Issue #16061: Speed up str.replace() for replacing 1-character strings.

Makefile.pre.in

@@ -726,6 +726,7 @@ UNICODE_DEPS = \
 		$(srcdir)/Objects/stringlib/find_max_char.h \
 		$(srcdir)/Objects/stringlib/localeutil.h \
 		$(srcdir)/Objects/stringlib/partition.h \
+		$(srcdir)/Objects/stringlib/replace.h \
 		$(srcdir)/Objects/stringlib/split.h \
 		$(srcdir)/Objects/stringlib/ucs1lib.h \
 		$(srcdir)/Objects/stringlib/ucs2lib.h \

Misc/NEWS

@@ -10,6 +10,8 @@ What's New in Python 3.4.0 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #16061: Speed up str.replace() for replacing 1-character strings.
+
 - Issue #17715: Fix segmentation fault from raising an exception in a __trunc__
   method.
 

Objects/stringlib/replace.h

+/* stringlib: replace implementation */
+
+#ifndef STRINGLIB_FASTSEARCH_H
+#error must include "stringlib/fastsearch.h" before including this module
+#endif
+
+Py_LOCAL_INLINE(void)
+STRINGLIB(replace_1char_inplace)(STRINGLIB_CHAR* s, STRINGLIB_CHAR* end,
+                                 Py_UCS4 u1, Py_UCS4 u2, Py_ssize_t maxcount)
+{
+    *s = u2;
+    while (--maxcount && ++s != end) {
+        /* Find the next character to be replaced.
+
+           If it occurs often, it is faster to scan for it using an inline
+           loop.  If it occurs seldom, it is faster to scan for it using a
+           function call; the overhead of the function call is amortized
+           across the many characters that call covers.  We start with an
+           inline loop and use a heuristic to determine whether to fall back
+           to a function call. */
+        if (*s != u1) {
+            int attempts = 10;
+            /* search u1 in a dummy loop */
+            while (1) {
+                if (++s == end)
+                    return;
+                if (*s == u1)
+                    break;
+                if (!--attempts) {
+                    /* if u1 was not found for attempts iterations,
+                       use FASTSEARCH() or memchr() */
+#if STRINGLIB_SIZEOF_CHAR == 1
+                    s++;
+                    s = memchr(s, u1, end - s);
+                    if (s == NULL)
+                        return;
+#else
+                    Py_ssize_t i;
+                    STRINGLIB_CHAR ch1 = (STRINGLIB_CHAR) u1;
+                    s++;
+                    i = FASTSEARCH(s, end - s, &ch1, 1, 0, FAST_SEARCH);
+                    if (i < 0)
+                        return;
+                    s += i;
+#endif
+                    /* restart the dummy loop */
+                    break;
+                }
+            }
+        }
+        *s = u2;
+    }
+}

Objects/unicodeobject.c

@@ -605,6 +605,7 @@ make_bloom_mask(int kind, void* ptr, Py_ssize_t len)
 #include "stringlib/split.h"
 #include "stringlib/count.h"
 #include "stringlib/find.h"
+#include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
 #include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
@@ -615,6 +616,7 @@ make_bloom_mask(int kind, void* ptr, Py_ssize_t len)
 #include "stringlib/split.h"
 #include "stringlib/count.h"
 #include "stringlib/find.h"
+#include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
 #include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
@@ -625,6 +627,7 @@ make_bloom_mask(int kind, void* ptr, Py_ssize_t len)
 #include "stringlib/split.h"
 #include "stringlib/count.h"
 #include "stringlib/find.h"
+#include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
 #include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
@@ -9927,6 +9930,31 @@ anylib_count(int kind, PyObject *sstr, void* sbuf, Py_ssize_t slen,
     return 0;
 }
 
+static void
+replace_1char_inplace(PyObject *u, Py_ssize_t pos,
+                      Py_UCS4 u1, Py_UCS4 u2, Py_ssize_t maxcount)
+{
+    int kind = PyUnicode_KIND(u);
+    void *data = PyUnicode_DATA(u);
+    Py_ssize_t len = PyUnicode_GET_LENGTH(u);
+    if (kind == PyUnicode_1BYTE_KIND) {
+        ucs1lib_replace_1char_inplace((Py_UCS1 *)data + pos,
+                                      (Py_UCS1 *)data + len,
+                                      u1, u2, maxcount);
+    }
+    else if (kind == PyUnicode_2BYTE_KIND) {
+        ucs2lib_replace_1char_inplace((Py_UCS2 *)data + pos,
+                                      (Py_UCS2 *)data + len,
+                                      u1, u2, maxcount);
+    }
+    else {
+        assert(kind == PyUnicode_4BYTE_KIND);
+        ucs4lib_replace_1char_inplace((Py_UCS4 *)data + pos,
+                                      (Py_UCS4 *)data + len,
+                                      u1, u2, maxcount);
+    }
+}
+
 static PyObject *
 replace(PyObject *self, PyObject *str1,
         PyObject *str2, Py_ssize_t maxcount)
@@ -9943,7 +9971,7 @@ replace(PyObject *self, PyObject *str1,
     Py_ssize_t len1 = PyUnicode_GET_LENGTH(str1);
     Py_ssize_t len2 = PyUnicode_GET_LENGTH(str2);
     int mayshrink;
-    Py_UCS4 maxchar, maxchar_str2;
+    Py_UCS4 maxchar, maxchar_str1, maxchar_str2;
 
