diff --git a/Misc/NEWS.d/next/Windows/2020-10-18-18-43-45.bpo-38252.7Nlepg.rst b/Misc/NEWS.d/next/Windows/2020-10-18-18-43-45.bpo-38252.7Nlepg.rst
new file mode 100644
index 00000000000..c103e6cfcc7
--- /dev/null
+++ b/Misc/NEWS.d/next/Windows/2020-10-18-18-43-45.bpo-38252.7Nlepg.rst
@@ -0,0 +1 @@
+Use 8-byte step to detect ASCII sequence in 64-bit Windows build.
diff --git a/Objects/bytes_methods.c b/Objects/bytes_methods.c
index 72daa1fdd55..1512086e613 100644
--- a/Objects/bytes_methods.c
+++ b/Objects/bytes_methods.c
@@ -100,14 +100,14 @@ Return True if B is empty or all characters in B are ASCII,\n\
 False otherwise.");
 
 // Optimization is copied from ascii_decode in unicodeobject.c
-/* Mask to quickly check whether a C 'long' contains a
+/* Mask to quickly check whether a C 'size_t' contains a
    non-ASCII, UTF8-encoded char. */
-#if (SIZEOF_LONG == 8)
-# define ASCII_CHAR_MASK 0x8080808080808080UL
-#elif (SIZEOF_LONG == 4)
-# define ASCII_CHAR_MASK 0x80808080UL
+#if (SIZEOF_SIZE_T == 8)
+# define ASCII_CHAR_MASK 0x8080808080808080ULL
+#elif (SIZEOF_SIZE_T == 4)
+# define ASCII_CHAR_MASK 0x80808080U
 #else
-# error C 'long' size should be either 4 or 8!
+# error C 'size_t' size should be either 4 or 8!
 #endif
 
 PyObject*
@@ -115,20 +115,20 @@ _Py_bytes_isascii(const char *cptr, Py_ssize_t len)
 {
     const char *p = cptr;
     const char *end = p + len;
-    const char *aligned_end = (const char *) _Py_ALIGN_DOWN(end, SIZEOF_LONG);
+    const char *aligned_end = (const char *) _Py_ALIGN_DOWN(end, SIZEOF_SIZE_T);
 
     while (p < end) {
         /* Fast path, see in STRINGLIB(utf8_decode) in stringlib/codecs.h
            for an explanation. */
-        if (_Py_IS_ALIGNED(p, SIZEOF_LONG)) {
+        if (_Py_IS_ALIGNED(p, SIZEOF_SIZE_T)) {
             /* Help allocation */
             const char *_p = p;
             while (_p < aligned_end) {
-                unsigned long value = *(const unsigned long *) _p;
+                size_t value = *(const size_t *) _p;
                 if (value & ASCII_CHAR_MASK) {
                     Py_RETURN_FALSE;
                 }
-                _p += SIZEOF_LONG;
+                _p += SIZEOF_SIZE_T;
             }
             p = _p;
             if (_p == end)
diff --git a/Objects/stringlib/codecs.h b/Objects/stringlib/codecs.h
index 197605b012e..b6ca404b1a2 100644
--- a/Objects/stringlib/codecs.h
+++ b/Objects/stringlib/codecs.h
@@ -6,14 +6,14 @@
 
 #include "pycore_bitutils.h"      // _Py_bswap32()
 
-/* Mask to quickly check whether a C 'long' contains a
+/* Mask to quickly check whether a C 'size_t' contains a
    non-ASCII, UTF8-encoded char. */
-#if (SIZEOF_LONG == 8)
-# define ASCII_CHAR_MASK 0x8080808080808080UL
-#elif (SIZEOF_LONG == 4)
-# define ASCII_CHAR_MASK 0x80808080UL
+#if (SIZEOF_SIZE_T == 8)
+# define ASCII_CHAR_MASK 0x8080808080808080ULL
+#elif (SIZEOF_SIZE_T == 4)
+# define ASCII_CHAR_MASK 0x80808080U
 #else
-# error C 'long' size should be either 4 or 8!
+# error C 'size_t' size should be either 4 or 8!
 #endif
 
