Date: Wed, 15 Jan 2020 14:22:15 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH 2/2] Add big-endian support to ARM assembler memcpy On Wed, Jan 15, 2020 at 10:41:08AM -0800, Andre McCurdy 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/ I'm not sure of the details but the comparison was just between the armv6 version of Alpine and the armv7 version (so using musl's memcpy_le.S vs memcpy.c). Rich
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