bpo-43179: Generalise alignment for optimised string routines (GH-24624)

* Remove m68k-specific hack from ascii_decode

On m68k, alignments of primitives is more relaxed, with 4-byte and
8-byte types only requiring 2-byte alignment, thus using sizeof(size_t)
does not work. Instead, use the portable alternative.

Note that this is a minimal fix that only relaxes the assertion and the
condition for when to use the optimised version remains overly strict.
Such issues will be fixed tree-wide in the next commit.

NB: In C11 we could use _Alignof(size_t) instead, but for compatibility
we use autoconf.

* 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.

1 files changed, 71 insertions, 1 deletions

diff --git a/configure b/configure
index 6ba109b..34a19a1 100755
--- a/configure
+++ b/configure
@@ -8644,7 +8644,7 @@ $as_echo "#define HAVE_GCC_UINT128_T 1" >>confdefs.h
 fi
 
 
-# Sizes of various common basic types
+# Sizes and alignments of various common basic types
 # ANSI C requires sizeof(char) == 1, so no need to check it
 # 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
@@ -8712,6 +8712,41 @@ 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]];'.
@@ -8943,6 +8978,41 @@ 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 size_t" >&5
+$as_echo_n "checking alignment of size_t... " >&6; }
+if ${ac_cv_alignof_size_t+:} 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_size_t"        "$ac_includes_default
+#ifndef offsetof
+# define offsetof(type, member) ((char *) &((type *) 0)->member - (char *) 0)
+#endif
+typedef struct { char x; size_t y; } ac__type_alignof_;"; then :
+
+else
+  if test "$ac_cv_type_size_t" = 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 size_t
+See \`config.log' for more details" "$LINENO" 5; }
+   else
+     ac_cv_alignof_size_t=0
+   fi
+fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_alignof_size_t" >&5
+$as_echo "$ac_cv_alignof_size_t" >&6; }
+
+
+
+cat >>confdefs.h <<_ACEOF
+#define ALIGNOF_SIZE_T $ac_cv_alignof_size_t
+_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]];'.