toke.c: Add conditional to skip memcmp()

S_scan_str() calls memEQ() a bunch of times.  The general case is that
the strings are multiple bytes long, but most of the time, at least one
of the operands will be just a single byte.  We can avoid a libc call by
just comparing the single bytes when the length is 1.

Tony Cook is of the opinion that other uses of memEQ() will more likely
be multiple bytes, so adding this check to all calls would not be
beneficial.  I looked through the core source, and agree.  So this adds
a macro that tests for a single byte, and if multiple calls plain
memEQ().

Spotted by Daniel Dragan

Since the tokenizer is not hot code, this won't make a noticeable
difference in performance.  I think the reason to do it is mainly to
show it has been done for people in the future who would otherwise come
along and notice this

Author: khwilliamson

toke.c

@@ -11564,6 +11564,11 @@ S_scan_inputsymbol(pTHX_ char *start)
     return s;
 }
 
+/* In the function below, it's quite likely that the calls to memEQ() will have
+ * 'len' == 1.  So create a new macro that adds a conditional to skip the libc
+ * call. */
+#define memEQ_1(a, b, len)                                                  \
+   ((*(a) == *(b)) && ((LIKELY((len) <= 1) || memEQ((a)+1, (b)+1, (len)-1))))
 
 /* scan_str
    takes:
@@ -11616,7 +11621,6 @@ S_scan_inputsymbol(pTHX_ char *start)
    For convenience, the terminating delimiter character is stuffed into
    SvIVX of the SV.
 */
-
 char *
 Perl_scan_str(pTHX_ char *start, int keep_bracketed_quoted, int keep_delims, int re_reparse,
                  char **delimp
@@ -11796,18 +11800,18 @@ Perl_scan_str(pTHX_ char *start, int keep_bracketed_quoted, int keep_delims, int
                  * discard those that escape the closing delimiter, just
                  * discard this one */
                 if (   !  keep_bracketed_quoted
-                    &&   (    memEQ(s + 1,  open_delim_str, delim_byte_len)
+                    &&   (    memEQ_1(s + 1,  open_delim_str, delim_byte_len)
                           ||  (   PL_multi_open == PL_multi_close
                                && re_reparse && s[1] == '\\')
-                          ||  memEQ(s + 1, close_delim_str, delim_byte_len)))
+                          ||  memEQ_1(s + 1, close_delim_str, delim_byte_len)))
                 {
                     s++;
                 }
                 else /* any other escapes are simply copied straight through */
                     *to++ = *s++;
             }
             else if (   s < PL_bufend - (delim_byte_len - 1)
-                     && memEQ(s, close_delim_str, delim_byte_len)
+                     && memEQ_1(s, close_delim_str, delim_byte_len)
                      && --brackets <= 0)
             {
                 /* Found unescaped closing delimiter, unnested if we care about
@@ -11836,7 +11840,7 @@ Perl_scan_str(pTHX_ char *start, int keep_bracketed_quoted, int keep_delims, int
                         /* No nesting if open eq close */
             else if (   PL_multi_open != PL_multi_close
                      && s < PL_bufend - (delim_byte_len - 1)
-                     && memEQ(s, open_delim_str, delim_byte_len))
+                     && memEQ_1(s, open_delim_str, delim_byte_len))
             {
                 brackets++;
             }