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Re: SPARC LEON3 and CAS instruction


On Fri, Apr 25, 2014 at 1:31 PM, Eric Botcazou <ebotcazou@adacore.com> 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.
>
>> 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.

Since we are using RTEMS on Leon3 (and maybe moving to Leon4),
we're interested in getting this fixed too; feel free to assign us the
issue to fix it if you have no
time.

   Daniel.

>
> --
> Eric Botcazou



-- 

Daniel F. Gutson
Chief Engineering Officer, SPD


San Lorenzo 47, 3rd Floor, Office 5

CÃrdoba, Argentina


Phone: +54 351 4217888 / +54 351 4218211

Skype: dgutson


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