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Re: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled

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  Re: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled Vladimir Murzin <vladimir.murzin@arm.com> - 2016-01-21 13:40 +0100
    Re: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-01-21 15:40 +0100

#1314163 — Re: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled

On 21/12/15 13:37, Will Deacon wrote:
> On Mon, Dec 21, 2015 at 01:58:30PM +0100, Ard Biesheuvel wrote:
>> On 21 December 2015 at 13:51, Will Deacon <will.deacon@arm.com> wrote:
>>> On Mon, Dec 21, 2015 at 01:46:22PM +0100, Ard Biesheuvel wrote:
>>>> On 21 December 2015 at 13:38, Will Deacon <will.deacon@arm.com> wrote:
>>>>> On Fri, Dec 18, 2015 at 08:17:35PM -0800, Andrew Pinski wrote:
>>>>>> The problem here is that GCC 6 and above emits .arch now
>>>>>> for each function so now the global .arch_extension has
>>>>>> no effect.  This fixes the problem by putting
>>>>>> .arch_extension inside ARM64_LSE_ATOMIC_INSN so
>>>>>> it is enabled for each place where LSE is used.
>>>>>
>>>>> Hmm, this is going to affect arch/arm/ much more heavily than arch/arm64.
>>>>> .arch_extension is used for virt, mp and sec over there, and it may be
>>>>> tricky to isolate the actual instruction usage (at least, virt looks
>>>>> lost in kvm/arm.c).
>>>>>
>>>>> Why can't gas have an option to accept all instruction encodings that it
>>>>> knows about, inspite of any .arch directives?
>>>>>
>>>>
>>>> Modern GAS supports things like -march=armv7-a+mp+sec+virt, so it
>>>> probably makes sense to pass that on the command line when building
>>>> for v7 (or +sec only for v6) if the assembler is found to support it
>>>> at build time.
>>>
>>> Does that override a more restrictive .arch directive emitted by the
>>> compiler?
>>>
>>
>> It seems to be additive: -march=armv7-a+mp+sec allows a .S file
>> containing a virt arch_extension + both hvc and smc instructions to be
>> assembled.
> 
> The problem I'm seeing is if I have something like:
> 
>   .arch_extension lse
> 
> before something like:
> 
>   .cpu cortex-a57+fp+simd+crc
> 
> -or-
> 
>   .arch armv8-a+fp+simd+crc
> 
> then I can no longer assemble lse instructions. So the .cpu/.arch
> directive is undoing the .arch_extension. We can fix this by following
> Andrew's suggestion to have .arch_extension before every point of use,
> but the whole thing would be much simpler if we could just tell gas to
> assemble harder.
> 
> Maybe we just need to construct the mother of all -march options based
> on build-time checks in the Makefile?
> 

Since I've been bitten by this I'm curious what was conclusion on this
topic?

Thanks
Vladimir

> Will
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 
> 
> 

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#1314227

On 21 January 2016 at 13:32, Vladimir Murzin <vladimir.murzin@arm.com> wrote:
> On 21/12/15 13:37, Will Deacon wrote:
>> On Mon, Dec 21, 2015 at 01:58:30PM +0100, Ard Biesheuvel wrote:
>>> On 21 December 2015 at 13:51, Will Deacon <will.deacon@arm.com> wrote:
>>>> On Mon, Dec 21, 2015 at 01:46:22PM +0100, Ard Biesheuvel wrote:
>>>>> On 21 December 2015 at 13:38, Will Deacon <will.deacon@arm.com> wrote:
>>>>>> On Fri, Dec 18, 2015 at 08:17:35PM -0800, Andrew Pinski wrote:
>>>>>>> The problem here is that GCC 6 and above emits .arch now
>>>>>>> for each function so now the global .arch_extension has
>>>>>>> no effect.  This fixes the problem by putting
>>>>>>> .arch_extension inside ARM64_LSE_ATOMIC_INSN so
>>>>>>> it is enabled for each place where LSE is used.
>>>>>>
>>>>>> Hmm, this is going to affect arch/arm/ much more heavily than arch/arm64.
>>>>>> .arch_extension is used for virt, mp and sec over there, and it may be
>>>>>> tricky to isolate the actual instruction usage (at least, virt looks
>>>>>> lost in kvm/arm.c).
>>>>>>
>>>>>> Why can't gas have an option to accept all instruction encodings that it
>>>>>> knows about, inspite of any .arch directives?
>>>>>>
>>>>>
>>>>> Modern GAS supports things like -march=armv7-a+mp+sec+virt, so it
>>>>> probably makes sense to pass that on the command line when building
>>>>> for v7 (or +sec only for v6) if the assembler is found to support it
>>>>> at build time.
>>>>
>>>> Does that override a more restrictive .arch directive emitted by the
>>>> compiler?
>>>>
>>>
>>> It seems to be additive: -march=armv7-a+mp+sec allows a .S file
>>> containing a virt arch_extension + both hvc and smc instructions to be
>>> assembled.
>>
>> The problem I'm seeing is if I have something like:
>>
>>   .arch_extension lse
>>
>> before something like:
>>
>>   .cpu cortex-a57+fp+simd+crc
>>
>> -or-
>>
>>   .arch armv8-a+fp+simd+crc
>>
>> then I can no longer assemble lse instructions. So the .cpu/.arch
>> directive is undoing the .arch_extension. We can fix this by following
>> Andrew's suggestion to have .arch_extension before every point of use,
>> but the whole thing would be much simpler if we could just tell gas to
>> assemble harder.
>>
>> Maybe we just need to construct the mother of all -march options based
>> on build-time checks in the Makefile?
>>
>
> Since I've been bitten by this I'm curious what was conclusion on this
> topic?
>

None that I'm aware of

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