Issue #13134: optimize finding single-character strings using memchr

Lib/test/test_unicode.py

@@ -171,6 +171,15 @@ class UnicodeTest(string_tests.CommonTest,
 
     def test_find(self):
         string_tests.CommonTest.test_find(self)
+        # test implementation details of the memchr fast path
+        self.checkequal(100, 'a' * 100 + '\u0102', 'find', '\u0102')
+        self.checkequal(-1, 'a' * 100 + '\u0102', 'find', '\u0201')
+        self.checkequal(-1, 'a' * 100 + '\u0102', 'find', '\u0120')
+        self.checkequal(-1, 'a' * 100 + '\u0102', 'find', '\u0220')
+        self.checkequal(100, 'a' * 100 + '\U00100304', 'find', '\U00100304')
+        self.checkequal(-1, 'a' * 100 + '\U00100304', 'find', '\U00100204')
+        self.checkequal(-1, 'a' * 100 + '\U00100304', 'find', '\U00102004')
+        # check mixed argument types
         self.checkequalnofix(0,  'abcdefghiabc', 'find', 'abc')
         self.checkequalnofix(9,  'abcdefghiabc', 'find', 'abc', 1)
         self.checkequalnofix(-1, 'abcdefghiabc', 'find', 'def', 4)
@@ -180,6 +189,14 @@ class UnicodeTest(string_tests.CommonTest,
 
     def test_rfind(self):
         string_tests.CommonTest.test_rfind(self)
+        # test implementation details of the memrchr fast path
+        self.checkequal(0, '\u0102' + 'a' * 100 , 'rfind', '\u0102')
+        self.checkequal(-1, '\u0102' + 'a' * 100 , 'rfind', '\u0201')
+        self.checkequal(-1, '\u0102' + 'a' * 100 , 'rfind', '\u0120')
+        self.checkequal(-1, '\u0102' + 'a' * 100 , 'rfind', '\u0220')
+        self.checkequal(0, '\U00100304' + 'a' * 100, 'rfind', '\U00100304')
+        self.checkequal(-1, '\U00100304' + 'a' * 100, 'rfind', '\U00100204')
+        self.checkequal(-1, '\U00100304' + 'a' * 100, 'rfind', '\U00102004')
         # check mixed argument types
         self.checkequalnofix(9,   'abcdefghiabc', 'rfind', 'abc')
         self.checkequalnofix(12,  'abcdefghiabc', 'rfind', '')

Objects/stringlib/fastsearch.h

@@ -32,6 +32,60 @@
 #define STRINGLIB_BLOOM(mask, ch)     \
     ((mask &  (1UL << ((ch) & (STRINGLIB_BLOOM_WIDTH -1)))))
 
+
+Py_LOCAL_INLINE(Py_ssize_t)
+STRINGLIB(fastsearch_memchr_1char)(const STRINGLIB_CHAR* s, Py_ssize_t n,
+                                   STRINGLIB_CHAR ch, unsigned char needle,
+                                   Py_ssize_t maxcount, int mode)
+{
+    void *candidate;
+    const STRINGLIB_CHAR *found;
+
+#define DO_MEMCHR(memchr, s, needle, nchars) do { \
+    candidate = memchr((const void *) (s), (needle), (nchars) * sizeof(STRINGLIB_CHAR)); \
+    found = (const STRINGLIB_CHAR *) \
+        ((Py_ssize_t) candidate & (~ ((Py_ssize_t) sizeof(STRINGLIB_CHAR) - 1))); \
+    } while (0)
+
+    if (mode == FAST_SEARCH) {
+        const STRINGLIB_CHAR *_s = s;
+        const STRINGLIB_CHAR *e = s + n;
+        while (_s < e) {
+            DO_MEMCHR(memchr, _s, needle, e - _s);
+            if (found == NULL)
+                return -1;
+            if (sizeof(STRINGLIB_CHAR) == 1 || *found == ch)
+                return (found - _s);
+            /* False positive */
+            _s = found + 1;
+        }
+        return -1;
+    }
+#ifdef HAVE_MEMRCHR
+    /* memrchr() is a GNU extension, available since glibc 2.1.91.
+       it doesn't seem as optimized as memchr(), but is still quite
+       faster than our hand-written loop in FASTSEARCH below */
+    else if (mode == FAST_RSEARCH) {
+        while (n > 0) {
+            DO_MEMCHR(memrchr, s, needle, n);
+            if (found == NULL)
+                return -1;
+            n = found - s;
+            if (sizeof(STRINGLIB_CHAR) == 1 || *found == ch)
+                return n;
+            /* False positive */
+        }
+        return -1;
+    }
+#endif
+    else {
+        assert(0); /* Should never get here */
+        return 0;
+    }
+
+#undef DO_MEMCHR
+}
+
 Py_LOCAL_INLINE(Py_ssize_t)
 FASTSEARCH(const STRINGLIB_CHAR* s, Py_ssize_t n,
            const STRINGLIB_CHAR* p, Py_ssize_t m,
@@ -51,6 +105,25 @@ FASTSEARCH(const STRINGLIB_CHAR* s, Py_ssize_t n,
         if (m <= 0)
             return -1;
         /* use special case for 1-character strings */
+        if (n > 10 && (mode == FAST_SEARCH
+#ifdef HAVE_MEMRCHR
+                    || mode == FAST_RSEARCH
+#endif
+                    )) {
+            /* use memchr if we can choose a needle without two many likely
+               false positives */
+            unsigned char needle;
+            int use_needle = 1;
+            needle = p[0] & 0xff;
+            if (needle == 0 && sizeof(STRINGLIB_CHAR) > 1) {
+                needle = (p[0] >> 8) & 0xff;
+                if (needle >= 32)
+                    use_needle = 0;
+            }
+            if (use_needle)
+                return STRINGLIB(fastsearch_memchr_1char)
+                       (s, n, p[0], needle, maxcount, mode);
+        }
         if (mode == FAST_COUNT) {
             for (i = 0; i < n; i++)
                 if (s[i] == p[0]) {

configure.in

@@ -2566,7 +2566,8 @@ AC_CHECK_FUNCS(alarm accept4 setitimer getitimer bind_textdomain_codeset chown \
  getgrouplist getgroups getlogin getloadavg getpeername getpgid getpid \
  getpriority getresuid getresgid getpwent getspnam getspent getsid getwd \
  if_nameindex \
- initgroups kill killpg lchmod lchown lockf linkat lstat lutimes mbrtowc mkdirat mkfifo \
+ initgroups kill killpg lchmod lchown lockf linkat lstat lutimes memrchr \
+ mbrtowc mkdirat mkfifo \
  mkfifoat mknod mknodat mktime mremap nice openat pathconf pause pipe2 plock poll \
  posix_fallocate posix_fadvise pread \
  pthread_init pthread_kill putenv pwrite readlink readlinkat readv realpath renameat \

pyconfig.h.in

@@ -515,6 +515,9 @@
 /* Define to 1 if you have the <memory.h> header file. */
 #undef HAVE_MEMORY_H
 
+/* Define to 1 if you have the `memrchr' function. */
+#undef HAVE_MEMRCHR
+
 /* Define to 1 if you have the `mkdirat' function. */
 #undef HAVE_MKDIRAT