Is the check performed on the return value from force_const_mem in the plus_constant function correct?

[Andrew Bennett]

Hi,

I am debugging a segmentation fault when compiling some code for MIPS.  The
segmentation fault occurs in the plus_constant function in explow.c when it
tries to update a constant pool value.  The offending code is below:

case MEM:
      /* If this is a reference to the constant pool, try replacing it with
         a reference to a new constant.  If the resulting address isn't
         valid, don't return it because we have no way to validize it.  */
      if (GET_CODE (XEXP (x, 0)) == SYMBOL_REF
          && CONSTANT_POOL_ADDRESS_P (XEXP (x, 0)))
        {
          tem = plus_constant (mode, get_pool_constant (XEXP (x, 0)), c);
          tem = force_const_mem (GET_MODE (x), tem);
          if (memory_address_p (GET_MODE (tem), XEXP (tem, 0)))
            return tem;
        }


The code fails for MIPS because its cannot_force_const_mem target function does
not allow constants (so that the move expanders can deal with them later on),
this then causes the force_const_mem function to return a NULL_RTX.

What I can't understand is why the memory_address_p function is used to check
the result from force_const_mem?  From looking at the return values from
force_const_mem the only possibilies are what looks like a valid memory rtx,
or a NULL_RTX.  Also there is no other instances in the code where this happens.
I was therefore wondering is anyone knew why the check is done in this manner?

I think the fix should be to just check that the tem variable is not a NULL_RTX.
I was also wondering if anyone had any issues/objections to checking the result 
in this manner?


Many thanks,


Andrew