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, 6 insertions, 0 deletions

diff --git a/pyconfig.h.in b/pyconfig.h.in
index b65004e..9b713ac 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -16,6 +16,12 @@
    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
+
 /* Alternative SOABI used in debug build to load C extensions built in release
    mode */
 #undef ALT_SOABI