23667: Recent performance regression

[Paolo Carlini]

Hi,

this is just an heads-up: PR libstdc++/23667 is about a time-out, on
slower machines of tr1/6_containers/unordered/hashtable/23465.cc.

However, If I use together with the library an up to date compiler
(either mainline or 4_0-branch), I'm able to appreciate the horrible
slow-down of the generated code (1-2 orders of magnitude!) also on
x86-linux. I believe a recent correct-ness fix introduced an unintented
pessimization.

In order to avoid seeing a spurious correctness regression in the logs
(the testcase is in the correctness library testsuite, not in the
performance one), I simplified the testcase, thus made it much faster,
but now I see that the underlying issue is generic and very serious: in,
say, 3-4 days, I will revert the change in case the slow-down will be
still present.

Thanks,
Paolo.