Date: Thu, 16 Jan 2020 16:21:20 +0100 From: Natanael Copa <ncopa@...inelinux.org> To: Andre McCurdy <armccurdy@...il.com> Cc: musl@...ts.openwall.com Subject: Re: [PATCH 2/2] Add big-endian support to ARM assembler memcpy On Wed, 15 Jan 2020 10:41:08 -0800 Andre McCurdy <armccurdy@...il.com> wrote: > On Wed, Jan 15, 2020 at 7:46 AM Rich Felker <dalias@...c.org> wrote: > > On Fri, Sep 13, 2019 at 01:38:34PM -0700, Andre McCurdy wrote: > > > On Fri, Sep 13, 2019 at 11:59 AM Rich Felker <dalias@...c.org> wrote: > > > > On Fri, Sep 13, 2019 at 11:44:32AM -0700, Andre McCurdy wrote: > > > > > Allow the existing ARM assembler memcpy implementation to be used for > > > > > both big and little endian targets. > > > > > > > > Nice. I don't want to merge this just before release, but as long as > > > > it looks ok I should be able to review and merge it afterward. > > > > > > > > Note that I'd really like to replace this giant file with C using > > > > inline asm just for the inner block copies and C for all the flow > > > > control, but I don't mind merging this first as long as it's correct. > > > > > > Sounds good. I'll wait for your feedback after the upcoming release. > > > > Sorry this dropped off my radar. I'd like to merge at least the thumb > > part since it's simple enough to review quickly and users have > > actually complained about memcpy being slow on armv7 with -mthumb as > > default. > > Interesting. I wonder what the reference was against which the musl C > code was compared? From my own benchmarking I didn't find the musl > assembler to be much faster than the C code. There are armv6 and maybe > early armv7 CPUs where explicit prefetch instructions make a huge > difference (much more so than C -vs- assembler). Did the users who > complained about musl memcpy() compare against a memcpy() which uses > prefetch? For armv7 using NEON may help, although the latest armv7 > cores seem to perform very well with plain old C code too. There are > lots of trade offs so it's impossible for a single implementation to > be universally optimal. The "arm-mem" routines used on Raspberry Pi > seem to be a very fast for many targets, but unfortunately the armv6 > memcpy generates mis-aligned accesses so isn't suitable for armv5. > > https://github.com/bavison/arm-mem/ The Alpine user reported it here: https://gitlab.alpinelinux.org/alpine/aports/issues/11128 I don't know if you got the __builtin_memcpy or the libc version. I do know that qemu once got surprised that `memcpy` used libc's non-atomic version instead of gcc's atomic __builtin_memcpy. This happened due to alpine users fortify-headers as FORTIFY_SOURCE implementation. Not sure if something similar happened here. -nc
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