optimize & rmv from public API Perl_tmps_grow and related code

Previously in PUSH_EXTEND_MORTAL__SV_C, "PL_tmps_ix + 1" would execute
twice, once for the nonmutable if(>=), then again after the potential
tmps_grow call. tmps_grow has an unused return register/void proto, put it
to use by returning ix.  Also change tmps_grow to take the result of
"PL_tmps_ix + the constant (usually 1) or non-constant (EXTEND_MORTAL)".
This avoid having to put the constant twice in machine code, once for the
if test, 2nd time for extend length param for tmps_grow call. For
non-constant/EXTEND_MORTAL usage, it allows the C optimizer to have the
length var to go out of liveness sooner if possible. Also the var used for
the if(>=) test is more likely to be in a register than length var.
So "if test variable" is closer on hand to the CPU than length var. In some
cases, if non-const len var isn't used again, it becomes the "ix" variable
by having PL_tmps_ix added to it. Change sv_2mortal to return sv instead
of NULL to remove a unique branch/block of machine code that assigns 0 to
return variable (Visual C didn't figure out return sv == returned NULL,
not sv). See also [perl #121845].

1 files changed, 5 insertions, 3 deletions

diff --git a/sv.c b/sv.c
index dd0a97e2b4..665a0f6f89 100644
--- a/sv.c
+++ b/sv.c
@@ -8906,8 +8906,10 @@ Perl_sv_dec_nomg(pTHX_ SV *const sv)
  */
 #define PUSH_EXTEND_MORTAL__SV_C(AnSv) \
     STMT_START {      \
-	EXTEND_MORTAL(1); \
-	PL_tmps_stack[++PL_tmps_ix] = (AnSv); \
+	SSize_t ix = ++PL_tmps_ix;		\
+	if (UNLIKELY(ix >= PL_tmps_max))	\
+	    ix = tmps_grow_p(ix);			\
+	PL_tmps_stack[ix] = (AnSv); \
     } STMT_END
 
 /*
@@ -9029,7 +9031,7 @@ Perl_sv_2mortal(pTHX_ SV *const sv)
 {
     dVAR;
     if (!sv)
-	return NULL;
+	return sv;
     if (SvIMMORTAL(sv))
 	return sv;
     PUSH_EXTEND_MORTAL__SV_C(sv);