P';
+    $pkg .= "-$a1";
+    is($pkg, 'P-1', '$pkg .= "-$a1"');
+    $pkg = 'P';
+    $pkg .= "$a1-";
+    is($pkg, 'P1-', '$pkg .= "$a1-"');
+    $pkg = 'P';
+    $pkg .= "-$a1-";
+    is($pkg, 'P-1-', '$pkg .= "-$a1-"');
+    $pkg = 'P';
+    $pkg .= $a1.$a2;
+    is($pkg, 'P12', '$pkg .= $a1.$a2');
+    $pkg = 'P';
+    $pkg .= $a1."-".$a2;
+    is($pkg, 'P1-2', '$pkg .= $a1."-".$a2');
+    $pkg = 'P';
+    $pkg .= "-".$a1."-".$a2;
+    is($pkg, 'P-1-2', '$pkg .= "-".$a1."-".$a2');
+    $pkg = 'P';
+    $pkg .= $a1."-".$a2."-";
+    is($pkg, 'P1-2-', '$pkg .= $a1."-".$a2."-"');
+    $pkg = 'P';
+    $pkg .= "-".$a1."-".$a2."-";
+    is($pkg, 'P-1-2-', '$pkg .= "-".$a1."-".$a2."-"');
+    $pkg = 'P';
+    $pkg .= "$a1$a2";
+    is($pkg, 'P12', '$pkg .= "$a1$a2"');
+    $pkg = 'P';
+    $pkg .= "$a1-$a2";
+    is($pkg, 'P1-2', '$pkg .= "$a1-$a2"');
+    $pkg = 'P';
+    $pkg .= "-$a1-$a2";
+    is($pkg, 'P-1-2', '$pkg .= "-$a1-$a2"');
+    $pkg = 'P';
+    $pkg .= "$a1-$a2-";
+    is($pkg, 'P1-2-', '$pkg .= "$a1-$a2-"');
+    $pkg = 'P';
+    $pkg .= "-$a1-$a2-";
+    is($pkg, 'P-1-2-', '$pkg .= "-$a1-$a2-"');
+    $pkg = 'P';
+    $pkg .= $a1.$a2.$a3;
+    is($pkg, 'P123', '$pkg .= $a1.$a2.$a3');
+    $pkg = 'P';
+    $pkg .= $a1."-".$a2."-".$a3;
+    is($pkg, 'P1-2-3', '$pkg .= $a1."-".$a2."-".$a3');
+    $pkg = 'P';
+    $pkg .= "-".$a1."-".$a2."-".$a3;
+    is($pkg, 'P-1-2-3', '$pkg .= "-".$a1."-".$a2."-".$a3');
+    $pkg = 'P';
+    $pkg .= $a1."-".$a2."-".$a3."-";
+    is($pkg, 'P1-2-3-', '$pkg .= $a1."-".$a2."-".$a3."-"');
+    $pkg = 'P';
+    $pkg .= "-".$a1."-".$a2."-".$a3."-";
+    is($pkg, 'P-1-2-3-', '$pkg .= "-".$a1."-".$a2."-".$a3."-"');
+    $pkg = 'P';
+    $pkg .= "$a1$a2$a3";
+    is($pkg, 'P123', '$pkg .= "$a1$a2$a3"');
+    $pkg = 'P';
+    $pkg .= "$a1-$a2-$a3";
+    is($pkg, 'P1-2-3', '$pkg .= "$a1-$a2-$a3"');
+    $pkg = 'P';
+    $pkg .= "-$a1-$a2-$a3";
+    is($pkg, 'P-1-2-3', '$pkg .= "-$a1-$a2-$a3"');
+    $pkg = 'P';
+    $pkg .= "$a1-$a2-$a3-";
+    is($pkg, 'P1-2-3-', '$pkg .= "$a1-$a2-$a3-"');
+    $pkg = 'P';
+    $pkg .= "-$a1-$a2-$a3-";
+    is($pkg, 'P-1-2-3-', '$pkg .= "-$a1-$a2-$a3-"');
+    $lex  = "-";
+    is($lex, '-', '$lex  = "-"');
+    $lex  = "-";
+    is($lex, '-', '$lex  = "-"');
+    $lex  = "-";
+    is($lex, '-', '$lex  = "-"');
+    $lex  = "-";
+    is($lex, '-', '$lex  = "-"');
+    $lex  = $a1;
+    is($lex, '1', '$lex  = $a1');
