Better optimise my/local @a = split()

There are currently two optimisations for when the results of a split
are assigned to an array.

For the first,

    @array = split(...);

the aassign and padav/rv2av are optimised away, and pp_split() directly
assigns to the array attached to the split op (via op_pmtargetoff or
op_pmtargetgv).

For the second,

    my @array    = split(...);
    local @array = split(...);
    @{$expr}     = split(...);

The aassign is optimised away, but the padav/rv2av is kept as an additional
arg to split. pp_split itself then uses the first arg popped off the stack
as the array (This was introduced by FC with v5.21.4-409-gef7999f).

This commit moves these two:

    my @array    = split(...);
    local @array = split(...);

from the second case to the first case, by simply setting OPpLVAL_INTRO
on the OP_SPLIT, and making pp_split() do SAVECLEARSV() or save_ary()
as appropriate.

This makes my @a = split(...) a few percent faster.

1 files changed, 14 insertions, 5 deletions

diff --git a/pp.c b/pp.c
index eab970d3cc..00a577e278 100644
--- a/pp.c
+++ b/pp.c
@@ -5708,8 +5708,8 @@ PP(pp_reverse)
 PP(pp_split)
 {
     dSP; dTARG;
-    AV *ary = (   (PL_op->op_private & OPpSPLIT_ASSIGN)
-               && (PL_op->op_flags & OPf_STACKED))
+    AV *ary = (   (PL_op->op_private & OPpSPLIT_ASSIGN) /* @a = split */
+               && (PL_op->op_flags & OPf_STACKED))      /* @{expr} = split */
                ? (AV *)POPs : NULL;
     IV limit = POPi;			/* note, negative is forever */
     SV * const sv = POPs;
@@ -5733,7 +5733,7 @@ PP(pp_split)
     I32 base;
     const U8 gimme = GIMME_V;
     bool gimme_scalar;
-    const I32 oldsave = PL_savestack_ix;
+    I32 oldsave = PL_savestack_ix;
     U32 make_mortal = SVs_TEMP;
     bool multiline = 0;
     MAGIC *mg = NULL;
@@ -5743,10 +5743,14 @@ PP(pp_split)
     TAINT_IF(get_regex_charset(RX_EXTFLAGS(rx)) == REGEX_LOCALE_CHARSET &&
              (RX_EXTFLAGS(rx) & (RXf_WHITE | RXf_SKIPWHITE)));
 
+    /* handle @ary = split(...) optimisation */
     if (PL_op->op_private & OPpSPLIT_ASSIGN) {
         if (!(PL_op->op_flags & OPf_STACKED)) {
-            if (PL_op->op_private & OPpSPLIT_LEX)
+            if (PL_op->op_private & OPpSPLIT_LEX) {
+                if (PL_op->op_private & OPpLVAL_INTRO)
+                    SAVECLEARSV(PAD_SVl(pm->op_pmreplrootu.op_pmtargetoff));
                 ary = (AV *)PAD_SVl(pm->op_pmreplrootu.op_pmtargetoff);
+            }
             else {
                 GV *gv =
 #ifdef USE_ITHREADS
@@ -5754,8 +5758,13 @@ PP(pp_split)
 #else
                         pm->op_pmreplrootu.op_pmtargetgv;
 #endif
-                ary = GvAVn(gv);
+                if (PL_op->op_private & OPpLVAL_INTRO)
+                    ary = save_ary(gv);
+                else
+                    ary = GvAVn(gv);
             }
+            /* skip anything pushed by OPpLVAL_INTRO above */
+            oldsave = PL_savestack_ix;
         }
 
 	realarray = 1;