On 28 January 2015 at 17:20, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote:
> On 28 January 2015 at 17:08, Alex Elder <el...@linaro.org> wrote:
>> On 01/28/2015 10:17 AM, Ard Biesheuvel wrote:
>>> On 28 January 2015 at 14:11, Alex Elder <el...@linaro.org> wrote:
>>>> On 01/28/2015 05:15 AM, Ard Biesheuvel wrote:
>>>>> On 28 January 2015 at 05:18, Behan Webster <beh...@converseincode.com>
>>>>> wrote:
>>>>>> From: Alex Elder <el...@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 <el...@linaro.org>
>>>>>> Signed-off-by: Behan Webster <beh...@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?
-------->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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