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QHash: improve aeshash's page detection code

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We don't need to test for actually crossing a page boundary. It suffices
to check if we're in the upper half or the lower half of the page. In
the upper half, we load ending at the end; in the lower half, we load
starting at the current position. This way, it can never crash.

Pick-to: 6.2.3 6.2 6.3
Change-Id: Icad7c1bad46a449c8e8afffd16cb743e622b3405
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
This commit is contained in:
Thiago Macieira 2022-01-18 11:31:40 -08:00
parent e1654aeb2e
commit 517821b173
1 changed files with 5 additions and 5 deletions
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10
src/corelib/tools/qhash.cpp
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@ -597,10 +597,11 @@ lt16:
// including NULLs at the end because the length is in the key)
// WARNING: this may produce valgrind warnings, but it's safe
constexpr quintptr PageSize = 4096;
__m128i data;
if (Q_LIKELY(quintptr(src + 1) & 0xff0)) {
// same page, we definitely can't fault:
if ((quintptr(src) & (PageSize / 2)) == 0) {
// lower half of the page:
// load all 16 bytes and mask off the bytes past the end of the source
static const qint8 maskarray[] = {
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
@ -610,15 +611,14 @@ lt16:
data = _mm_loadu_si128(src);
data = _mm_and_si128(data, mask);
} else {
// too close to the end of the page, it could fault:
// upper half of the page:
// load 16 bytes ending at the data end, then shuffle them to the beginning
static const qint8 shufflecontrol[] = {
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1
};
__m128i control = _mm_loadu_si128(reinterpret_cast<const __m128i *>(shufflecontrol + 15 - len));
p = reinterpret_cast<const uchar *>(src - 1);
data = _mm_loadu_si128(reinterpret_cast<const __m128i *>(p + len));
data = _mm_loadu_si128(reinterpret_cast<const __m128i *>(p + len) - 1);
data = _mm_shuffle_epi8(data, control);
}