Re: [PATCH, RFC] LRA subreg handling

[Jeff Law <law at redhat dot com>]

On 01/15/15 03:13, Robert Suchanek wrote:
> > Robert, can you look at reload.c::reload_inner_reg_of_subreg and verify
> > that the comment just before its return statement is effectively the
> > situation you're in.
> >
> > There are certainly cases where a SUBREG needs to be treated as an
> > in-out operand.  We walked through them eons ago when we were poking at
> > SSA for RTL.  But the details have long since faded from memory.
>
> The comment pretty much applies to my situation.  The only difference I can
> see is that reload would have had hard registers at this point.  In the testcase,
> LRA does not have hard registers assigned to the concerned pseudo(s), thus,
> it can't rely on the information in hard_regno_nregs to check if the number of
> registers in INNER is different to the number of words in INNER.
The differences (hard vs pseudo regs) are primarily an implementation 
detail.  I was really looking to see if there was existing code which 
would turn an output reload into an in-out reload for these subregs.

The in-out nature of certain subregs is something I've personally 
stumbled over in various contexts (for example, this also came up during 
RTL-SSA investigations years ago).

Jeff