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Optimize str2wcs_internal more

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Prior to this change, str2wcs_internal had an optimization for ASCII
inputs. However the main cost was the repeated bounds checks when
performing push_back() on the resulting wcstring.

Switch to determining the number of ASCII characters, and then appending
those all in one go. This improves the time in the 'convert_ascii' test
from ~450 usec to ~75 usec.
This commit is contained in:
ridiculousfish 2020-09-19 15:01:38 -07:00
parent 4d5d90d828
commit 4f0ade7a73
1 changed files with 21 additions and 7 deletions
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28
src/common.cpp
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@ -269,6 +269,14 @@ inline __attribute__((always_inline)) bool is_ascii(const char *in, size_t len)
(bitmask2 & 0x8080808080808080ULL)) == 0ULL;
}
/// \return the count of initial characters in \p in, which are ASCII.
static size_t count_ascii_prefix(const char *in, size_t in_len) {
for (size_t i = 0; i < in_len; i++) {
if (in[i] & 0x80) return i;
}
return in_len;
}
/// Converts the narrow character string \c in into its wide equivalent, and return it.
///
/// The string may contain embedded nulls.
@ -281,19 +289,25 @@ static wcstring str2wcs_internal(const char *in, const size_t in_len) {
wcstring result;
result.reserve(in_len);
size_t in_pos = 0;
if (MB_CUR_MAX == 1 || is_ascii(in, in_len)) {
// Single-byte locale, all values are legal.
while (in_pos < in_len) {
result.push_back(static_cast<unsigned char>(in[in_pos]));
in_pos++;
}
// In the unlikely event that MB_CUR_MAX is 1, then we are just going to append.
if (MB_CUR_MAX == 1) {
result.insert(result.end(), in, in + in_len);
return result;
}
size_t in_pos = 0;
mbstate_t state = {};
while (in_pos < in_len) {
// Note we do not support character sets which are not supersets of ASCII.
// Append any initial sequence of ascii characters.
size_t ascii_prefix_length = count_ascii_prefix(&in[in_pos], in_len - in_pos);
result.insert(result.end(), &in[in_pos], &in[in_pos + ascii_prefix_length]);
in_pos += ascii_prefix_length;
assert(in_pos <= in_len && "Position overflowed length");
if (in_pos == in_len) break;
// We have found a non-ASCII character.
bool use_encode_direct = false;
size_t ret = 0;
wchar_t wc = 0;