+    $lex  = "-".$a1;
+    is($lex, '-1', '$lex  = "-".$a1');
+    $lex  = $a1."-";
+    is($lex, '1-', '$lex  = $a1."-"');
+    $lex  = "-".$a1."-";
+    is($lex, '-1-', '$lex  = "-".$a1."-"');
+    $lex  = "$a1";
+    is($lex, '1', '$lex  = "$a1"');
+    $lex  = "-$a1";
+    is($lex, '-1', '$lex  = "-$a1"');
+    $lex  = "$a1-";
+    is($lex, '1-', '$lex  = "$a1-"');
+    $lex  = "-$a1-";
+    is($lex, '-1-', '$lex  = "-$a1-"');
+    $lex  = $a1.$a2;
+    is($lex, '12', '$lex  = $a1.$a2');
+    $lex  = $a1."-".$a2;
+    is($lex, '1-2', '$lex  = $a1."-".$a2');
+    $lex  = "-".$a1."-".$a2;
+    is($lex, '-1-2', '$lex  = "-".$a1."-".$a2');
+    $lex  = $a1."-".$a2."-";
+    is($lex, '1-2-', '$lex  = $a1."-".$a2."-"');
+    $lex  = "-".$a1."-".$a2."-";
+    is($lex, '-1-2-', '$lex  = "-".$a1."-".$a2."-"');
+    $lex  = "$a1$a2";
+    is($lex, '12', '$lex  = "$a1$a2"');
+    $lex  = "$a1-$a2";
+    is($lex, '1-2', '$lex  = "$a1-$a2"');
+    $lex  = "-$a1-$a2";
+    is($lex, '-1-2', '$lex  = "-$a1-$a2"');
+    $lex  = "$a1-$a2-";
+    is($lex, '1-2-', '$lex  = "$a1-$a2-"');
+    $lex  = "-$a1-$a2-";
+    is($lex, '-1-2-', '$lex  = "-$a1-$a2-"');
+    $lex  = $a1.$a2.$a3;
+    is($lex, '123', '$lex  = $a1.$a2.$a3');
+    $lex  = $a1."-".$a2."-".$a3;
+    is($lex, '1-2-3', '$lex  = $a1."-".$a2."-".$a3');
+    $lex  = "-".$a1."-".$a2."-".$a3;
+    is($lex, '-1-2-3', '$lex  = "-".$a1."-".$a2."-".$a3');
+    $lex  = $a1."-".$a2."-".$a3."-";
+    is($lex, '1-2-3-', '$lex  = $a1."-".$a2."-".$a3."-"');
+    $lex  = "-".$a1."-".$a2."-".$a3."-";
+    is($lex, '-1-2-3-', '$lex  = "-".$a1."-".$a2."-".$a3."-"');
+    $lex  = "$a1$a2$a3";
+    is($lex, '123', '$lex  = "$a1$a2$a3"');
+    $lex  = "$a1-$a2-$a3";
+    is($lex, '1-2-3', '$lex  = "$a1-$a2-$a3"');
+    $lex  = "-$a1-$a2-$a3";
+    is($lex, '-1-2-3', '$lex  = "-$a1-$a2-$a3"');
+    $lex  = "$a1-$a2-$a3-";
+    is($lex, '1-2-3-', '$lex  = "$a1-$a2-$a3-"');
+    $lex  = "-$a1-$a2-$a3-";
+    is($lex, '-1-2-3-', '$lex  = "-$a1-$a2-$a3-"');
+    $lex = 'L';
+    $lex .= "-";
+    is($lex, 'L-', '$lex .= "-"');
+    $lex = 'L';
+    $lex .= "-";
+    is($lex, 'L-', '$lex .= "-"');
+    $lex = 'L';
+    $lex .= "-";
+    is($lex, 'L-', '$lex .= "-"');
+    $lex = 'L';
+    $lex .= "-";
+    is($lex, 'L-', '$lex .= "-"');
+    $lex = 'L';
+    $lex .= $a1;
+    is($lex, 'L1', '$lex .= $a1');
+    $lex = 'L';
+    $lex .= "-".$a1;
+    is($lex, 'L-1', '$lex .= "-".$a1');
