linux-kernel - Re: [PATCH] bcm: address clang inline asm incompatibility

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Date:	Wed, 28 Jan 2015 11:30:12 -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 11:17, 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?
>
> -------->8----------
> diff --git a/arch/arm/include/asm/compiler.h b/arch/arm/include/asm/compiler.h
> index 8155db2f7fa1..f99c674b3751 100644
> --- a/arch/arm/include/asm/compiler.h
> +++ b/arch/arm/include/asm/compiler.h
> @@ -9,7 +9,8 @@
>   * will cause compilation to stop on mismatch.
>   * (for details, see gcc PR 15089)
>   */
> -#define __asmeq(x, y)  ".ifnc " x "," y " ; .err ; .endif\n\t"
> +#define __asmeq(x, y)  ".ifnc " x "," y " ; .ifnc " x y ",ipr12 ; " \
> +       ".ifnc " x y ",r12ip ; .err ; .endif ; .endif ; .endif\n\t"
>
>
>  #endif /* __ASM_ARM_COMPILER_H */
> -------->8----------
If that is acceptable, that's fine by me.

In principal none of us *want* to use #ifdefs.

Behan

-- 
Behan Webster
behanw@...verseincode.com

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