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leveldb: Fix alignment code in SSE4.2-optimized CRC32C.

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When faced with a pointer that is misaligned by K bytes (pointer % 8 ==
K), the code previously moved forward by K bytes. In order to end up
with an aligned pointer, the code must move by 8 - K bytes.

This lands https://github.com/google/leveldb/pull/488

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=166295921
This commit is contained in:
costan 2017-08-23 20:59:46 -07:00 committed by Victor Costan
parent 02f43c0fcd
commit 2964b803b8
1 changed files with 6 additions and 2 deletions
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8
port/port_posix_sse.cc
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@ -92,8 +92,12 @@ uint32_t AcceleratedCRC32C(uint32_t crc, const char* buf, size_t size) {
} while (0) } while (0)
if (size > 16) { if (size > 16) {
// Process unaligned bytes // Point x at first 8-byte aligned byte in string. This must be inside the
for (unsigned int i = reinterpret_cast<uintptr_t>(p) % 8; i; --i) { // string, due to the size check above.
const uintptr_t pval = reinterpret_cast<uintptr_t>(p);
const uint8_t* x = reinterpret_cast<const uint8_t*>(((pval + 7) >> 3) << 3);
// Process bytes until p is 8-byte aligned.
while (p != x) {
STEP1; STEP1;
} }