Add Perl_bytes_cmp_utf8() to compare character sequences in different encodings

Convert sv_eq_flags() and sv_cmp_flags() to use it.

Previously, to compare two strings of characters, where was was in UTF-8, and
one was not, you had to either:

1: Upgrade the second to UTF-8
2: Compare the resulting octet sequence
3: Free the temporary UTF-8 string

or:

1: Attempt to downgrade the first to bytes. If it can't be, they aren't equal
2: Else compare the resulting octet sequence
3: Free the temporary byte string

Which for the general case involves a malloc()/free() and at least two O(n)
scans per comparison.

Whereas this approach has no allocation, a single O(n) scan, which terminates
as early as the best case for the second approach.

1 files changed, 12 insertions, 21 deletions

diff --git a/sv.c b/sv.c
index 27b4bd6fa8..a1ca186b6f 100644
--- a/sv.c
+++ b/sv.c
@@ -7044,28 +7044,15 @@ Perl_sv_eq_flags(pTHX_ register SV *sv1, register SV *sv2, const U32 flags)
 	      }
 	 }
 	 else {
-	      bool is_utf8 = TRUE;
-
 	      if (SvUTF8(sv1)) {
-		   /* sv1 is the UTF-8 one,
-		    * if is equal it must be downgrade-able */
-		   char * const pv = (char*)bytes_from_utf8((const U8*)pv1,
-						     &cur1, &is_utf8);
-		   if (pv != pv1)
-			pv1 = tpv = pv;
+		  /* sv1 is the UTF-8 one  */
+		  return bytes_cmp_utf8((const U8*)pv2, cur2,
+					(const U8*)pv1, cur1) == 0;
 	      }
 	      else {
-		   /* sv2 is the UTF-8 one,
-		    * if is equal it must be downgrade-able */
-		   char * const pv = (char *)bytes_from_utf8((const U8*)pv2,
-						      &cur2, &is_utf8);
-		   if (pv != pv2)
-			pv2 = tpv = pv;
-	      }
-	      if (is_utf8) {
-		   /* Downgrade not possible - cannot be eq */
-		   assert (tpv == 0);
-		   return FALSE;
+		  /* sv2 is the UTF-8 one  */
+		  return bytes_cmp_utf8((const U8*)pv1, cur1,
+					(const U8*)pv2, cur2) == 0;
 	      }
 	 }
     }
@@ -7140,7 +7127,9 @@ Perl_sv_cmp_flags(pTHX_ register SV *const sv1, register SV *const sv2,
 		 pv2 = SvPV_const(svrecode, cur2);
 	    }
 	    else {
-		 pv2 = tpv = (char*)bytes_to_utf8((const U8*)pv2, &cur2);
+		const int retval = -bytes_cmp_utf8((const U8*)pv2, cur2,
+						   (const U8*)pv1, cur1);
+		return retval ? retval < 0 ? -1 : +1 : 0;
 	    }
 	}
 	else {
@@ -7150,7 +7139,9 @@ Perl_sv_cmp_flags(pTHX_ register SV *const sv1, register SV *const sv2,
 		 pv1 = SvPV_const(svrecode, cur1);
 	    }
 	    else {
-		 pv1 = tpv = (char*)bytes_to_utf8((const U8*)pv1, &cur1);
+		const int retval = bytes_cmp_utf8((const U8*)pv1, cur1,
+						  (const U8*)pv2, cur2);
+		return retval ? retval < 0 ? -1 : +1 : 0;
 	    }
 	}
     }