 /* 10xxxxxx */
@@ -26,7 +26,7 @@ STRINGLIB(utf8_decode)(const char **inptr, const char *end,
 {
     Py_UCS4 ch;
     const char *s = *inptr;
-    const char *aligned_end = (const char *) _Py_ALIGN_DOWN(end, SIZEOF_LONG);
+    const char *aligned_end = (const char *) _Py_ALIGN_DOWN(end, SIZEOF_SIZE_T);
     STRINGLIB_CHAR *p = dest + *outpos;
 
     while (s < end) {
@@ -36,19 +36,19 @@ STRINGLIB(utf8_decode)(const char **inptr, const char *end,
             /* Fast path for runs of ASCII characters. Given that common UTF-8
                input will consist of an overwhelming majority of ASCII
                characters, we try to optimize for this case by checking
-               as many characters as a C 'long' can contain.
+               as many characters as a C 'size_t' can contain.
                First, check if we can do an aligned read, as most CPUs have
                a penalty for unaligned reads.
             */
-            if (_Py_IS_ALIGNED(s, SIZEOF_LONG)) {
+            if (_Py_IS_ALIGNED(s, SIZEOF_SIZE_T)) {
                 /* Help register allocation */
                 const char *_s = s;
                 STRINGLIB_CHAR *_p = p;
                 while (_s < aligned_end) {
-                    /* Read a whole long at a time (either 4 or 8 bytes),
+                    /* Read a whole size_t at a time (either 4 or 8 bytes),
                        and do a fast unrolled copy if it only contains ASCII
                        characters. */
-                    unsigned long value = *(const unsigned long *) _s;
+                    size_t value = *(const size_t *) _s;
                     if (value & ASCII_CHAR_MASK)
                         break;
 #if PY_LITTLE_ENDIAN
@@ -56,14 +56,14 @@ STRINGLIB(utf8_decode)(const char **inptr, const char *end,
                     _p[1] = (STRINGLIB_CHAR)((value >> 8) & 0xFFu);
                     _p[2] = (STRINGLIB_CHAR)((value >> 16) & 0xFFu);
                     _p[3] = (STRINGLIB_CHAR)((value >> 24) & 0xFFu);
-# if SIZEOF_LONG == 8
+# if SIZEOF_SIZE_T == 8
                     _p[4] = (STRINGLIB_CHAR)((value >> 32) & 0xFFu);
                     _p[5] = (STRINGLIB_CHAR)((value >> 40) & 0xFFu);
                     _p[6] = (STRINGLIB_CHAR)((value >> 48) & 0xFFu);
                     _p[7] = (STRINGLIB_CHAR)((value >> 56) & 0xFFu);
 # endif
 #else
-# if SIZEOF_LONG == 8
+# if SIZEOF_SIZE_T == 8
                     _p[0] = (STRINGLIB_CHAR)((value >> 56) & 0xFFu);
                     _p[1] = (STRINGLIB_CHAR)((value >> 48) & 0xFFu);
                     _p[2] = (STRINGLIB_CHAR)((value >> 40) & 0xFFu);
@@ -79,8 +79,8 @@ STRINGLIB(utf8_decode)(const char **inptr, const char *end,
                     _p[3] = (STRINGLIB_CHAR)(value & 0xFFu);
 # endif
 #endif
-                    _s += SIZEOF_LONG;
-                    _p += SIZEOF_LONG;
+                    _s += SIZEOF_SIZE_T;
+                    _p += SIZEOF_SIZE_T;
                 }
                 s = _s;
                 p = _p;
diff --git a/Objects/stringlib/find_max_char.h b/Objects/stringlib/find_max_char.h
index f4e0a7761d3..3319a464614 100644
--- a/Objects/stringlib/find_max_char.h
+++ b/Objects/stringlib/find_max_char.h
@@ -4,14 +4,14 @@
 # error "find_max_char.h is specific to Unicode"
 #endif
 
-/* Mask to quickly check whether a C 'long' contains a
+/* Mask to quickly check whether a C 'size_t' contains a
    non-ASCII, UTF8-encoded char. */
-#if (SIZEOF_LONG == 8)
-# define UCS1_ASCII_CHAR_MASK 0x8080808080808080UL
-#elif (SIZEOF_LONG == 4)
-# define UCS1_ASCII_CHAR_MASK 0x80808080UL
+#if (SIZEOF_SIZE_T == 8)
+# define UCS1_ASCII_CHAR_MASK 0x8080808080808080ULL
+#elif (SIZEOF_SIZE_T == 4)
+# define UCS1_ASCII_CHAR_MASK 0x80808080U
 #else
-# error C 'long' size should be either 4 or 8!
+# error C 'size_t' size should be either 4 or 8!
 #endif
 
