This thread got a lot of replies on mpir-devel which didn't show up
here for various random reasons.
The gist is that the problem only occurs with debian gcc 4.3.2, and
only with fstrict-aliasing which is enabled by -O2.
There's a discussion there about what best to do about it in Sage and
MPIR. I refer you to mpir-devel for the follow up.
Bill.