optimize pp_length for simple PVs

Previously in pp_length, for non-magic, non-utf8 PVs, a number of
conditional branches based on bitfield testing had to execute, and finally
the length of a non-utf8 SVPV was fetched with sv_len_utf8_nomg.
sv_len_utf8_nomg checks SvUTF8, and if false, use the len from
SvPV_nomg_const call in sv_len_utf8_nomg. Note in pp_length,
SvPV_nomg_const already exists for the "use bytes" branch.

After this patch, a !SvGMAGICAL && (!SvUTF8  || use_bytes) SV will take
only 1 conditional branch and no func calls before reaching sv_setiv.
svflags is reused for the mg_get test since partially masked svflags will
already be in a register vs fetching the whole flags from SV head again.

SETS(TARG) was factored out from all the SETI/SETTARG macros. If targ is
not set/written to (the PL_sv_undef branch), do not check/call for
set magic on it, just return from the opcode. Also a putback was removed
since the operand SV is replaced inplace on Perl stack with a SV with IV
length in it.

For profiling info, profiling stats, and rejected implementations see
[perl #122835]

2 files changed, 32 insertions, 10 deletions

diff --git a/pod/perldelta.pod b/pod/perldelta.pod
index 608c1df506..65c495aa0e 100644
--- a/pod/perldelta.pod
+++ b/pod/perldelta.pod
@@ -104,7 +104,8 @@ There may well be none in a stable release.
 
 =item *
 
-XXX
+C<length> is upto 20% faster for non-magical/non-tied containing a string and
+either non-utf8 or C<use bytes;> scalars.
 
 =back
 
diff --git a/pp.c b/pp.c
index d33914b449..18c3f03464 100644
--- a/pp.c
+++ b/pp.c
@@ -2956,24 +2956,45 @@ PP(pp_length)
     dSP; dTARGET;
     SV * const sv = TOPs;
 
-    SvGETMAGIC(sv);
+    U32 in_bytes = IN_BYTES;
+    /* simplest case shortcut */
+    /* turn off SVf_UTF8 in tmp flags if HINT_BYTES on*/
+    U32 svflags = (SvFLAGS(sv) ^ (in_bytes << 26)) & (SVf_POK|SVs_GMG|SVf_UTF8);
+    assert(HINT_BYTES == 0x00000008 && SVf_UTF8 == 0x20000000 && (SVf_UTF8 == HINT_BYTES << 26));
+    SETs(TARG);
+
+    if(LIKELY(svflags == SVf_POK))
+        goto simple_pv;
+    if(svflags & SVs_GMG)
+        mg_get(sv);
     if (SvOK(sv)) {
-	if (!IN_BYTES)
-	    SETi(sv_len_utf8_nomg(sv));
+	if (!IN_BYTES) /* reread to avoid using an C auto/register */
+	    sv_setiv(TARG, (IV)sv_len_utf8_nomg(sv));
 	else
 	{
 	    STRLEN len;
-	    (void)SvPV_nomg_const(sv,len);
-	    SETi(len);
+            /* unrolled SvPV_nomg_const(sv,len) */
+            if(SvPOK_nog(sv)){
+                simple_pv:
+                len = SvCUR(sv);
+            } else  {
+                (void)sv_2pv_flags(sv, &len, 0|SV_CONST_RETURN);
+            }
+	    sv_setiv(TARG, (IV)(len));
 	}
     } else {
 	if (!SvPADTMP(TARG)) {
 	    sv_setsv_nomg(TARG, &PL_sv_undef);
-	    SETTARG;
-	}
-	SETs(&PL_sv_undef);
+	} else { /* TARG is on stack at this point and is overwriten by SETs.
+                   This branch is the odd one out, so put TARG by default on
+                   stack earlier to let local SP go out of liveness sooner */
+            SETs(&PL_sv_undef);
+            goto no_set_magic;
+        }
     }
-    RETURN;
+    SvSETMAGIC(TARG);
+    no_set_magic:
+    return NORMAL; /* no putback, SP didn't move in this opcode */
 }
 
 /* Returns false if substring is completely outside original string.