Re: [FFmpeg-devel] [PATCH] configure: link to libatomic when it's present

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

James Almer:
> 
> 
> On 1/19/2022 8:54 AM, Anton Khirnov wrote:
>> C11 atomics in some configurations (e.g. 64bit operations on ppc64 with
>> GCC) require linking to libatomic.
>> ---
>> Testing welcome, especially in configurations where
>> * libatomic is not present
>> * libatomic is actually needed
>> ---
>>   configure | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/configure b/configure
>> index 1413122d87..1ff5dbee5b 100755
>> --- a/configure
>> +++ b/configure
>> @@ -6324,7 +6324,14 @@ check_headers asm/types.h
>>   # it seems there are versions of clang in some distros that try to
>> use the
>>   # gcc headers, which explodes for stdatomic
>>   # so we also check that atomics actually work here
>> -check_builtin stdatomic stdatomic.h "atomic_int foo, bar =
>> ATOMIC_VAR_INIT(-1); atomic_store(&foo, 0); foo += bar"
>> +#
>> +# some configurations also require linking to libatomic, so try
>> +# both with -latomic and without
>> +for LATOMIC in "-latomic" ""; do
> 
> Shouldn't you try without it first? On my toolchain libatomic is
> present, but libraries compile without linking to it just fine. That
> changes after this patch, where it starts linking to it explicitly.
> 

This would work if this test checked for all the atomic operations that
might be needed; but it doesn't: It checks just for atomic increment and
only for atomic_int. What is if atomic_increment can be done without
recourse to libatomic because of hardware support whereas another atomic
operation needs libatomic?

>> +    check_builtin stdatomic
>> stdatomic.h                                                 \
>> +        "atomic_int foo, bar = ATOMIC_VAR_INIT(-1);
>> atomic_store(&foo, 0); foo += bar"  \
>> +        $LATOMIC && add_extralibs $LATOMIC && break
> 
> You should probably add it to the required libraries' extralibs only.
> Just replace the add_extralibs part with setting stdatomic_extralibs to
> $LATOMIC, and then add stdatomic to all the libraries' _suggest lists,
> same as we do for libm.
> 
>> +done
>>     check_lib advapi32 "windows.h"            RegCloseKey         
>> -ladvapi32
>>   check_lib bcrypt   "windows.h bcrypt.h"   BCryptGenRandom     
>> -lbcrypt &&
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