Andi Kleen a écrit :
From: "Bryan O'Sullivan" <[email protected]>
This copy routine is memcpy-compatible, but on some architectures will use
cache-bypassing loads to avoid bringing the source data into the cache.
One case where this is useful is when a device issues a DMA to a memory
region, and the CPU must copy the DMAed data elsewhere before doing any work
with it. Since the source data is read-once, write-never from the CPU's
perspective, caching the data at those addresses can only evict potentially
useful data.
We provide an x86_64 implementation that uses SSE non-temporal loads, and a
generic version that falls back to plain memcpy.
+ movq %r11, 56(%rdi)
+ addq %rcx, %rdi
+ cmpq %rdx, %rcx /* is rdx >= 64? */
+ jbe .L42
+ sfence
+ orl %edx, %edx
+ je .L33
I have three questions/remarks
1) Just curious why sfence is necessary here ?
2) Shouldnt we use this for large buffers, and restrict them to a size
multiple of 64, to avoid all these conditional branches ?
3) Also, the first 128 bytes of the source buffer will be bring into cache.
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