Re: SPARC LEON3 and CAS instruction

[Sebastian Huber <sebastian dot huber at embedded-brains dot de>]

On 2014-04-25 18:31, Eric Botcazou wrote:
> > recent GCC versions support the C11 atomic operations for the SPARC LEON3
> > processor via the CASA instruction.  GCC emits CASA instructions with an ASI
> > of 0x80.  I think this is due to the usage of "cas" if I get the stuff in
> > sync.md right:
> >
> > "(define_insn "*atomic_compare_and_swap<mode>_1"
> >     [(set (match_operand:I48MODE 0 "register_operand" "=r")
> > 	(match_operand:I48MODE 1 "mem_noofs_operand" "+w"))
> >      (set (match_dup 1)
> > 	(unspec_volatile:I48MODE
> > 	  [(match_operand:I48MODE 2 "register_operand" "r")
> > 	   (match_operand:I48MODE 3 "register_operand" "0")]
> > 	  UNSPECV_CAS))]
> >     "(TARGET_V9 || TARGET_LEON3) && (<MODE>mode != DImode || TARGET_ARCH64)"
> >     "cas<modesuffix>\t%1, %2, %0"
> >     [(set_attr "type" "multi")])"
>
> Right, this is a bug, both in the compiler and the assembler, since an ASI of
> 0x80 is not allowed for SPARC-V8.

Ok, I didn't notice this before since it worked well on the LEON4FT in 
supervisor mode.  I work currently with the XtratuM hypervisor and noticed this 
problem with CAS in user mode.

> > According to the LEON3 manual we have:
> >
> > "6.2.7 Compare and Swap instruction (CASA)
> >
> > LEON4 implements the SPARC V9 Compare and Swap Alternative (CASA)
> > instruction. The CASA operates as described in the SPARC V9 manual. The
> > instruction is privileged, except when setting ASI = 0xA (user data)."
> >
> > I would like to use atomic operations in user mode.  Is it possible to add a
> > machine option to GCC to use an ASI of 0x0A for the atomic operations via
> > CASA on LEON3?
>
> Yes, I guess we actually want to emit an ASI of either 0xA (user data) or 0xB
> (supervisor data), predicated on -muser-mode.  I'll prepare a patch.

Thanks, since this -muser-mode seems to be something new, maybe we should 
instead use -mcas=supervisor|user to make it more specific?

--
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone   : +49 89 189 47 41-16
Fax     : +49 89 189 47 41-09
E-Mail  : sebastian.huber@embedded-brains.de
PGP     : Public key available on request.

Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.