+    $lex = 'L';
+    $lex .= $a1."-";
+    is($lex, 'L1-', '$lex .= $a1."-"');
+    $lex = 'L';
+    $lex .= "-".$a1."-";
+    is($lex, 'L-1-', '$lex .= "-".$a1."-"');
+    $lex = 'L';
+    $lex .= "$a1";
+    is($lex, 'L1', '$lex .= "$a1"');
+    $lex = 'L';
+    $lex .= "-$a1";
+    is($lex, 'L-1', '$lex .= "-$a1"');
+    $lex = 'L';
+    $lex .= "$a1-";
+    is($lex, 'L1-', '$lex .= "$a1-"');
+    $lex = 'L';
+    $lex .= "-$a1-";
+    is($lex, 'L-1-', '$lex .= "-$a1-"');
+    $lex = 'L';
+    $lex .= $a1.$a2;
+    is($lex, 'L12', '$lex .= $a1.$a2');
+    $lex = 'L';
+    $lex .= $a1."-".$a2;
+    is($lex, 'L1-2', '$lex .= $a1."-".$a2');
+    $lex = 'L';
+    $lex .= "-".$a1."-".$a2;
+    is($lex, 'L-1-2', '$lex .= "-".$a1."-".$a2');
+    $lex = 'L';
+    $lex .= $a1."-".$a2."-";
+    is($lex, 'L1-2-', '$lex .= $a1."-".$a2."-"');
+    $lex = 'L';
+    $lex .= "-".$a1."-".$a2."-";
+    is($lex, 'L-1-2-', '$lex .= "-".$a1."-".$a2."-"');
+    $lex = 'L';
+    $lex .= "$a1$a2";
+    is($lex, 'L12', '$lex .= "$a1$a2"');
+    $lex = 'L';
+    $lex .= "$a1-$a2";
+    is($lex, 'L1-2', '$lex .= "$a1-$a2"');
+    $lex = 'L';
+    $lex .= "-$a1-$a2";
+    is($lex, 'L-1-2', '$lex .= "-$a1-$a2"');
+    $lex = 'L';
+    $lex .= "$a1-$a2-";
+    is($lex, 'L1-2-', '$lex .= "$a1-$a2-"');
+    $lex = 'L';
+    $lex .= "-$a1-$a2-";
+    is($lex, 'L-1-2-', '$lex .= "-$a1-$a2-"');
+    $lex = 'L';
+    $lex .= $a1.$a2.$a3;
+    is($lex, 'L123', '$lex .= $a1.$a2.$a3');
+    $lex = 'L';
+    $lex .= $a1."-".$a2."-".$a3;
+    is($lex, 'L1-2-3', '$lex .= $a1."-".$a2."-".$a3');
+    $lex = 'L';
+    $lex .= "-".$a1."-".$a2."-".$a3;
+    is($lex, 'L-1-2-3', '$lex .= "-".$a1."-".$a2."-".$a3');
+    $lex = 'L';
+    $lex .= $a1."-".$a2."-".$a3."-";
+    is($lex, 'L1-2-3-', '$lex .= $a1."-".$a2."-".$a3."-"');
+    $lex = 'L';
+    $lex .= "-".$a1."-".$a2."-".$a3."-";
+    is($lex, 'L-1-2-3-', '$lex .= "-".$a1."-".$a2."-".$a3."-"');
+    $lex = 'L';
+    $lex .= "$a1$a2$a3";
+    is($lex, 'L123', '$lex .= "$a1$a2$a3"');
+    $lex = 'L';
+    $lex .= "$a1-$a2-$a3";
+    is($lex, 'L1-2-3', '$lex .= "$a1-$a2-$a3"');
+    $lex = 'L';
+    $lex .= "-$a1-$a2-$a3";
+    is($lex, 'L-1-2-3', '$lex .= "-$a1-$a2-$a3"');
+    $lex = 'L';
+    $lex .= "$a1-$a2-$a3-";
+    is($lex, 'L1-2-3-', '$lex .= "$a1-$a2-$a3-"');
+    $lex = 'L';
+    $lex .= "-$a1-$a2-$a3-";
+    is($lex, 'L-1-2-3-', '$lex .= "-$a1-$a2-$a3-"');
