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Enhanced REP MOVSB feature of CPUs starting from Ivy Bridge makes REP MOVSB the fastest way to copy memory in most of cases. However Intel Optimization Reference Manual says: "setting the DF to force REP MOVSB to copy bytes from high towards low addresses will expe- rience significant performance degradation". Measurements on Intel Cascade Lake and Alder Lake, same as on AMD Zen3 show that it can drop throughput to as low as 2.5-3.5GB/s, comparing to ~10-30GB/s of REP MOVSQ or hand-rolled loop, used for non-ERMS CPUs. This patch keeps ERMS use for forward ordered memory copies, but removes it for backward overlapped moves where it does not work. This is just a cosmetic sync with kernel, since libc does not use ERMS at this time. Reviewed by: mjg MFC after: 2 weeks |
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| .. | ||
| gen | ||
| stdlib | ||
| string | ||
| sys | ||
| _fpmath.h | ||
| arith.h | ||
| gd_qnan.h | ||
| Makefile.inc | ||
| static_tls.h | ||
| Symbol.map | ||
| SYS.h | ||