Re: [PATCH] MdePkg/Include: AARCH64: disable outline atomics on GCC 10.2+

["Ard Biesheuvel" <ard.biesheuvel@arm.com>]

On 5/22/20 2:14 PM, Leif Lindholm wrote:
> On Fri, May 22, 2020 at 14:05:02 +0200, Ard Biesheuvel wrote:
>> On 5/22/20 1:41 PM, Leif Lindholm wrote:
>>> On Fri, May 22, 2020 at 12:12:02 +0200, Ard Biesheuvel wrote:
>>>> GCC 10 enabled a feature by default that was introduced in GCC 9,
>>>> which results in atomic operations to be emitted as function calls
>>>> to intrinsics provided by a runtime library.
>>>>
>>>> Atomics are hardly used in EDK2, which runs on a single CPU anyway,
>>>> and any benefit that would result from reusing library code that
>>>> implements these operations is defeated by the fact that every EDK2
>>>> module will need to have its own copy anyway.
>>>>
>>>> So let's disable this feature on GCC versions that support the
>>>> pragma to do so (GCC 10.2 and up)
>>>>
>>>> Link: https://bugzilla.tianocore.org/show_bug.cgi?id=2723
>>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
>>>>
>>>> ---
>>>> The GCC support for this pragma has already been pulled into the 10.2
>>>> release branch. I think we should consider adding this to the stable
>>>> tag, so that the issue can easily be resolved by upgrading the compiler.
>>>> Whether we add the intrinsics too is a separate matter, but we can
>>>> revisit that later.
>>>
>>> Hmm. I am just slightly concerned over referencing GCC 10.2 before GCC
>>> 10.2 is released.
>>> Presumably, there will be no ill effects if that pragma is enabled also
>>> on 10.2, we just won't need it? If so ...
>>>
>>
>> No the pragma is only enabled on 10.2 and later. Older GCCs will choke on
>> it.
> 
> Urgh, right. I guess I'm having a "can't read" day.
> 
> Still. That means that if the pragma gets pulled last second from
> 10.2, we now explicitly break builds for 10.2.
> No matter how low the risk of that happening, this doesn't make me
> comfortable.
> 

Indeed. So assuming that we won't take the atomics patch for the stable 
tag, we already know it will be broken for all GCC 10 and up.