+    {
+        my $l = "-";
+        is($l, '-', 'my $l = "-"');
+    }
+    {
+        my $l = "-";
+        is($l, '-', 'my $l = "-"');
+    }
+    {
+        my $l = "-";
+        is($l, '-', 'my $l = "-"');
+    }
+    {
+        my $l = "-";
+        is($l, '-', 'my $l = "-"');
+    }
+    {
+        my $l = $a1;
+        is($l, '1', 'my $l = $a1');
+    }
+    {
+        my $l = "-".$a1;
+        is($l, '-1', 'my $l = "-".$a1');
+    }
+    {
+        my $l = $a1."-";
+        is($l, '1-', 'my $l = $a1."-"');
+    }
+    {
+        my $l = "-".$a1."-";
+        is($l, '-1-', 'my $l = "-".$a1."-"');
+    }
+    {
+        my $l = "$a1";
+        is($l, '1', 'my $l = "$a1"');
+    }
+    {
+        my $l = "-$a1";
+        is($l, '-1', 'my $l = "-$a1"');
+    }
+    {
+        my $l = "$a1-";
+        is($l, '1-', 'my $l = "$a1-"');
+    }
+    {
+        my $l = "-$a1-";
+        is($l, '-1-', 'my $l = "-$a1-"');
+    }
+    {
+        my $l = $a1.$a2;
+        is($l, '12', 'my $l = $a1.$a2');
+    }
+    {
+        my $l = $a1."-".$a2;
+        is($l, '1-2', 'my $l = $a1."-".$a2');
+    }
+    {
+        my $l = "-".$a1."-".$a2;
+        is($l, '-1-2', 'my $l = "-".$a1."-".$a2');
+    }
+    {
+        my $l = $a1."-".$a2."-";
+        is($l, '1-2-', 'my $l = $a1."-".$a2."-"');
+    }
+    {
+        my $l = "-".$a1."-".$a2."-";
+        is($l, '-1-2-', 'my $l = "-".$a1."-".$a2."-"');
+    }
+    {
+        my $l = "$a1$a2";
+        is($l, '12', 'my $l = "$a1$a2"');
+    }
+    {
+        my $l = "$a1-$a2";
+        is($l, '1-2', 'my $l = "$a1-$a2"');
+    }
+    {
+        my $l = "-$a1-$a2";
+        is($l, '-1-2', 'my $l = "-$a1-$a2"');
+    }
+    {
+        my $l = "$a1-$a2-";
+        is($l, '1-2-', 'my $l = "$a1-$a2-"');
+    }
+    {
+        my $l = "-$a1-$a2-";
+        is($l, '-1-2-', 'my $l = "-$a1-$a2-"');
+    }
+    {
+        my $l = $a1.$a2.$a3;
+        is($l, '123', 'my $l = $a1.$a2.$a3');
+    }
+    {
+        my $l = $a1."-".$a2."-".$a3;
+        is($l, '1-2-3', 'my $l = $a1."-".$a2."-".$a3');
+    }
+    {
+        my $l = "-".$a1."-".$a2."-".$a3;
+        is($l, '-1-2-3', 'my $l = "-".$a1."-".$a2."-".$a3');
+    }
+    {
+        my $l = $a1."-".$a2."-".$a3."-";
+        is($l, '1-2-3-', 'my $l = $a1."-".$a2."-".$a3."-"');
+    }
+    {
+        my $l = "-".$a1."-".$a2."-".$a3."-";
+        is($l, '-1-2-3-', 'my $l = "-".$a1."-".$a2."-".$a3."-"');
+    }
