Optimise string routines for architectures with non-natural alignment

C only requires that sizeof(x) is a multiple of alignof(x), not that the
two are equal. Thus anywhere where we optimise based on alignment we
should be using alignof(x) not sizeof(x).

This is more annoying than it would be in C11 where we could just use
_Alignof(x) (and alignof(x) in C++11), but since we still require only
C99 we must plumb the information all the way from autoconf through the
various typedefs and defines.

Author: jrtc27

Objects/bytes_methods.c

@@ -115,15 +115,14 @@ _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_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_SIZE_T)) {
+        if (_Py_IS_ALIGNED(p, ALIGNOF_SIZE_T)) {
             /* Help allocation */
             const char *_p = p;
-            while (_p < aligned_end) {
+            while (_p + SIZEOF_SIZE_T <= end) {
                 size_t value = *(const size_t *) _p;
                 if (value & ASCII_CHAR_MASK) {
                     Py_RETURN_FALSE;

Objects/stringlib/codecs.h

@@ -26,7 +26,6 @@ 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_SIZE_T);
     STRINGLIB_CHAR *p = dest + *outpos;
 
     while (s < end) {
@@ -40,11 +39,11 @@ STRINGLIB(utf8_decode)(const char **inptr, const char *end,
                First, check if we can do an aligned read, as most CPUs have
                a penalty for unaligned reads.
             */
-            if (_Py_IS_ALIGNED(s, SIZEOF_SIZE_T)) {
+            if (_Py_IS_ALIGNED(s, ALIGNOF_SIZE_T)) {
                 /* Help register allocation */
                 const char *_s = s;
                 STRINGLIB_CHAR *_p = p;
-                while (_s < aligned_end) {
+                while (_s + SIZEOF_SIZE_T <= end) {
                     /* 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. */
@@ -496,8 +495,6 @@ STRINGLIB(utf16_decode)(const unsigned char **inptr, const unsigned char *e,
                         int native_ordering)
 {
     Py_UCS4 ch;
-    const unsigned char *aligned_end =
-            (const unsigned char *) _Py_ALIGN_DOWN(e, SIZEOF_LONG);
     const unsigned char *q = *inptr;
     STRINGLIB_CHAR *p = dest + *outpos;
     /* Offsets from q for retrieving byte pairs in the right order. */
@@ -512,10 +509,10 @@ STRINGLIB(utf16_decode)(const unsigned char **inptr, const unsigned char *e,
         Py_UCS4 ch2;
         /* First check for possible aligned read of a C 'long'. Unaligned
            reads are more expensive, better to defer to another iteration. */
-        if (_Py_IS_ALIGNED(q, SIZEOF_LONG)) {
+        if (_Py_IS_ALIGNED(q, ALIGNOF_LONG)) {
             /* Fast path for runs of in-range non-surrogate chars. */
             const unsigned char *_q = q;
-            while (_q < aligned_end) {
+            while (_q + SIZEOF_LONG <= e) {
                 unsigned long block = * (const unsigned long *) _q;
                 if (native_ordering) {
                     /* Can use buffer directly */

Objects/stringlib/find_max_char.h

@@ -20,14 +20,12 @@ Py_LOCAL_INLINE(Py_UCS4)
 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_SIZE_T);
 
     while (p < end) {
-        if (_Py_IS_ALIGNED(p, SIZEOF_SIZE_T)) {
+        if (_Py_IS_ALIGNED(p, ALIGNOF_SIZE_T)) {
             /* Help register allocation */
             const unsigned char *_p = p;
-            while (_p < aligned_end) {
+            while (_p + SIZEOF_SIZE_T <= end) {
                 size_t value = *(const size_t *) _p;
                 if (value & UCS1_ASCII_CHAR_MASK)
                     return 255;

Objects/unicodeobject.c

@@ -5069,17 +5069,16 @@ 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_SIZE_T);
 
 #if SIZEOF_SIZE_T <= SIZEOF_VOID_P
     assert(_Py_IS_ALIGNED(dest, ALIGNOF_SIZE_T));
-    if (_Py_IS_ALIGNED(p, SIZEOF_SIZE_T)) {
+    if (_Py_IS_ALIGNED(p, ALIGNOF_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) {
+        while (_p + SIZEOF_SIZE_T <= end) {
             size_t value = *(const size_t *) _p;
             if (value & ASCII_CHAR_MASK)
                 break;
@@ -5099,10 +5098,10 @@ 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_SIZE_T)) {
+        if (_Py_IS_ALIGNED(p, ALIGNOF_SIZE_T)) {
             /* Help allocation */
             const char *_p = p;
-            while (_p < aligned_end) {
+            while (_p + SIZEOF_SIZE_T <= end) {
                 size_t value = *(const size_t *) _p;
                 if (value & ASCII_CHAR_MASK)
                     break;

PC/pyconfig.h

@@ -323,6 +323,7 @@ Py_NO_ENABLE_SHARED to find out.  Also support MS_NO_COREDLL for b/w compat */
 #define SIZEOF_SHORT 2
 #define SIZEOF_INT 4
 #define SIZEOF_LONG 4
+#define ALIGNOF_LONG 4
 #define SIZEOF_LONG_LONG 8
 #define SIZEOF_DOUBLE 8
 #define SIZEOF_FLOAT 4

configure

@@ -8705,6 +8705,41 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
+# The cast to long int works around a bug in the HP C Compiler,
+# see AC_CHECK_SIZEOF for more information.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking alignment of long" >&5
+$as_echo_n "checking alignment of long... " >&6; }
+if ${ac_cv_alignof_long+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if ac_fn_c_compute_int "$LINENO" "(long int) offsetof (ac__type_alignof_, y)" "ac_cv_alignof_long"        "$ac_includes_default
+#ifndef offsetof
+# define offsetof(type, member) ((char *) &((type *) 0)->member - (char *) 0)
+#endif
+typedef struct { char x; long y; } ac__type_alignof_;"; then :
+
+else
+  if test "$ac_cv_type_long" = yes; then
+     { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error 77 "cannot compute alignment of long
+See \`config.log' for more details" "$LINENO" 5; }
+   else
+     ac_cv_alignof_long=0
+   fi
+fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_alignof_long" >&5
+$as_echo "$ac_cv_alignof_long" >&6; }
+
+
+
+cat >>confdefs.h <<_ACEOF
+#define ALIGNOF_LONG $ac_cv_alignof_long
+_ACEOF
+
+
 # The cast to long int works around a bug in the HP C Compiler
 # version HP92453-01 B.11.11.23709.GP, which incorrectly rejects
 # declarations like `int a3[[(sizeof (unsigned char)) >= 0]];'.

configure.ac

@@ -2351,6 +2351,7 @@ AC_CHECK_TYPE(__uint128_t,
 # ANSI C requires sizeof(char) == 1, so no need to check it
 AC_CHECK_SIZEOF(int, 4)
 AC_CHECK_SIZEOF(long, 4)
+AC_CHECK_ALIGNOF(long)
 AC_CHECK_SIZEOF(long long, 8)
 AC_CHECK_SIZEOF(void *, 4)
 AC_CHECK_SIZEOF(short, 2)

pyconfig.h.in

@@ -16,6 +16,9 @@
    support for AIX C++ shared extension modules. */
 #undef AIX_GENUINE_CPLUSPLUS
 
+/* The normal alignment of `long', in bytes. */
+#undef ALIGNOF_LONG
+
 /* The normal alignment of `size_t', in bytes. */
 #undef ALIGNOF_SIZE_T