I found that PRE is not done in gcc-3.3.3, though the code for doing PRE exists in source code.
gcc-3.3 is not interesting for work like this. The PRE support in gcc-3.3 has already been effectively obsoleted by other work in current gcc sources, though it is still there.
You failed to specify the target. The target often makes a big difference on whether optimizations are performed. I am assuming the target is ia64-linux.
Your testcase incidentally is serious flawed. The varible c is both used while uninitialized, and set using an unitialized value. It doesn't make much sense to talk about compiler optimizations when you have a testcase that invokes undefined behaviour.
The testcase is also flawed in that it takes the address of both a and b, forcing them into memory, and thus making optimization of them difficult, particularly in pre-gcc-4 sources.
I seem to get the same behaviour from both gcc-3.3 and gcc-3.4. In neither case is the partially redundant expression eliminated.
gcc-4 does perform the optimization. -- Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
