Re: Using memcpy() on an object

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++

Date:

Wed, 28 Apr 2010 20:25:22 +0200

Message-ID:

<83rcsiFmm4U1@mid.individual.net>

Thomas J. Gritzan wrote:

Am 28.04.2010 18:55, schrieb Peter Olcott:

"Thomas J. Gritzan" <phygon_antispam@gmx.de> wrote in

I just looked into the memcpy implementation on my
platform (Visual
Studio 2008 SP1): First, it does some testing and
alignment tricks, then
it jumps to a function that uses fast 128-bit SSE2
instructions if
availible and the object is big enough (so 16 bytes are
copied at a
time), otherwise it uses "rep movsd" to copy 32-bit at a
time.

I guess you can only be faster if you can remove all the
testing
overhead. If you let the compiler generate the copy
constructor, it can
unroll the loop and remove the testing, since it knows the
size of the
object and can assume non-overlapping storage regions.

Well, I just compiled an example ;-)
For assignment of a struct { int a[100]; } the compiler
generates 4
lines of x86 asm code with another "rep movsd" in it:

mov esi,eax
mov ecx,64h
mov edi,ebx
rep movsd

That's faster than calling memcpy.

According to one of the members of the C++ standards
committee, memcpy() is supposed to be intrinsic which is
supposed to mean that it is inlined, thus no function call
overhead.

Then VS 2008 ignores this in release build, because it calls memcpy
from the CRT that is defined in an ASM file.

It only does that if it cannot statically determine the alignment of
the copied buffers, or the alignment is not optimal. In that case,
calling the asm code results in a smaller program, which can also be
faster than a large inlined routine.

Bo Persson