Jakub Jelinek <ja...@redhat.com> writes: > Hi! > > When working on PR57130, I've wondered why we don't simplify it much earlier > and end up with creating such weirdness. > > The following patch fixes that, by using nonzero_bits to see if all the low > bits must be zero and in that case just return zero. > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? > > 2013-05-02 Jakub Jelinek <ja...@redhat.com> > > * simplify-rtx.c (simplify_truncation): If nonzero_bits > on op shows all bits zero in mode, return zero.
I'm probably wrong, but just in case: it looks like this might fall foul of: /* Try to simplify a MODE truncation of OP, which has OP_MODE. Only handle cases where the truncated value is inherently an rvalue. If op is a register, it might be being used as an lvalue instead of an rvalue. We don't want to simplify (subreg (reg ..)) to (const_int 0) in that case. It should be OK to put this under the TRUNCATE case in simplify_unary_operation_1, but I don't know if that'd be good enough for your testcase. Thanks, Richard
