[Mesa-dev] [Bug 107670] Massive slowdown under specific memcpy implementations (32bit, no-SIMD, backward copy).

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Fri Aug 24 01:55:49 UTC 2018


--- Comment #2 from iive at yahoo.com ---
(In reply to Timothy Arceri from comment #1)
> There already is asm optimized version of memcpy() in glibc. Why would we
> want to reinvent that in Mesa?
> glibc should pick the right implementation for you system.

Because some implementations copy data backwards and this creates a huge
problem when it is written over PCIe.

To be clear:
for(i=0;i<size;i++)    dst[i]=src[i]; // forward copy.
for(i=size-1;i>=0;i--) dst[i]=src[i]; // backwards copy

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