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Visual Studio のデバッガでmemcpyに飛び込んでみた。
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かたやコチラ↓が C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.12.25827\crt\src\x64\memcpy.asm
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```
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mov r11, rcx ; save destination address
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mov r10, rdx ; save source address
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cmp r8, 16 ; if 16 bytes or less
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jbe MoveBytes16 ; go move them quick
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cmp r8, 32 ; check for length <= 32 (we know its > 16)
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jbe Move17to32 ; go handle lengths 17-32 as a special case
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sub rdx, rcx ; compute offset to source buffer
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jae CopyUp ; if above or equal, go move up
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mov rax, r10 ; else check that src+count < dst
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add rax, r8 ; src + count
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cmp rcx, rax ; (src + count) < dst
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jl CopyDown ; no, buffers overlap go move downward
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CopyUp:
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cmp r8, 128
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jbe XmmCopySmall
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bt __favor, __FAVOR_ENFSTRG ; check for ENFSTRG (enhanced fast strings)
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jnc XmmCopyUp ; If Enhanced Fast String not available, use XMM
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; use Enhanced Fast Strings
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; but first align the destination dst to 16 byte alignment
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StringCopy:
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mov rax, r11 ; return original destination pointer
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mov r11, rdi ; save rdi in r11
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mov rdi, rcx ; move destination pointer to rdi
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mov rcx, r8 ; move length to rcx
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mov r8, rsi ; save rsi in r8
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mov rsi, r10 ; move source pointer to rsi
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rep movsb ; copy source to destination buffer
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mov rsi, r8 ; restore rsi
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mov rdi, r11 ; restore rdi
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ret
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```
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同じもんみたいやね、ッタリマエだけど。
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