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

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Message-ID: <CAKv+Gu9fc_F1iC5NAwaG_ed3RRhYnxEBLbuLgW0Oa_qJ96j-gA@mail.gmail.com>
Date:	Wed, 28 Jan 2015 19:17:51 +0000
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Alex Elder <elder@...aro.org>
Cc:	Behan Webster <behanw@...verseincode.com>, 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 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----------
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