SSE2 provides functionality for performing faster on aligned memory. By copying the first and last bytes of an unaligned memory destination using the conventional unaligned functionality, and copying everything in between as aligned, it is possible to make use of this performance improvement on large unaligned memory blocks as well.
In this graph the green lines are the conventional memcpy available in Microsoft Visual Studio 2008, the red lines are the SSE memcpy function available in Nevrax NeL, and the blue lines are the custom SSE2 function. The bright colored lines are the performance on alinged memory blocks, while the dark colored lines are tested on differently unaligned blocks of memory. Horizontally the copy function is tested on different sizes of memory, on the vertical axis the copy speed is displayed in MB/s.
As you can see, NeL’s SSE memcpy performs very well on aligned memory, but gives horrible performance on unaligned memory, as it does not take the aligning of the memory blocks into account. The builtin memcpy function is fastest of all at copying blocks below 128 bytes, but also reaches it’s speed limit there. The SSE2 memcpy takes larger sizes to get to it’s maximum performance, but peaks above NeL’s aligned SSE memcpy even for unaligned memory blocks.
Code is available below, ask before using.
void *memcpy_kaetemi_sse2(void *dst, void *src, int nBytes)
{
__asm
{
// Copyright (C) 2009 Jan Boon (Kaetemi)
// optimized on Intel Core 2 Duo T7500
mov ecx, nBytes
mov edi, dst
mov esi, src
add ecx, edi
prefetchnta [esi]
prefetchnta [esi+32]
prefetchnta [esi+64]
prefetchnta [esi+96]
// handle nBytes lower than 128
cmp nBytes, 512
jge fast
slow:
mov bl, [esi]
mov [edi], bl
inc edi
inc esi
cmp ecx, edi
jnz slow
jmp end
fast:
// align dstEnd to 128 bytes
and ecx, 0xFFFFFF80
// get srcEnd aligned to dstEnd aligned to 128 bytes
mov ebx, esi
sub ebx, edi
add ebx, ecx
// skip unaligned copy if dst is aligned
mov eax, edi
and edi, 0xFFFFFF80
cmp eax, edi
jne first
jmp more
first:
// copy the first 128 bytes unaligned
movdqu xmm0, [esi]
movdqu xmm1, [esi+16]
movdqu xmm2, [esi+32]
movdqu xmm3, [esi+48]
movdqu xmm4, [esi+64]
movdqu xmm5, [esi+80]
movdqu xmm6, [esi+96]
movdqu xmm7, [esi+112]
movdqu [eax], xmm0
movdqu [eax+16], xmm1
movdqu [eax+32], xmm2
movdqu [eax+48], xmm3
movdqu [eax+64], xmm4
movdqu [eax+80], xmm5
movdqu [eax+96], xmm6
movdqu [eax+112], xmm7
// add 128 bytes to edi aligned earlier
add edi, 128
// offset esi by the same value
sub eax, edi
sub esi, eax
// last bytes if dst at dstEnd
cmp ecx, edi
jnz more
jmp last
more:
// handle equally aligned arrays
mov eax, esi
and eax, 0xFFFFFF80
cmp eax, esi
jne unaligned4k
aligned4k:
mov eax, esi
add eax, 4096
cmp eax, ebx
jle aligned4kin
cmp ecx, edi
jne alignedlast
jmp last
aligned4kin:
prefetchnta [esi]
prefetchnta [esi+32]
prefetchnta [esi+64]
prefetchnta [esi+96]
add esi, 128
cmp eax, esi
jne aligned4kin
sub esi, 4096
alinged4kout:
