Re: [rfc] multi-word subreg lowering pass

[Paul Schlie <schlie at comcast dot net>]

> From: Paul Schlie <schlie@comcast.net>
>> Richard Henderson writes:
>> ...
>> Where it fails is when the target has patterns that take multi-word
>> inputs.  In this case we have to stop and leave the data in the
>> multi-word pseudo.  This approach does have the advantage of being
>> able to work incrementally, though it's also true that incomplete
>> conversion can hurt
> 
> - Out of curiosity, why not leave all decomposed operations in their
>   subreg form, thereby maintaining the logical integrity of their operand
>   modes? i.e. the above decomposes to something like:
> 
>      (set (subreg:SI 100 0) (and:SI (reg:SI x) (subreg:SI y 0)))
>      (set (subreg:SI 100 1) (and:SI (const_int 0) (subreg:SI y 1)))
> 
>   which seems both simpler, and does not require introduction of new
>   semantics which complicate multi-word/sub-reg input operand expressions.

- where then I'd guess:

       char foo (char x, long long y)
       {
         return x & y;
       }

  could be then be correspondingly "lowered" to something like:
  
       (set (reg:QI 100) (and:QI (reg:QI x) (subreg:QI y 0))

  whereby then it should be sufficiently clear that (subreg:QI y 0)
  would only require the "loading" of the QI subreg component of "y"
  if no subreg components are referenced?