+    {
+        my $l = "$a1$a2$a3";
+        is($l, '123', 'my $l = "$a1$a2$a3"');
+    }
+    {
+        my $l = "$a1-$a2-$a3";
+        is($l, '1-2-3', 'my $l = "$a1-$a2-$a3"');
+    }
+    {
+        my $l = "-$a1-$a2-$a3";
+        is($l, '-1-2-3', 'my $l = "-$a1-$a2-$a3"');
+    }
+    {
+        my $l = "$a1-$a2-$a3-";
+        is($l, '1-2-3-', 'my $l = "$a1-$a2-$a3-"');
+    }
+    {
+        my $l = "-$a1-$a2-$a3-";
+        is($l, '-1-2-3-', 'my $l = "-$a1-$a2-$a3-"');
+    }
+}
+
+# multiconcat optimises away scalar assign, and is responsible
+# for handling the assign itself. If the LHS is something weird,
+# make sure it's handled ok
+
+{
+    my $a = 'a';
+    my $b = 'b';
+    my $o = 'o';
+
+    my $re = qr/abc/;
+    $$re = $a . $b;
+    is($$re, "ab", '$$re = $a . $b');
+
+    #passing a hash elem to a sub creates a PVLV
+    my $s = sub { $_[0] = $a . $b; };
+    my %h;
+    $s->($h{foo});
+    is($h{foo}, "ab", "PVLV");
+
+    # assigning a string to a typeglob creates an alias
+    $Foo = 'myfoo';
+    *Bar = ("F" . $o . $o);
+    is($Bar, "myfoo", '*Bar = "Foo"');
+
+    # while that same typeglob also appearing on the RHS returns
+    # a stringified value
+
+    package QPR {
+        ${'*QPR::Bar*QPR::BarBaz'} = 'myfoobarbaz';
+        *Bar = (*Bar  . *Bar . "Baz");
+        ::is($Bar, "myfoobarbaz", '*Bar =  (*Bar  . *Bar . "Baz")');
+    }
+}
+
+# distinguish between '=' and  '.=' where the LHS has the OPf_MOD flag
+
+{
+    my $foo = "foo";
+    my $a . $foo; # weird but legal
+    is($a, '', 'my $a . $foo');
+    my $b; $b .= $foo;
+    is($b, 'foo', 'my $b; $b .= $foo');
+}
+
+# distinguish between nested appends and concats; the former is
+# affected by the change of value of the target on each concat.
+# This is why multiconcat shouldn't be used in that case
+
+{
+    my $a = "a";
+    (($a .= $a) .= $a) .= $a;
+    is($a, "aaaaaaaa", '(($a .= $a) .= $a) .= $a;');
+}
+
+# check everything works ok near the max arg size of a multiconcat
+
+{
+    my @a = map "<$_>", 0..99;
+    for my $i (60..68) { # check each side of 64 threshold
+        my $c = join '.', map "\$a[$_]", 0..$i;
+        my $got = eval $c or die $@;
+        my $empty = ''; # don't use a const string in case join'' ever
+                        # gets optimised into a multiconcat
+        my $expected = join $empty, @a[0..$i];
+        is($got, $expected, "long concat chain $i");
+    }
+}