Re: clang build error on i686

[Richard Henderson <richard.henderson@linaro.org>]

On 7/3/21 10:45 AM, Peter Maydell wrote:
> On Sat, 3 Jul 2021 at 15:37, Cole Robinson <crobinso@redhat.com> wrote:
>>
>> Hi, I'm hitting build errors with clang on i686 userspace on x86_64
>> kernel. Affects both qemu 6.0.0 and qemu.git, tested with fedora
>> clang-12.0.1~rc3-1.fc35.i686.
>>
>> Full build log from the 6.0.0 build:
>> https://gist.githubusercontent.com/crobinso/7b1206044eac7326490b2adce829e861/raw/9dddef968051fd6383ba7adb9e595081ad4f8fa4/gistfile1.txt
>>
>> Lots of errors like:
>>
>> /usr/bin/ld: libqemu-aarch64-softmmu.fa.p/accel_tcg_cputlb.c.o: in
>> function `helper_atomic_cmpxchgq_le_mmu':
>> /builddir/build/BUILD/qemu-6.0.0/accel/tcg/atomic_template.h:86:
>> undefined reference to `__atomic_compare_exchange_8'
>> /usr/bin/ld: libqemu-aarch64-softmmu.fa.p/accel_tcg_cputlb.c.o: in
>> function `helper_atomic_xchgq_le_mmu':
>> /builddir/build/BUILD/qemu-6.0.0/accel/tcg/atomic_template.h:134:
>> undefined reference to `__atomic_exchange_8'
>>
>> Also warnings like:
>>
>> /builddir/build/BUILD/qemu-6.0.0/include/qemu/stats64.h:58:21: warning:
>> misaligned atomic operation may incur significant performance penalty;
>> the expected alignment (8 bytes) exceeds the actual alignment (4 bytes)
>> [-Watomic-alignment]
>>      uint64_t orig = qatomic_read__nocheck(&s->value);
>>                      ^
>> /builddir/build/BUILD/qemu-6.0.0/include/qemu/atomic.h:129:5: note:
>> expanded from macro 'qatomic_read__nocheck'
>>      __atomic_load_n(ptr, __ATOMIC_RELAXED)
> 
> I think at least part of what is happening here is that this compiler/host
> doesn't support native 64-bit atomics, but configure has selected
> CONFIG_ATOMIC64 anyway.

Not true.  The host certainly supports it.

This is a new alignment warning in clang-12 wrt the alignment of the atomic operation. 
Which may be complicated by the fact that the i386 abi does not normally align structures 
beyond 4 bytes.

We may need to disable this warning for i386 (but not x86_64).


r~