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Re: [Xen-devel] [XEN PATCH 8/8] xen: Move GCC_HAS_VISIBILITY_ATTRIBUTE to Kconfig and common

On 12.12.2019 20:04, Andrew Cooper wrote:
> On 12/12/2019 18:27, Anthony PERARD wrote:
>> diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
>> index ff6c0f5cdd18..8c846261d241 100644
>> --- a/xen/include/xen/compiler.h
>> +++ b/xen/include/xen/compiler.h
>> @@ -78,7 +78,7 @@
>>  #define __must_be_array(a) \
>>    BUILD_BUG_ON_ZERO(__builtin_types_compatible_p(typeof(a), typeof(&a[0])))
>>  /* Results in more efficient PIC code (no indirections through GOT or PLT). 
>> */
>>  #pragma GCC visibility push(hidden)
> (I realise we are getting into archaeology, but) Why do we have this as
> a pragma gcc?
> Surely it would be simpler to just feed -fvisibility=hidden into CFLAGS?

No, as per the (admittedly not very explicit) commit message of the
change introducing it. Quoting gcc documentation for making it more

"extern declarations are not affected by ‘-fvisibility’, so a lot of code
 can be recompiled with ‘-fvisibility=hidden’ with no modifications.
 However, this means that calls to extern functions with no explicit
 visibility use the PLT, so it is more effective to use
 __attribute ((visibility)) and/or #pragma GCC visibility to tell the
 compiler which extern declarations should be treated as hidden."


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