Re: string word-at-a-time and atomic.h FAQ on twitter

[Rich Felker <dalias@libc.org>]

On Sat, Jan 09, 2016 at 01:39:10AM +0300, Alexander Cherepanov wrote:
> >>>>this takes care of oob access, but the bytes outside the passed
> >>>>object might change concurrently i.e. strlen might introduce a
> >>>>data race: again this is a problem on the abstract c language
> >>>>level that may be solved e.g. by making all accesses to those
> >>>>bytes relaxed atomic, but user code is not under libc control.
> >>>>in practice the code works if HASZERO reads the word once so it
> >>>>does arithmetics with a consistent value (because the memory
> >>>>model of the underlying machine does not treat such race
> >>>>undefined and it does not propagate unspecified value bits nor
> >>>>has trap representations).
> >>>
> >>>Indeed, this seems like less of a practical concern.
> >>
> >>HASZERO reads the word twice so this should be a problem for
> >>unoptimized code on big-endian platforms.
> >
> >The number of abstract-machine reads is irrelevant unless we use
> >volatile here. A good compiler will always reduce it to one read, and
> >a bad compiler is always free to turn it into multiple reads.
> 
> Ok, I'll reformulate: is compiling musl on a big-endian platform
> with optimizations turned off officially supported?

Yes, and I don't see why you expect this case to break due to data
race issues.

Rich