From: Alex Elder <elder@linaro.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Behan Webster <behanw@converseincode.com>,
bcm@fixthebug.org, Florian Fainelli <f.fainelli@gmail.com>,
Russell King - ARM Linux <linux@arm.linux.org.uk>,
Matt Porter <mporter@linaro.org>,
bcm-kernel-feedback-list@broadcom.com,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] bcm: address clang inline asm incompatibility
Date: Wed, 28 Jan 2015 13:27:57 -0600 [thread overview]
Message-ID: <54C9383D.10901@linaro.org> (raw)
In-Reply-To: <CAKv+Gu9fc_F1iC5NAwaG_ed3RRhYnxEBLbuLgW0Oa_qJ96j-gA@mail.gmail.com>
On 01/28/2015 01:17 PM, Ard Biesheuvel wrote:
> On 28 January 2015 at 17:20, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>> On 28 January 2015 at 17:08, Alex Elder <elder@linaro.org> wrote:
>>> On 01/28/2015 10:17 AM, Ard Biesheuvel wrote:
>>>> On 28 January 2015 at 14:11, Alex Elder <elder@linaro.org> wrote:
>>>>> On 01/28/2015 05:15 AM, Ard Biesheuvel wrote:
>>>>>> On 28 January 2015 at 05:18, Behan Webster <behanw@converseincode.com> wrote:
>>>>>>> From: Alex Elder <elder@linaro.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@linaro.org>
>>>>>>> Signed-off-by: Behan Webster <behanw@converseincode.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?
-Alex
> -------->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----------
>
next prev parent reply other threads:[~2015-01-28 20:36 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-28 5:18 [PATCH] bcm: address clang inline asm incompatibility Behan Webster
2015-01-28 11:15 ` Ard Biesheuvel
2015-01-28 14:11 ` Alex Elder
2015-01-28 16:17 ` Ard Biesheuvel
2015-01-28 17:08 ` Alex Elder
2015-01-28 17:20 ` Ard Biesheuvel
2015-01-28 19:17 ` Ard Biesheuvel
2015-01-28 19:27 ` Alex Elder [this message]
2015-01-28 19:38 ` Ard Biesheuvel
2015-01-28 20:11 ` Ard Biesheuvel
2015-01-28 20:15 ` Alex Elder
2015-01-28 21:18 ` Behan Webster
2015-01-28 21:07 ` Behan Webster
2015-01-28 19:30 ` Behan Webster
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=54C9383D.10901@linaro.org \
--to=elder@linaro.org \
--cc=ard.biesheuvel@linaro.org \
--cc=bcm-kernel-feedback-list@broadcom.com \
--cc=bcm@fixthebug.org \
--cc=behanw@converseincode.com \
--cc=f.fainelli@gmail.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux@arm.linux.org.uk \
--cc=mporter@linaro.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).