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Re: SPARC LEON3 and CAS instruction

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 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)
	  [(match_operand:I48MODE 2 "register_operand" "r")
	   (match_operand:I48MODE 3 "register_operand" "0")]
    "(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

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

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