     if (maxcount < 0)
         maxcount = PY_SSIZE_T_MAX;
@@ -9952,15 +9980,16 @@ replace(PyObject *self, PyObject *str1,
 
     if (str1 == str2)
         goto nothing;
-    if (skind < kind1)
-        /* substring too wide to be present */
-        goto nothing;
 
     maxchar = PyUnicode_MAX_CHAR_VALUE(self);
+    maxchar_str1 = PyUnicode_MAX_CHAR_VALUE(str1);
+    if (maxchar < maxchar_str1)
+        /* substring too wide to be present */
+        goto nothing;
     maxchar_str2 = PyUnicode_MAX_CHAR_VALUE(str2);
     /* Replacing str1 with str2 may cause a maxchar reduction in the
        result string. */
-    mayshrink = (maxchar_str2 < maxchar);
+    mayshrink = (maxchar_str2 < maxchar_str1) && (maxchar == maxchar_str1);
     maxchar = MAX_MAXCHAR(maxchar, maxchar_str2);
 
     if (len1 == len2) {
@@ -9970,36 +9999,19 @@ replace(PyObject *self, PyObject *str1,
         if (len1 == 1) {
             /* replace characters */
             Py_UCS4 u1, u2;
-            int rkind;
-            Py_ssize_t index, pos;
-            char *src, *rbuf;
+            Py_ssize_t pos;
 
             u1 = PyUnicode_READ(kind1, buf1, 0);
-            pos = findchar(sbuf, PyUnicode_KIND(self), slen, u1, 1);
+            pos = findchar(sbuf, skind, slen, u1, 1);
             if (pos < 0)
                 goto nothing;
             u2 = PyUnicode_READ(kind2, buf2, 0);
             u = PyUnicode_New(slen, maxchar);
             if (!u)
                 goto error;
-            _PyUnicode_FastCopyCharacters(u, 0, self, 0, slen);
-            rkind = PyUnicode_KIND(u);
-            rbuf = PyUnicode_DATA(u);
 
-            PyUnicode_WRITE(rkind, rbuf, pos, u2);
-            index = 0;
-            src = sbuf;
-            while (--maxcount)
-            {
-                pos++;
-                src += pos * PyUnicode_KIND(self);
-                slen -= pos;
-                index += pos;
-                pos = findchar(src, PyUnicode_KIND(self), slen, u1, 1);
-                if (pos < 0)
-                    break;
-                PyUnicode_WRITE(rkind, rbuf, index + pos, u2);
-            }
+            _PyUnicode_FastCopyCharacters(u, 0, self, 0, slen);
+            replace_1char_inplace(u, pos, u1, u2, maxcount);
         }
         else {
             int rkind = skind;

PC/VS9.0/pythoncore.vcproj

@@ -1586,6 +1586,10 @@
 				RelativePath="..\..\Objects\rangeobject.c"
 				>
 			</File>
+			<File
+				RelativePath="..\..\Objects\stringlib\replace.h"
+				>
+			</File>
 			<File
 				RelativePath="..\..\Objects\setobject.c"
 				>

PCbuild/pythoncore.vcxproj

@@ -475,6 +475,7 @@
     <ClInclude Include="..\Objects\stringlib\fastsearch.h" />
     <ClInclude Include="..\Objects\stringlib\find.h" />
     <ClInclude Include="..\Objects\stringlib\partition.h" />
+    <ClInclude Include="..\Objects\stringlib\replace.h" />
     <ClInclude Include="..\Objects\stringlib\split.h" />
     <ClInclude Include="..\Objects\unicodetype_db.h" />
     <ClInclude Include="..\Parser\parser.h" />

PCbuild/pythoncore.vcxproj.filters

@@ -378,6 +378,9 @@
     <ClInclude Include="..\Objects\stringlib\partition.h">
       <Filter>Objects</Filter>
     </ClInclude>
+    <ClInclude Include="..\Objects\stringlib\replace.h">
+      <Filter>Objects</Filter>
+    </ClInclude>
     <ClInclude Include="..\Objects\stringlib\split.h">
       <Filter>Objects</Filter>
     </ClInclude>