On Tue, 2 Aug 2011, Richard Guenther wrote:
> On Tue, Aug 2, 2011 at 2:06 PM, Mikael Pettersson <mikpe@it.uu.se> wrote:
> > Michael Walle writes:
> >  >
> >  > Hi,
> >  >
> >  > > To confirm that try -fno-tree-ter.
> >  >
> >  > "lm32-gcc -O1 -fno-tree-ter -S -c test.c" generates the following working
> >  > assembly code:
> >  >
> >  > f2:
> >  >      addi     sp, sp, -4
> >  >      sw       (sp+4), ra
> >  >      addi     r2, r0, 10
> >  >      calli    __ashrsi3
> >  >      addi     r8, r0, 10
> >  >      scall
> >  >      lw       ra, (sp+4)
> >  >      addi     sp, sp, 4
> >  >      b        ra
> >
> > -fno-tree-ter also unbreaks the ARM test case in PR48863 comment #4.
>
> It's of course only a workaround, not a real fix as nothing prevents
> other optimizers from performing the re-scheduling TER does.
>
> I suggest to amend the documentation for local call-clobbered register
> variables to say that the only valid sequence using them is from a
> non-inlinable function that contains only direct initializations of the
> register variables from constants or parameters.

I'd be ok with that, FWIW; I see the problem with keeping the
scheduling of operations in a working order (yuck) and I don't
see how else to keep it working ...except perhaps make gcc flag
functions with register asms as non-inlinable, maybe even flag
down any of the dangerous re-scheduling?

Maybe I can do that with some hand-holding?

> Or go one step further and deprecate local register variables alltogether
> (they IMHO don't make much sense, and rather the targets should provide
> a way to properly constrain asm inputs and outputs).

They do make sense when implementing e.g. system calls, and
they're documented to work as discussed.  (I almost regret
making that happen, though.)  Fortunately such functions are
small, and not relatively much helped by inlining (it's a
*syscall*; much more happening beyond the call than is affected
by inlining some parameter initialization).  Sure, new targets
are much better off by implementing that through other means,
but preferably intrinsic functions to asms.

brgds, H-P