Re: splitting add instructions

[Paul Koning <paul_koning@dell.com>]

On Jul 18, 2011, at 12:53 PM, Richard Henderson wrote:

> On 07/18/2011 08:01 AM, Paulo J. Matos wrote:
>> The problem is, if addhi3 expands into two insn:
>> (define_insn "addqi3"
>>   [(set (match_operand:HI 0 "nonimmediate_operand" "=c")
>>         (plus:HI (match_operand:HI 1 "general_operand" "%0")
>>                  (match_operand:HI 2 "general_operand" "cwmi")))]
>>   "get_attr_CARRY(insn) == 0"
>>   "add  %b0,%b2")
>> 
>> (define_insn "addqi3_carry"
>>   [(set (match_operand:HI 0 "nonimmediate_operand" "=c")
>>         (plus:HI (match_operand:HI 1 "general_operand" "%0")
>>                  (match_operand:HI 2 "general_operand" "cwmi")))]
>>   "get_attr_CARRY(insn) == 1"
>>   "addc %t0,%t2")
>> 
>> _One_ of the problems with this is that if GCC sees that op2 is 0, it will remove the insn because it will see: R0 = R0 + 0. However, it can't do this in this specific case because the plus is actually also adding the carry flag.
>> 
>> Any suggestions on how to deal with this situation?
> 
> You need to specifically represent the other output, i.e. the carry flag.
> 
> Depending on the constraints of your processor, you may or may not be able
> to expose this before reload.  Reload requires the ability to issue move
> instructions (all of loads, stores, reg-reg) and addition instructions.
> It must be able to do this between any pair of instructions in order to
> handle register spilling.  Therefore in order to expose the carry flag
> before reload, you must have an add instruction that does not modify the
> carry. Some processors have this in the form of a "load-effective-address"
> instruction.

Why an add instruction?  Is that in the case where address arithmetic requires separate adds?  If the machine is CISC, then I would think that reload just needs to be able to generate loads and stores that don't modify the carry, which would be a different requirement.  For example, on a pdp11 load/store (move) has that property, while add doesn't (it always touches carry).

"Specifically represent... the carry flag" means using the CCmode style of condition code handling, right?

	paul