movdqa xmm0, [esi]
movdqa xmm1, [esi+16]
movdqa xmm2, [esi+32]
movdqa xmm3, [esi+48]
movdqa xmm4, [esi+64]
movdqa xmm5, [esi+80]
movdqa xmm6, [esi+96]
movdqa xmm7, [esi+112]
movntdq [edi], xmm0
movntdq [edi+16], xmm1
movntdq [edi+32], xmm2
movntdq [edi+48], xmm3
movntdq [edi+64], xmm4
movntdq [edi+80], xmm5
movntdq [edi+96], xmm6
movntdq [edi+112], xmm7
add esi, 128
add edi, 128
cmp eax, esi
jne alinged4kout
jmp aligned4k
alignedlast:
mov eax, esi
alignedlastin:
prefetchnta [esi]
prefetchnta [esi+32]
prefetchnta [esi+64]
prefetchnta [esi+96]
add esi, 128
cmp ebx, esi
jne alignedlastin
mov esi, eax
alignedlastout:
movdqa xmm0, [esi]
movdqa xmm1, [esi+16]
movdqa xmm2, [esi+32]
movdqa xmm3, [esi+48]
movdqa xmm4, [esi+64]
movdqa xmm5, [esi+80]
movdqa xmm6, [esi+96]
movdqa xmm7, [esi+112]
movntdq [edi], xmm0
movntdq [edi+16], xmm1
movntdq [edi+32], xmm2
movntdq [edi+48], xmm3
movntdq [edi+64], xmm4
movntdq [edi+80], xmm5
movntdq [edi+96], xmm6
movntdq [edi+112], xmm7
add esi, 128
add edi, 128
cmp ecx, edi
jne alignedlastout
jmp last
unaligned4k:
mov eax, esi
add eax, 4096
cmp eax, ebx
jle unaligned4kin
cmp ecx, edi
jne unalignedlast
jmp last
unaligned4kin:
prefetchnta [esi]
prefetchnta [esi+32]
prefetchnta [esi+64]
prefetchnta [esi+96]
add esi, 128
cmp eax, esi
jne unaligned4kin
sub esi, 4096
unalinged4kout:
movdqu xmm0, [esi]
movdqu xmm1, [esi+16]
movdqu xmm2, [esi+32]
movdqu xmm3, [esi+48]
movdqu xmm4, [esi+64]
movdqu xmm5, [esi+80]
movdqu xmm6, [esi+96]
movdqu xmm7, [esi+112]
movntdq [edi], xmm0
movntdq [edi+16], xmm1
movntdq [edi+32], xmm2
movntdq [edi+48], xmm3
movntdq [edi+64], xmm4
movntdq [edi+80], xmm5
movntdq [edi+96], xmm6
movntdq [edi+112], xmm7
add esi, 128
add edi, 128
cmp eax, esi
jne unalinged4kout
jmp unaligned4k
unalignedlast:
mov eax, esi
unalignedlastin:
prefetchnta [esi]
prefetchnta [esi+32]
prefetchnta [esi+64]
prefetchnta [esi+96]
add esi, 128
cmp ebx, esi
jne unalignedlastin
mov esi, eax
unalignedlastout:
movdqu xmm0, [esi]
movdqu xmm1, [esi+16]
movdqu xmm2, [esi+32]
movdqu xmm3, [esi+48]
movdqu xmm4, [esi+64]
movdqu xmm5, [esi+80]
movdqu xmm6, [esi+96]
movdqu xmm7, [esi+112]
movntdq [edi], xmm0
movntdq [edi+16], xmm1
movntdq [edi+32], xmm2
movntdq [edi+48], xmm3
movntdq [edi+64], xmm4
movntdq [edi+80], xmm5
movntdq [edi+96], xmm6
movntdq [edi+112], xmm7
add esi, 128
add edi, 128
cmp ecx, edi
jne unalignedlastout
jmp last
last:
// get the last 128 bytes
mov ecx, nBytes
mov edi, dst
mov esi, src
add edi, ecx
add esi, ecx
sub edi, 128
sub esi, 128
// copy the last 128 bytes unaligned
movdqu xmm0, [esi]
movdqu xmm1, [esi+16]
movdqu xmm2, [esi+32]
movdqu xmm3, [esi+48]
movdqu xmm4, [esi+64]
movdqu xmm5, [esi+80]
movdqu xmm6, [esi+96]
movdqu xmm7, [esi+112]
movdqu [edi], xmm0
movdqu [edi+16], xmm1
movdqu [edi+32], xmm2
movdqu [edi+48], xmm3
movdqu [edi+64], xmm4
movdqu [edi+80], xmm5
movdqu [edi+96], xmm6
movdqu [edi+112], xmm7
end:
}
return dst;
}
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