offset PL_savestack_max by SS_MAXPUSH

The newer SS_ADD macros expect there to always be SS_MAXPUSH slots
free on the savestack; so they can push multiple items, then only check
once at the end whether stack needs expanding.

This commit makes savestack_grow() set PL_savestack_max to SS_MAXPUSH
short of what has actually been allocated. This makes the tests
to see whether the stack needs growing slightly simpler, i.e.

    PL_savestack_ix > PL_savestack_max

rather than

    PL_savestack_ix + SS_MAXPUSH > PL_savestack_max

1 files changed, 7 insertions, 3 deletions

diff --git a/scope.c b/scope.c
index f1d1ccbe80..78a465bf66 100644
--- a/scope.c
+++ b/scope.c
@@ -140,15 +140,19 @@ Perl_markstack_grow(pTHX)
 void
 Perl_savestack_grow(pTHX)
 {
-    PL_savestack_max = GROW(PL_savestack_max) + 4;
-    Renew(PL_savestack, PL_savestack_max, ANY);
+    PL_savestack_max = GROW(PL_savestack_max);
+    /* Note that we allocate SS_MAXPUSH slots higher than ss_max
+     * so that SS_ADD_END(), SSGROW() etc can do a simper check */
+    Renew(PL_savestack, PL_savestack_max + SS_MAXPUSH, ANY);
 }
 
 void
 Perl_savestack_grow_cnt(pTHX_ I32 need)
 {
     PL_savestack_max = PL_savestack_ix + need;
-    Renew(PL_savestack, PL_savestack_max, ANY);
+    /* Note that we allocate SS_MAXPUSH slots higher than ss_max
+     * so that SS_ADD_END(), SSGROW() etc can do a simper check */
+    Renew(PL_savestack, PL_savestack_max + SS_MAXPUSH, ANY);
 }
 
 #undef GROW