WORD_REGISTER_OPERATIONS/LOAD_EXTEND_OP

[Bernd Schmidt <bernds at redhat dot com>]

Can someone tell me what assumptions are made by this piece of code in
combine.c (nonzero_bits):

#if defined (WORD_REGISTER_OPERATIONS) && defined (LOAD_EXTEND_OP)
          /* If this is a typical RISC machine, we only have to worry
             about the way loads are extended.  */
          if (LOAD_EXTEND_OP (GET_MODE (SUBREG_REG (x))) == SIGN_EXTEND
              ? (((nonzero
                   & (((unsigned HOST_WIDE_INT) 1
                       << (GET_MODE_BITSIZE (GET_MODE (SUBREG_REG (x))) - 1))))
                  != 0))
              : LOAD_EXTEND_OP (GET_MODE (SUBREG_REG (x))) != ZERO_EXTEND)
#endif

I've got a sparc testcase here (PR 1061) where a masking operation is missing
in the output.  It appears that this piece of code assumes that if W_R_O and
L_E_O are defined, an expression like (subreg:SI (reg:HI x)) always has all
bits zero in the upper half.  This isn't the case, though - this seems like
it's just an invalid optimization and should be removed.

Comments?


Bernd