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gh-126024: optimize UTF-8 decoder for short non-ASCII string (#126025)

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Inada Naoki 2024-11-29 19:48:02 +09:00 committed by GitHub
parent bfabf96b50
commit 322b486010
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2 changed files with 261 additions and 45 deletions
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2
Misc/NEWS.d/next/Core_and_Builtins/2024-10-27-04-47-28.gh-issue-126024.XCQSqT.rst Normal file
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@ -0,0 +1,2 @@
Optimize decoding of short UTF-8 sequences containing non-ASCII characters
by approximately 15%.

304
Objects/unicodeobject.c
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@ -4979,39 +4979,228 @@ PyUnicode_DecodeUTF8(const char *s,
#include "stringlib/codecs.h"
#include "stringlib/undef.h"
#if (SIZEOF_SIZE_T == 8)
/* Mask to quickly check whether a C 'size_t' contains a
non-ASCII, UTF8-encoded char. */
#if (SIZEOF_SIZE_T == 8)
# define ASCII_CHAR_MASK 0x8080808080808080ULL
// used to count codepoints in UTF-8 string.
# define VECTOR_0101 0x0101010101010101ULL
# define VECTOR_00FF 0x00ff00ff00ff00ffULL
#elif (SIZEOF_SIZE_T == 4)
# define ASCII_CHAR_MASK 0x80808080U
# define VECTOR_0101 0x01010101U
# define VECTOR_00FF 0x00ff00ffU
#else
# error C 'size_t' size should be either 4 or 8!
#endif
#if (defined(__clang__) || defined(__GNUC__))
#define HAVE_CTZ 1
static inline unsigned int
ctz(size_t v)
{
return __builtin_ctzll((unsigned long long)v);
}
#elif defined(_MSC_VER)
#define HAVE_CTZ 1
static inline unsigned int
ctz(size_t v)
{
unsigned long pos;
#if SIZEOF_SIZE_T == 4
_BitScanForward(&pos, v);
#else
_BitScanForward64(&pos, v);
#endif /* SIZEOF_SIZE_T */
return pos;
}
#endif
#if HAVE_CTZ
// load p[0]..p[size-1] as a little-endian size_t
// without unaligned access nor read ahead.
static size_t
load_unaligned(const unsigned char *p, size_t size)
{
assert(size <= SIZEOF_SIZE_T);
union {
size_t s;
unsigned char b[SIZEOF_SIZE_T];
} u;
u.s = 0;
switch (size) {
case 8:
u.b[7] = p[7];
_Py_FALLTHROUGH;
case 7:
u.b[6] = p[6];
_Py_FALLTHROUGH;
case 6:
u.b[5] = p[5];
_Py_FALLTHROUGH;
case 5:
u.b[4] = p[4];
_Py_FALLTHROUGH;
case 4:
u.b[3] = p[3];
_Py_FALLTHROUGH;
case 3:
u.b[2] = p[2];
_Py_FALLTHROUGH;
case 2:
u.b[1] = p[1];
_Py_FALLTHROUGH;
case 1:
u.b[0] = p[0];
break;
case 0:
break;
default:
Py_UNREACHABLE();
}
return u.s;
}
#endif
/*
* Find the first non-ASCII character in a byte sequence.
*
* This function scans a range of bytes from `start` to `end` and returns the
* index of the first byte that is not an ASCII character (i.e., has the most
* significant bit set). If all characters in the range are ASCII, it returns
* `end - start`.
*/
static Py_ssize_t
find_first_nonascii(const unsigned char *start, const unsigned char *end)
{
const unsigned char *p = start;
if (end - start >= SIZEOF_SIZE_T) {
const unsigned char *p2 = _Py_ALIGN_UP(p, SIZEOF_SIZE_T);
if (p < p2) {
#if HAVE_CTZ
#if defined(_M_AMD64) || defined(_M_IX86) || defined(__x86_64__) || defined(__i386__)
// x86 and amd64 are little endian and can load unaligned memory.
