The next release of pacman (after pacman 5.1.3-1.3) will set the architecture to "pentium4" if you have

Architecture = auto

in your /etc/pacman.conf and your machine supports sse2 instructions.
This requires reinstallation (and if necessary a downgrade) of all installed packages, e.g. by running

pacman -Syy
pacman -Qqn | pacman -S -

Note, that this is only possible and necessary after pacman > 5.1.3-1.3 gets installed.
Also note, that it's not a good idea to stick with i686 if your machine supports sse2, as more and more packages require sse2 and will be dropped from the i686 repositories, thus.

Edit: There seems to be a flaw in the sse2 detection logic of pacman 5.1.3-1.4 - it correctly switches to pentium4 on my test vm, but not on my test real hardware (both with sse2). You can check with

pacman-conf Architecture

if it changes to pentium4 for you - and apply the above steps only if it does so.
I'm working on it ... stay tuned.

A new architecture is available, called 'pentium4'. This one is for CPUs supporting at least SSE2.

For installation and transition information head to:

Installation of pentium4 optimized packages

libreadline 8 made it into [core], although not all packages were rebuilt against it yet.
This breaks several packages, some of them vital for the system.

Before proceeding to update, please check the package database for the updated packages if it mentions a 'not satisfiable dependency: "libreadline.so.7"'.
I'll update this notice when all/most packages have been rebuilt against the new readline package and have been moved to core/extra/community.

Sorry for the trouble,
deep42thought

Edit:
most packages have been rebuilt and are in core/extra/community. Exceptions:
gap
js60
libgda
percona-server
percona-server-clients
sagemath

It seems, I managed to break several packages by updating icu. The current list (known to our database / me) is:

i686/community/gitlab-10.8.4-1.0-i686
i686/community/mysql-workbench-6.3.10-7.0-i686
i686/community/0ad-a23-4.0-i686 (0ad-a23-6.0-i686 now available in [community])
i686/community/aegisub-3.2.2-28.0-i686 (aegisub-3.2.2-30.0-i686 now available in [community])
i686/community/calibre-3.31.0-1.0-i686 (calibre-3.34.0-1.0-i686 now available in [community])
i686/community/haskell-hakyll-4.12.3.0-117.0-i686 (haskell-hakyll-4.12.4.0-67.0-i686 now available in [community])
i686/community/kbibtex-1:0.8.1-2.1-i686 (kbibtex-1:0.8.1-3.0-i686 now available in [community])
i686/community/pandoc-citeproc-0.14.3.1-133.0-i686 (pandoc-citeproc-0.14.8.1-1.0-i686 now available in [community])
i686/community/scribus-1.5.4-1.0-i686 (scribus-1.5.4-5.0-i686 now available in [community])
i686/extra/chromium-69.0.3497.92-1.0-i686 (chromium-70.0.3538.77-2.0-i686 now available in [extra])
i686/extra/firefox-62.0-1-i686 (firefox-63.0.3-1.0-i686 now available in [extra])
i686/extra/libreoffice-fresh-6.1.1-1.0-i686 (libreoffice-fresh-6.1.3-1.0-i686 now available in [extra])
i686/extra/libreoffice-still-6.0.6-3.0-i686 (libreoffice-still-6.0.7-1.0-i686 now available in [extra])
i686/extra/mpd-0.20.21-1.0-i686 (mpd-0.21.1-1.0-i686 now available in [extra])
i686/extra/thunderbird-60.0-4-i686 (thunderbird-60.3.1-2.0-i686 now available in [extra])

If you require any of those packages (or packages which depend on those), please postpone updates, until we rebuilt those packages.

A few days ago some (crucial) packages linked against glibc 2.28 were falsely moved to [core] [extra] [community], while glibc-2.28 was still in [testing].
This resulted in all kinds of serious errors.

glibc-2.28 is now in [core], so these issues should be fixable by running

pacman -Syu

I'm sorry for any caused troubles.

Updating the system may result in:

error: failed to prepare transaction (could not satisfy dependencies)
:: installing x265 (2.8-1.0) breaks dependency 'libx265.so=151-32' required by ffmpeg2.8

simply remove ffmpeg2.8 and the repeat the update:

pacman -Rs ffmpeg2.8
pacman -Syyu

Some keys in archlinux32-keyring expired on short notice and were replaced / extended.
To get the latest keys, run

pacman-key --refresh-keys

and update archlinux32-keyring before any other packages:

pacman -S archlinux32-keyring
pacman -Syu

Archlinux mainline just announced the EOL for i686 in archlinux:

Transition from the Official Repositories

If you plan to move from the official repositories to our community maintained repositories, follow these steps:

• Put any mirror from our mirrolist into /etc/pacman.d/mirrorlist.

• Execute pacman -Syy archlinux32-keyring-transition to install our keyring transition package signed by one of the x86_64 archlinux devs.

• Run pacman -Syuu for a full transition.

  • Note, that some packages are currently newer in the official repositories and will be downgraded, therefore.

  • Also note, that packages found in your package cache won't match the signatures from archlinux32 - either allow pacman to delete these cached packages or run pacman -Sc to clean your cache before you start and after you finish upgrading.

Available Mirrors:

• Canada

  • http://arch32.mirrors.simplysam.us/$arch/$repo

• France

  • https://archlinux32.mirror.roelf.org/$arch/$repo

• Germany

  • https://mirror.archlinux32.org/$arch/$repo

• United States

  • https://32.arlm.tyzoid.com/$arch/$repo

See https://archlinux32.org/download/ for more information, including installation media.

The November ISOs are now available for download, and should be syncing to the various mirrors shortly.

Master Mirror:
http://mirror.archlinux32.org/archisos/ (EU/Germany)

Tier 1 Mirrors:
https://archlinux32.mirror.roelf.org/archisos/ (EU/France)
http://32.arlm.tyzoid.com/archisos/ (North America/United States)
http://arch32.mirrors.simplysam.us/archisos/ (North America/Canada)

The August ISOs are now available for download, and should be syncing to the various mirrors shortly.

Master Mirror:
http://mirror.archlinux32.org/archisos/ (EU/Germany)

Tier 1 Mirrors:
https://archlinux32.mirror.roelf.org/archisos/ (EU/France)
http://32.arlm.tyzoid.com/archisos/ (North America/United States)
http://arch32.mirrors.simplysam.us/archisos/ (North America/Canada)