Re: [Xen-devel] Ping: [PATCH] x86emul: keep compiler from using {x, y, z}mm registers itself

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 11/06/2017 11:59 AM, Jan Beulich wrote:
>>>> On 16.10.17 at 14:42,  wrote:
>>>>> On 16.10.17 at 14:37, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 16/10/17 13:32, Jan Beulich wrote:
>>>> Since the emulator acts on the live hardware registers, we need to
>>>> prevent the compiler from using them e.g. for inlined memcpy() /
>>>> memset() (as gcc7 does). We can't, however, set this from the command
>>>> line, as otherwise the 64-bit build would face issues with functions
>>>> returning floating point values and being declared in standard headers.
>>>>
>>>> As the pragma isn't available prior to gcc6, we need to invoke it
>>>> conditionally. Luckily up to gcc6 we haven't seen generated code access
>>>> SIMD registers beyond what our asm()s do.
>>>>
>>>> Reported-by: George Dunlap <george.dunlap@xxxxxxxxxx>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>> ---
>>>> While this doesn't affect core functionality, I think it would still be
>>>> nice for it to be allowed in for 4.10.
>>>
>>> Agreed.
>>>
>>> Has this been tested with Clang?
>>
>> Sorry, no - still haven't got around to set up a suitable Clang
>> locally.
>>
>>>  It stands a good chance of being
>>> compatible, but we may need an && !defined(__clang__) included.
>>
>> Should non-gcc silently ignore "#pragma GCC ..." it doesn't
>> recognize, or not define __GNUC__ in the first place if it isn't
>> sufficiently compatible? I.e. if anything I'd expect we need
>> "#elif defined(__clang__)" to achieve the same for Clang by
>> some different pragma (if such exists).
> 
> Not having received any reply so far, I'm wondering whether
> being able to build the test harness with clang is more
> important than for it to work correctly when built with gcc. I
> can't predict when I would get around to set up a suitable
> clang on my dev systems.

I agree with the argument you make above.  On the unlikely chance
there's a problem Travis should catch it, and someone who actually has a
clang setup can help sort it out.

 -George

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