size_t u = *(const size_t*)p & ASCII_CHAR_MASK;
#else
size_t u = load_unaligned(p, p2 - p) & ASCII_CHAR_MASK;
#endif
if (u) {
return p - start + (ctz(u) - 7) / 8;
}
p = p2;
}
#else
while (p < p2) {
if (*p & 0x80) {
return p - start;
}
p++;
}
#endif
const unsigned char *e = end - SIZEOF_SIZE_T;
while (p <= e) {
size_t u = (*(const size_t *)p) & ASCII_CHAR_MASK;
if (u) {
#if PY_LITTLE_ENDIAN && HAVE_CTZ
return p - start + (ctz(u) - 7) / 8;
#else
// big endian and minor compilers are difficult to test.
// fallback to per byte check.
break;
#endif
}
p += SIZEOF_SIZE_T;
}
}
#if HAVE_CTZ
// we can not use *(const size_t*)p to avoid buffer overrun.
size_t u = load_unaligned(p, end - p) & ASCII_CHAR_MASK;
if (u) {
return p - start + (ctz(u) - 7) / 8;
}
return end - start;
#else
while (p < end) {
if (*p & 0x80) {
break;
}
p++;
}
return p - start;
#endif
}
static inline int
scalar_utf8_start_char(unsigned int ch)
{
// 0xxxxxxx or 11xxxxxx are first byte.
return (~ch >> 7 | ch >> 6) & 1;
}
static inline size_t
vector_utf8_start_chars(size_t v)
{
return ((~v >> 7) | (v >> 6)) & VECTOR_0101;
}
// Count the number of UTF-8 code points in a given byte sequence.
static Py_ssize_t
utf8_count_codepoints(const unsigned char *s, const unsigned char *end)
{
Py_ssize_t len = 0;
if (end - s >= SIZEOF_SIZE_T) {
while (!_Py_IS_ALIGNED(s, ALIGNOF_SIZE_T)) {
len += scalar_utf8_start_char(*s++);
}
while (s + SIZEOF_SIZE_T <= end) {
const unsigned char *e = end;
if (e - s > SIZEOF_SIZE_T * 255) {
e = s + SIZEOF_SIZE_T * 255;
}
Py_ssize_t vstart = 0;
while (s + SIZEOF_SIZE_T <= e) {
size_t v = *(size_t*)s;
size_t vs = vector_utf8_start_chars(v);
vstart += vs;
s += SIZEOF_SIZE_T;
}
vstart = (vstart & VECTOR_00FF) + ((vstart >> 8) & VECTOR_00FF);
vstart += vstart >> 16;
#if SIZEOF_SIZE_T == 8
vstart += vstart >> 32;
#endif
len += vstart & 0x7ff;
}
}
while (s < end) {
len += scalar_utf8_start_char(*s++);
}
return len;
}
static Py_ssize_t
ascii_decode(const char *start, const char *end, Py_UCS1 *dest)
{
const char *p = start;
#if SIZEOF_SIZE_T <= SIZEOF_VOID_P
if (_Py_IS_ALIGNED(p, ALIGNOF_SIZE_T)
if (_Py_IS_ALIGNED(start, ALIGNOF_SIZE_T)
&& _Py_IS_ALIGNED(dest, 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 + SIZEOF_SIZE_T <= end) {
size_t value = *(const size_t *) _p;
const char *p = start;
Py_UCS1 *q = dest;
while (p + SIZEOF_SIZE_T <= end) {
size_t value = *(const size_t *) p;
if (value & ASCII_CHAR_MASK)
break;
*((size_t *)q) = value;
_p += SIZEOF_SIZE_T;
p += SIZEOF_SIZE_T;
q += SIZEOF_SIZE_T;
}
p = _p;
while (p < end) {
if ((unsigned char)*p & 0x80)
break;
@ -5020,31 +5209,12 @@ ascii_decode(const char *start, const char *end, Py_UCS1 *dest)
return p - start;
}
#endif
while (p < end) {
/* Fast path, see in STRINGLIB(utf8_decode) in stringlib/codecs.h
for an explanation. */
if (_Py_IS_ALIGNED(p, ALIGNOF_SIZE_T)) {
/* Help allocation */
const char *_p = p;
while (_p + SIZEOF_SIZE_T <= end) {
size_t value = *(const size_t *) _p;
if (value & ASCII_CHAR_MASK)
break;
_p += SIZEOF_SIZE_T;
}
p = _p;
if (_p == end)
break;
}
if ((unsigned char)*p & 0x80)
break;
++p;
}
memcpy(dest, start, p - start);
return p - start;
Py_ssize_t pos = find_first_nonascii((const unsigned char*)start,
(const unsigned char*)end);
memcpy(dest, start, pos);
return pos;
}
static int
unicode_decode_utf8_impl(_PyUnicodeWriter *writer,
const char *starts, const char *s, const char *end,
@ -5188,27 +5358,69 @@ unicode_decode_utf8(const char *s, Py_ssize_t size,
return get_latin1_char((unsigned char)s[0]);
}
// fast path: try ASCII string.
