[perl #82111] de-pessimise some my @array = ...

Due to obscure closure and goto tricks, it's sometimes possible for the
array or hash in the LHS of 'my @a = ...' and 'my %h = ...' to be
non-empty. At compile-time, these conditions are detected and the assign
op is compiled with the OPpASSIGN_COMMON, making the assignment slower.

This commit speeds it up again by adding a run-time check to pp_aassign
to only do the OPpASSIGN_COMMON code-branch if the LHS isn't an empty
array or hash.

See also #70171.

1 files changed, 12 insertions, 1 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 852ff80d43..f8a61cbdb2 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -989,8 +989,19 @@ PP(pp_aassign)
     /* If there's a common identifier on both sides we have to take
      * special care that assigning the identifier on the left doesn't
      * clobber a value on the right that's used later in the list.
+     * Don't bother if LHS is just an empty hash or array.
      */
-    if (PL_op->op_private & (OPpASSIGN_COMMON)) {
+
+    if (    (PL_op->op_private & OPpASSIGN_COMMON)
+	&&  (
+	       firstlelem != lastlelem
+	    || ! ((sv = *firstlelem))
+	    || SvMAGICAL(sv)
+	    || ! (SvTYPE(sv) == SVt_PVAV || SvTYPE(sv) == SVt_PVHV)
+	    || (SvTYPE(sv) == SVt_PVAV && AvFILL((AV*)sv) != -1)
+	    || (SvTYPE(sv) == SVt_PVHV && HvKEYS((HV*)sv) != 0)
+	    )
+    ) {
 	EXTEND_MORTAL(lastrelem - firstrelem + 1);
 	for (relem = firstrelem; relem <= lastrelem; relem++) {
 	    if ((sv = *relem)) {