 #if STRINGLIB_SIZEOF_CHAR == 1
@@ -21,17 +21,17 @@ STRINGLIB(find_max_char)(const STRINGLIB_CHAR *begin, const STRINGLIB_CHAR *end)
 {
     const unsigned char *p = (const unsigned char *) begin;
     const unsigned char *aligned_end =
-            (const unsigned char *) _Py_ALIGN_DOWN(end, SIZEOF_LONG);
+            (const unsigned char *) _Py_ALIGN_DOWN(end, SIZEOF_SIZE_T);
 
     while (p < end) {
-        if (_Py_IS_ALIGNED(p, SIZEOF_LONG)) {
+        if (_Py_IS_ALIGNED(p, SIZEOF_SIZE_T)) {
             /* Help register allocation */
             const unsigned char *_p = p;
             while (_p < aligned_end) {
-                unsigned long value = *(const unsigned long *) _p;
+                size_t value = *(const size_t *) _p;
                 if (value & UCS1_ASCII_CHAR_MASK)
                     return 255;
-                _p += SIZEOF_LONG;
+                _p += SIZEOF_SIZE_T;
             }
             p = _p;
             if (p == end)
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index c4e73ebd45d..f963deb0201 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -5025,21 +5025,21 @@ PyUnicode_DecodeUTF8(const char *s,
 #include "stringlib/codecs.h"
 #include "stringlib/undef.h"
 
-/* Mask to quickly check whether a C 'long' contains a
+/* Mask to quickly check whether a C 'size_t' contains a
    non-ASCII, UTF8-encoded char. */
-#if (SIZEOF_LONG == 8)
-# define ASCII_CHAR_MASK 0x8080808080808080UL
-#elif (SIZEOF_LONG == 4)
-# define ASCII_CHAR_MASK 0x80808080UL
+#if (SIZEOF_SIZE_T == 8)
+# define ASCII_CHAR_MASK 0x8080808080808080ULL
+#elif (SIZEOF_SIZE_T == 4)
+# define ASCII_CHAR_MASK 0x80808080U
 #else
-# error C 'long' size should be either 4 or 8!
+# error C 'size_t' size should be either 4 or 8!
 #endif
 
 static Py_ssize_t
 ascii_decode(const char *start, const char *end, Py_UCS1 *dest)
 {
     const char *p = start;
-    const char *aligned_end = (const char *) _Py_ALIGN_DOWN(end, SIZEOF_LONG);
+    const char *aligned_end = (const char *) _Py_ALIGN_DOWN(end, SIZEOF_SIZE_T);
 
     /*
      * Issue #17237: m68k is a bit different from most architectures in
@@ -5049,21 +5049,21 @@ ascii_decode(const char *start, const char *end, Py_UCS1 *dest)
      * version" will even speed up m68k.
      */
 #if !defined(__m68k__)
-#if SIZEOF_LONG <= SIZEOF_VOID_P
-    assert(_Py_IS_ALIGNED(dest, SIZEOF_LONG));
-    if (_Py_IS_ALIGNED(p, SIZEOF_LONG)) {
+#if SIZEOF_SIZE_T <= SIZEOF_VOID_P
+    assert(_Py_IS_ALIGNED(dest, SIZEOF_SIZE_T));
+    if (_Py_IS_ALIGNED(p, SIZEOF_SIZE_T)) {
         /* Fast path, see in STRINGLIB(utf8_decode) for
            an explanation. */
         /* Help allocation */
         const char *_p = p;
         Py_UCS1 * q = dest;
         while (_p < aligned_end) {
-            unsigned long value = *(const unsigned long *) _p;
+            size_t value = *(const size_t *) _p;
             if (value & ASCII_CHAR_MASK)
                 break;
-            *((unsigned long *)q) = value;
-            _p += SIZEOF_LONG;
-            q += SIZEOF_LONG;
+            *((size_t *)q) = value;
+            _p += SIZEOF_SIZE_T;
+            q += SIZEOF_SIZE_T;
         }
         p = _p;
         while (p < end) {
@@ -5078,14 +5078,14 @@ ascii_decode(const char *start, const char *end, Py_UCS1 *dest)
     while (p < end) {
         /* Fast path, see in STRINGLIB(utf8_decode) in stringlib/codecs.h
            for an explanation. */
-        if (_Py_IS_ALIGNED(p, SIZEOF_LONG)) {
+        if (_Py_IS_ALIGNED(p, SIZEOF_SIZE_T)) {
             /* Help allocation */
             const char *_p = p;
             while (_p < aligned_end) {
-                unsigned long value = *(const unsigned long *) _p;
+                size_t value = *(const size_t *) _p;
                 if (value & ASCII_CHAR_MASK)
                     break;
-                _p += SIZEOF_LONG;
+                _p += SIZEOF_SIZE_T;
             }
             p = _p;
             if (_p == end)