const char *starts = s;
const char *end = s + size;
PyObject *u = PyUnicode_New(size, 127);
if (u == NULL) {
// I don't know this check is necessary or not. But there is a test
// case that requires size=PY_SSIZE_T_MAX cause MemoryError.
if (PY_SSIZE_T_MAX - sizeof(PyCompactUnicodeObject) < (size_t)size) {
PyErr_NoMemory();
return NULL;
}
Py_ssize_t decoded = ascii_decode(s, end, PyUnicode_1BYTE_DATA(u));
if (decoded == size) {
const char *starts = s;
const char *end = s + size;
Py_ssize_t pos = find_first_nonascii((const unsigned char*)starts, (const unsigned char*)end);
if (pos == size) { // fast path: ASCII string.
PyObject *u = PyUnicode_New(size, 127);
if (u == NULL) {
return NULL;
}
memcpy(PyUnicode_1BYTE_DATA(u), s, size);
if (consumed) {
*consumed = size;
}
return u;
}
s += decoded;
size -= decoded;
int maxchr = 127;
Py_ssize_t maxsize = size;
unsigned char ch = (unsigned char)(s[pos]);
// error handler other than strict may remove/replace the invalid byte.
// consumed != NULL allows 1~3 bytes remainings.
// 0x80 <= ch < 0xc2 is invalid start byte that cause UnicodeDecodeError.
// otherwise: check the input and decide the maxchr and maxsize to reduce
// reallocation and copy.
if (error_handler == _Py_ERROR_STRICT && !consumed && ch >= 0xc2) {
// we only calculate the number of codepoints and don't determine the exact maxchr.
// This is because writing fast and portable SIMD code to find maxchr is difficult.
// If reallocation occurs for a larger maxchar, knowing the exact number of codepoints
// means that it is no longer necessary to allocate several times the required amount
// of memory.
maxsize = utf8_count_codepoints((const unsigned char *)s, (const unsigned char *)end);
if (ch < 0xc4) { // latin1
maxchr = 0xff;
}
else if (ch < 0xf0) { // ucs2
maxchr = 0xffff;
}
else { // ucs4
maxchr = 0x10ffff;
}
}
PyObject *u = PyUnicode_New(maxsize, maxchr);
if (!u) {
return NULL;
}
// Use _PyUnicodeWriter after fast path is failed.
_PyUnicodeWriter writer;
_PyUnicodeWriter_InitWithBuffer(&writer, u);
writer.pos = decoded;
if (maxchr <= 255) {
memcpy(PyUnicode_1BYTE_DATA(u), s, pos);
s += pos;
size -= pos;
writer.pos = pos;
}
if (unicode_decode_utf8_impl(&writer, starts, s, end,
error_handler, errors,
@ -5268,7 +5480,9 @@ PyUnicode_DecodeUTF8Stateful(const char *s,
const char *errors,
Py_ssize_t *consumed)
{
return unicode_decode_utf8(s, size, _Py_ERROR_UNKNOWN, errors, consumed);
return unicode_decode_utf8(s, size,
errors ? _Py_ERROR_UNKNOWN : _Py_ERROR_STRICT,
errors, consumed);
}