Issue #15144: Fix possible integer overflow when handling pointers as integer values, by using Py_uintptr_t instead of size_t.
Patch by Serhiy Storchaka.
diff --git a/Objects/stringlib/fastsearch.h b/Objects/stringlib/fastsearch.h
index 5b8d5db..ecf885e 100644
--- a/Objects/stringlib/fastsearch.h
+++ b/Objects/stringlib/fastsearch.h
@@ -43,8 +43,7 @@
 
 #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))); \
+    found = (const STRINGLIB_CHAR *) _Py_ALIGN_DOWN(candidate, sizeof(STRINGLIB_CHAR)); \
     } while (0)
 
     if (mode == FAST_SEARCH) {