> I thought the idea of per-user /tmp was that every user got his own /tmp, sure, but this was implemented via subdirectories of the *real*, tmpfs, cleared-on-boot /tmp.
You don't need tmpfs then. This will work with /tmp anywhere (disk, ram, separate partition, nfs, etc). I mean this is neither a reason to use tmpfs nor it's a reason to avoid it.
> One application that becomes a lot faster with /tmp on tmpfs is GCC without -pipe, or, even with -pipe, at the LTO link step.
Faster linking? Let's check that with something having a lot of binaries:
mount tmpfs or ext3 to /mnt/test, then
$ cd /mnt/test
$ wget http://ftp.gnu.org/gnu/coreutils/coreutils-8.17.tar.xz
$ export CFLAGS='-O2 -g -flto' TMPDIR=/mnt/test
$ time sh -c "tar xf coreutils-8.17.tar.xz; cd coreutils-8.17; ./configure; make install DESTDIR=/mnt/test/root; cd ../root; tar czf ../coreutils-package.tar.gz *; cd ..; rm -rf coreutils-8.17 root"
real 882.876s user 760.111s sys 110.353s
real 884.456s user 761.408s sys 110.603s
real 885.245s user 762.770s sys 110.525s
real 884.914s user 762.417s sys 110.395s
real 885.352s user 762.865s sys 110.360s
real 895.244s user 762.620s sys 115.027s
real 893.134s user 762.447s sys 114.841s
real 898.353s user 763.645s sys 116.369s
real 898.010s user 763.472s sys 116.074s
real 897.525s user 763.671s sys 116.219s
If my test is correct, it's still same 1-2%. It is faster, but not a lot.