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Optimize the Latin1 conversion code in the JSON parser with SSE2

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This also reduces the number of variables modified in each loop from two
(l and i) to one (just i) and avoids calling str.length() all the
time. Those should be no-op changes, but why not help the optimizer?

Change-Id: I8895c35f84d545dba45bffff13bc4147ae53eaab
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
This commit is contained in:
Thiago Macieira 2015-01-23 23:07:34 -08:00
parent dbd21d0879
commit 8d9d2a7b85
1 changed files with 28 additions and 5 deletions
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33
src/corelib/json/qjson_p.h
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@ -54,6 +54,8 @@
#include <qendian.h>
#include <qnumeric.h>
#include "private/qsimd_p.h"
#include <limits.h>
#include <limits>
@ -374,13 +376,34 @@ public:
inline Latin1String &operator=(const QString &str)
{
d->length = str.length();
int len = d->length = str.length();
uchar *l = (uchar *)d->latin1;
const ushort *uc = (const ushort *)str.unicode();
for (int i = 0; i < str.length(); ++i)
*l++ = uc[i];
while ((quintptr)l & 0x3)
*l++ = 0;
int i = 0;
#ifdef __SSE2__
for ( ; i + 16 < len; i += 16) {
__m128i chunk1 = _mm_loadu_si128((__m128i*)&uc[i]); // load
__m128i chunk2 = _mm_loadu_si128((__m128i*)&uc[i + 8]); // load
// pack the two vector to 16 x 8bits elements
const __m128i result = _mm_packus_epi16(chunk1, chunk2);
_mm_storeu_si128((__m128i*)&l[i], result); // store
}
# ifdef Q_PROCESSOR_X86_64
// we can do one more round, of 8 characters
if (i + 8 < len) {
__m128i chunk = _mm_loadu_si128((__m128i*)&uc[i]); // load
// pack with itself, we'll discard the high part anyway
chunk = _mm_packus_epi16(chunk, chunk);
// unaligned 64-bit store
*(quint64*)&l[i] = _mm_cvtsi128_si64(chunk);
i += 8;
}
# endif
#endif
for ( ; i < len; ++i)
l[i] = uc[i];
for ( ; (quintptr)(l+i) & 0x3; ++i)
l[i] = 0;
return *this;
}