does the instruction combiner regards (foo & 0xff) as a special case?

[James E Wilson]

ibanez@sunplus.com wrote:
> I guess the combiner generates something like
> a trucation pattern when special constant are detected.
> The combiner also takse a similiar action in pattern

See the section of the documentation that talks about instruction 
canonicalization.
     http://gcc.gnu.org/onlinedocs/gccint/Insn-Canonicalizations.html
See in particular the last bullet.

Also, as Joern mentioned, you should try stepping through try_combine to 
see what is really happening.
-- 
Jim Wilson, GNU Tools Support, http://www.specifix.com