> Actually, simplify-rtx.c now uses nonzero_bits and num_sign_bit_copies: > these ask combine for the value in the case of pseudos, via the "RTL > hooks" mechanism.
Right. That was certainly a step (and was discussed a while ago), but doing it more globally would make it even easier. > 1) what to do with (clobber (const_int 0)). This should be not so much > of a problem thanks to validate_change, but I'd be weary of having such > CLOBBER rtx-en in REG_EQUAL notes! Just return NULL. The philosophy of simplify_rtx is different from combine. In the former, you're being asked "can you simplify this?" while in the latter, there's a lot of intertwining between simplification and substitution. So you can just return "no, I can't simplify it" and combine would then do what it wants do with that result. > 2) a lot of this is hard to be done incrementally. For example, it is > hard to move any one of simplify_and_const_int, make_compound_operation, > expand_compound_operation, force_to_mode, simplify_shift_const without > moving the others. Indeed. And you can't just move them intact because of the change of philosphy. I originally wrote those routines and so am the most familiar with them. Once the infrastructure is ready, I probably should take a look at what's involved. > > That means we could merge CSE and combine and do a *lot* more than we ever > > were able to do before while having the code for both passes be much > > simpler. > > Unfortunately, we should not forget CSE's secondary functionality, which > is to act as garbageman for the GCSE pass. Right, but much of that functionality is more in the arena of operand selection than true "cse".
