[Rd] 32 vs 64 bit difference?

[Terry Therneau]

I've spent the last few hours baffled by a test suite inconsistency.

The exact same library code gives slightly different answers on the home 
and work machines  - found in my R CMD check run.  I've recopied the entire
directory to make sure it's really identical code. 
  The data set and fit in question has a pretty flat "top" to the likelihood.
I put print statements in to the "f()" function called by optim, and the
two parameters and the likelihood track perfectly for 48 iterations, then
start to drift ever so slightly:
< theta= -3.254176 -6.201119 ilik= -16.64806 
> theta= -3.254176 -6.201118 ilik= -16.64806 

And at the end of the iteration:
< theta= -3.207488 -8.583329 ilik= -16.70139 
> theta= -3.207488 -8.583333 ilik= -16.70139 

As you can see, they get to the same max, but with just a slightly
different path.

  The work machine is running 64 bit Unix (CentOS) and the home one 32 bit
Ubuntu.
Could this be enough to cause the difference?  Most of my tests are
based on all.equal, but I also print out 1 or 2 full solutions; perhaps 
I'll have to modify that?

Terry Therneau