multiconcat: don't fold adjacent constants

RT #132385

In something like

    $overloaded . "a" . "b"

perl used to do

    $overloaded->concat("a")->concat("b")

but since the introduction of OP_MULTICONCAT, started doing:

    $overloaded->concat("ab")

This commit restores the old behaviour, by keeping every second adjacent
OP_CONST as an arg rather than optimising it away and adding its contents
to the constant string in the aux struct.

But note that

    $overloaded .=  "a" . "b"

originally, and still, constant folds.

1 files changed, 30 insertions, 1 deletions

diff --git a/t/perf/opcount.t b/t/perf/opcount.t
index 0ded6cd360..2d0ade56b0 100644
--- a/t/perf/opcount.t
+++ b/t/perf/opcount.t
@@ -20,7 +20,7 @@ BEGIN {
 use warnings;
 use strict;
 
-plan 2579;
+plan 2582;
 
 use B ();
 
@@ -634,3 +634,32 @@ test_opcount(0, "state works with multiconcat",
                     once        => 1,
                     padsv       => 2, # one each for the next/once branches
                 });
+
+# multiple concats of constants preceded by at least one non-constant
+# shouldn't get constant-folded so that a concat overload method is called
+# for each arg. So every second constant string is left as an OP_CONST
+
+test_opcount(0, "multiconcat: 2 adjacent consts",
+                sub { my ($a, $b); $a = $b . "c" . "d" },
+                {
+                    const       => 1,
+                    multiconcat => 1,
+                    concat      => 0,
+                    sassign     => 0,
+                });
+test_opcount(0, "multiconcat: 3 adjacent consts",
+                sub { my ($a, $b); $a = $b . "c" . "d" . "e" },
+                {
+                    const       => 1,
+                    multiconcat => 1,
+                    concat      => 0,
+                    sassign     => 0,
+                });
+test_opcount(0, "multiconcat: 4 adjacent consts",
+                sub { my ($a, $b); $a = $b . "c" . "d" . "e" ."f" },
+                {
+                    const       => 2,
+                    multiconcat => 1,
+                    concat      => 0,
+                    sassign     => 0,
+                });