Re: [Qemu-devel] [PATCH RFC] configure: fix clang failure for libatomic

[Nikunj A Dadhania]

Peter Maydell <address@hidden> writes:

> On 25 April 2017 at 09:35, Nikunj A Dadhania <address@hidden> wrote:
>> Travis builds failure was reported for powernv boot-serial test with
>> qemu built with clang.
>>
>> Debugging revealed that CONFIG_ATOMIC64 wasnt getting set for the clang
>> build because of that atomic operations weren't being used and was
>> resulting in MTTCG failure in the powernv boot-serial test.
>>
>> libatomic is required to successfully test atomic64 and atomic128 for
>> clang. Introduced newer checks for the same. And on failure default to
>> single threaded tcg support in PPC64.
>>
>> Signed-off-by: Nikunj A Dadhania <address@hidden>
>
> Do we really need libatomic?

I was trying out the program in the configure script with clang and I do
get errors without libatomic:

    $ clang /tmp/atomic.c 
    /tmp/atomic.c:6:7: warning: implicit declaration of function 
'__atomic_load_8' is invalid in C99 [-Wimplicit-function-declaration]
      y = __atomic_load_8(&x, 0);
          ^
    /tmp/atomic.c:7:3: warning: implicit declaration of function 
'__atomic_store_8' is invalid in C99 [-Wimplicit-function-declaration]
      __atomic_store_8(&x, y, 0);
      ^
    /tmp/atomic.c:8:3: warning: implicit declaration of function 
'__atomic_compare_exchange_8' is invalid in C99 
[-Wimplicit-function-declaration]
      __atomic_compare_exchange_8(&x, &y, x, 0, 0, 0);
      ^
    /tmp/atomic.c:9:3: warning: implicit declaration of function 
'__atomic_exchange_8' is invalid in C99 [-Wimplicit-function-declaration]
      __atomic_exchange_8(&x, y, 0);
      ^
    /tmp/atomic.c:10:3: warning: implicit declaration of function 
'__atomic_fetch_add_8' is invalid in C99 [-Wimplicit-function-declaration]
      __atomic_fetch_add_8(&x, y, 0);
      ^
    5 warnings generated.
    /tmp/atomic-1660e0.o: In function `main':
    /tmp/atomic.c:(.text+0x28): undefined reference to `__atomic_load_8'
    /tmp/atomic.c:(.text+0x40): undefined reference to `__atomic_store_8'
    /tmp/atomic.c:(.text+0x69): undefined reference to 
`__atomic_compare_exchange_8'
    /tmp/atomic.c:(.text+0x7d): undefined reference to `__atomic_exchange_8'
    /tmp/atomic.c:(.text+0x91): undefined reference to `__atomic_fetch_add_8'
    clang-3.9: error: linker command failed with exit code 1 (use -v to see 
invocation)

With -latomic, the linker succeeds in getting the binary.

> I thought the intention here was that atomic operations were only done
> on types of width of the pointer or less, which should all be doable
> by the compiler inline, and thus don't need the external library.

You are right, even without -latomic in libs_softmmu, tests are passing.
But then __atomic_load_8() isn't used in qemu, if it was using them,
build would fail.

> In the past "doesn't work without libatomic" usually meant
>"accidentally tried to do an atomic operation on a type that is too
>wide".

Regards
Nikunj