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Message-ID: <54C94F9C.7030309@converseincode.com>
Date: Wed, 28 Jan 2015 13:07:40 -0800
From: Behan Webster <behanw@...verseincode.com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Alex Elder <elder@...aro.org>
CC: bcm@...thebug.org, Florian Fainelli <f.fainelli@...il.com>,
Russell King - ARM Linux <linux@....linux.org.uk>,
Matt Porter <mporter@...aro.org>,
bcm-kernel-feedback-list@...adcom.com,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] bcm: address clang inline asm incompatibility
On 01/28/15 11:38, Ard Biesheuvel wrote:
> On 28 January 2015 at 19:27, Alex Elder <elder@...aro.org> wrote:
>> On 01/28/2015 01:17 PM, Ard Biesheuvel wrote:
>>> On 28 January 2015 at 17:20, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>>>> On 28 January 2015 at 17:08, Alex Elder <elder@...aro.org> wrote:
>>>>> On 01/28/2015 10:17 AM, Ard Biesheuvel wrote:
>>>>>> On 28 January 2015 at 14:11, Alex Elder <elder@...aro.org> wrote:
>>>>>>> On 01/28/2015 05:15 AM, Ard Biesheuvel wrote:
>>>>>>>> On 28 January 2015 at 05:18, Behan Webster <behanw@...verseincode.com> wrote:
>>>>>>>>> From: Alex Elder <elder@...aro.org>
>>>>>>>>>
>>>>>>>>> My GCC-based build environment likes to call register r12 by the
>>>>>>>>> name "ip" in inline asm. Behan Webster informed me that his Clang-
>>>>>>>>> based build environment likes "r12" instead.
>>>>>>>>>
>>>>>>>>> Try to make them both happy.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Alex Elder <elder@...aro.org>
>>>>>>>>> Signed-off-by: Behan Webster <behanw@...verseincode.com>
>>>>>>>>> ---
>>>>>>>>> arch/arm/mach-bcm/bcm_kona_smc.c | 9 +++++++--
>>>>>>>>> 1 file changed, 7 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/arch/arm/mach-bcm/bcm_kona_smc.c b/arch/arm/mach-bcm/bcm_kona_smc.c
>>>>>>>>> index a55a7ec..3937bd5 100644
>>>>>>>>> --- a/arch/arm/mach-bcm/bcm_kona_smc.c
>>>>>>>>> +++ b/arch/arm/mach-bcm/bcm_kona_smc.c
>>>>>>>>> @@ -106,9 +106,14 @@ int __init bcm_kona_smc_init(void)
>>>>>>>>> * request result appropriately. This result value is found in r0
>>>>>>>>> * when the "smc" request completes.
>>>>>>>>> */
>>>>>>>>> +#ifdef __clang__
>>>>>>>>> +#define R12 "r12"
>>>>>>>>> +#else /* !__clang__ */
>>>>>>>>> +#define R12 "ip" /* gcc calls r12 "ip" */
>>>>>>>>> +#endif /* !__clang__ */
>>>>>>>> Why not just use r12 for both?
>>>>>>> Yes, that would have been an obvious fix. But the
>>>>>>> assembler (in the GCC environment) doesn't accept that.
>>>>>>>
>>>>>> Mine has no problems with it at all
>>>>>>
>>>>>> $ echo 'mov r12, #0' | arm-linux-gnueabihf-gcc -c -x assembler-with-cpp -
>>>>>>
>>>>>> and grepping for r12 under arch/arm suggests the same
>>>>> The use of "r12" is fine. But it's not just the assembler,
>>>>> I believe it also involves gcc.
>>>>>
>>>>> The problem is with the use of the __asmeq(x, y) macro.
>>>>>
>>>> Ah right. Apologies for assuming that you had missed something obvious here.
>>>> But __asmeq is not the toolchain, it is a local construct #define'd in
>>>> compiler.h
>>>>
>>>>> If I assign the "ip" variable with "r12":
>>>>> register u32 ip asm("r12"); /* Also called ip */
>>>>>
>>>>> Then that's fine. However, this line then causes an error:
>>>>> __asmeq("%0", "r12")
>>>>>
>>>>> Apparently gcc uses register "ip" when it sees asm("r12"). So
>>>>> attempting to verify the desired register got used with __asmeq()
>>>>> causes a string mismatch--"ip" is not equal to "r12".
>>>>>
>>>>> So I could use:
>>>>>
>>>>> register u32 ip asm("r12"); /* Also called ip */
>>>>> ...
>>>>> __asmeq("%0", "ip")
>>>>>
>>>>> And that will build. But it's a little non-intuitive, and
>>>>> I suspect that clang might (rightfully) have a failure in
>>>>> this __asmeq() call.
>>>>>
>>>> In that case, I would strongly suggest fixing the __asmeq () macro
>>>> instead, and teach it that ("r12","ip") and ("ip","r12") are fine too.
>>>>
>>>> The thing is, inline asm is a dodgy area to begin with in terms of
>>>> clang-to-gcc compatibility. On arm64, we have been seeing issues where
>>>> the width of the register -which is fixed on gcc- is selected based on
>>>> the size of that variable, i.e., an int32 gets a w# register and int64
>>>> gets a x# register. Imagine debugging that, e.g., a str %0, [xx] that
>>>> writes 8 bytes on GCC suddenly only writing 4 bytes when built with
>>>> clang.
>>>>
>>>> If we also start using the preprocessor to conditionalise what is
>>>> emitted by inline asm, the waters get even murkier and it becomes even
>>>> harder to claim parity between the two.
>>>>
>>> Something like this perhaps?
>> So __asmeq() yields true if the register names (strings) are
>> equal, or if one is "ip" and the other is "r12" (in either order).
>>
>> I can't comment on whether it's right in all build environments but
>> this looks OK to me, to handle this special case.
>>
>> I would much rather you generate that patch. Is that OK?
>>
> Sure, I can cook up a patch if you guys can confirm that it fixes your
> use case. (I tested GCC myself but I don't have clang installed)
>
That appears to work with clang as well.
All in all a much better solution.
Thank you,
Behan
--
Behan Webster
behanw@...verseincode.com
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