[Bug rtl-optimization/114766] ^ constraint modifier unexpectedly affects register class selection.

["vmakarov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114766

--- Comment #3 from Vladimir Makarov <vmakarov at gcc dot gnu.org> ---
(In reply to Tamar Christina from comment #2)
> (In reply to Vladimir Makarov from comment #1)
> > (In reply to Tamar Christina from comment #0)
> > > The documentation for ^ states:
> >
> > If it works for you, we could try to use the patch (although it needs some
> > investigation how other targets uses the hint).  In any case, the
> > documentation should be modified or made more clear depending on applying or
> > not applying the patch.
> 
> Yeah, using the patch gives us the behavior we expected, we added a
> workaround for now so we can investigate what other targets do in GCC 15.
> 
> But while looking at this we also got some unexpected behavior with using ?
> 

> 
>   r103 costs: W8_W11_REGS:2000 W12_W15_REGS:2000 TAILCALL_ADDR_REGS:2000
> STUB_REGS:2000 GENERAL_REGS:2000 FP_LO8_REGS:0 FP_LO_REGS:0 FP_REGS:0
> POINTER_AND_FP_REGS:7000 MEM:9000
> 
> In this particular pattern the ? isn't needed so we're removing it, but the
> behavior is still unexpected.

'?' is a very old hint (unlike ^ and @).  It is used by all targets for many
years.  IRA cost calculation uses exactly the same algorithm as it was in now
non-existing regclass.c file.  Changing code related to processing '?' would
have very unpredictable consequences for many targets.  After many years
working on RA, I am still surprised how fragile code calculating costs and reg
classes and how insignificant changes can result in a cascade of GCC test
failures.

There are many factors which still can result in zero cost code even when '?'
is used.  You can try to use more one '?' and see what happens.  If the cost is
still zero, I could look what is going on in the cost calculation.