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

From: vladimir.murzin@arm•com (Vladimir Murzin)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled
Date: Thu, 21 Jan 2016 12:32:09 +0000	[thread overview]
Message-ID: <56A0CFC9.1070401@arm.com> (raw)
In-Reply-To: <20151221133730.GP23092@arm.com>

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
> 
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