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

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* 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.
This commit is contained in:
Jessica Clarke 2021-03-31 11:12:39 +01:00 • committed by GitHub
parent cfa176685a
commit dec0757549
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GPG key ID: 4AEE18F83AFDEB23
8 changed files with 96 additions and 31 deletions
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20
Objects/unicodeobject.c
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@ -5070,25 +5070,16 @@ 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);
/*
* Issue #17237: m68k is a bit different from most architectures in
* that objects do not use "natural alignment" - for example, int and
* long are only aligned at 2-byte boundaries. Therefore the assert()
* won't work; also, tests have shown that skipping the "optimised
* version" will even speed up m68k.
*/
#if !defined(__m68k__)
#if SIZEOF_SIZE_T <= SIZEOF_VOID_P
assert(_Py_IS_ALIGNED(dest, SIZEOF_SIZE_T));
if (_Py_IS_ALIGNED(p, SIZEOF_SIZE_T)) {
assert(_Py_IS_ALIGNED(dest, ALIGNOF_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;
@ -5104,15 +5095,14 @@ ascii_decode(const char *start, const char *end, Py_UCS1 *dest)
}
return p - start;
}
#endif
#endif
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;