Date: Wed, 1 Aug 2012 00:27:22 -0400 From: Rich Felker <dalias@...ifal.cx> To: Kim Walisch <kim.walisch@...il.com> Cc: musl@...ts.openwall.com Subject: Re: musl libc, memcpy On Tue, Jul 31, 2012 at 12:19:13AM +0200, Kim Walisch wrote: > > I'd like to know what block sizes you were looking at, because for > > memcpy that makes all the difference in the world: > > I copied blocks of 16 kilobytes. OK, that sounds (off-hand) like a good size for testing. > > I don't think this is necessary or useful. If we want better > > performance on these archs, a tiny asm file that does almost nothing > > but "rep movsd" is known to be the fastest solution on 32-bit x86, and > > is at least the second-fastest on 64-bit, with the faster solutions > > not being available on all cpus. On pretty much all other archs, > > unaligned access is illegal. > > My point is that your code uses byte (char) copying for unaligned data > but on x86 this is not necessary. Using a simple macro in your memcpy > implementation that always uses the size_t copying path for x86 speeds > up your memcpy implementation by about 500% for unaligned data on my > PC (Intel i5-670 3.46GHz, gcc-4.7, SL Linux 6.2 x86_64). You can also > use a separate asm file with "rep movsd" for x86, I guess it will run > at the same speed as my macro solution. I'm attaching a (possibly buggy; not heavily tested) rep-movsd-based version. I'd be interested in hearing how it performs. > Another interesting thing to mention is that gcc-4.5 vectorizes the 3 > copying loops of your memcpy implementation if it is compiled with the > -ftree-vectorize flag (add -ftree-vectorizer-verbose=1 for > vectorization report) but not if simply compiled with -O2 or -O3. With Odd, the gcc manual claims -ftree-vectorize is included in -O3: http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html > $ gcc -O2 -ftree-vectorize -ftree-vectorizer-verbose=1 memcpy.c main.c -o memcpy > > memcpy.c:25: note: created 1 versioning for alias checks. > memcpy.c:25: note: LOOP VECTORIZED. > memcpy.c:21: note: created 1 versioning for alias checks. > memcpy.c:21: note: LOOP VECTORIZED. > memcpy.c:9: note: vectorized 2 loops in function. >From the sound of those notes, I suspect duplicate code (and wasteful conditional branches) are getting generated to handle the possibility that the source and destination pointers might alias. I think this means it would be a good idea to add proper use of "restrict" pointers (per C99 requirements) in musl sooner rather than later; it might both reduce code size and improve performance. Rich
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