Re: [PATCH 0/3] Atomic cleanup + clang-12 build fix

[Richard Henderson]

On 7/12/21 5:37 PM, Richard Henderson wrote:
> On 7/12/21 2:30 PM, Cole Robinson wrote:
> > On 7/12/21 11:59 AM, Richard Henderson wrote:
> > > The first two patches are not strictly required, but they
> > > were useful in tracking down the root problem here.
> > >
> > > I understand the logic behind the clang-12 warning, but I think
> > > it's a clear mistake that it should be enabled by default for a
> > > target where alignment is not enforced by default.
> > >
> > > I found over a dozen places where we would have to manually add
> > > QEMU_ALIGNED(8) to uint64_t declarations in order to suppress
> > > all of the instances.  IMO there's no point fighting this.
> >
> > I tested your patches, they seem to get rid of the warnings. The errors
> > persist.
> >
> > FWIW here's my reproduce starting from fedora 34 x86_64 host:
> >
> > $ sudo mock --root fedora-35-i386 --install dnf --install dnf-utils
> > --install fedora-packager --install clang
> > $ sudo mock --root fedora-35-i386 --shell --enable-network
> > # dnf builddep -y qemu
> > # git clone https://github.com/qemu/qemu
> > # cd qemu
> > # CC=clang CXX=clang++ ./configure --disable-werror
> > # make V=1
>
> Ho hum.  So, the warnings are where clang has decided to insert calls to 
> libatomic.
>
> So we either have to
>
> (1) work around all of the places, which, unless we set up an i386 clang-12 builder will 
> quickly bitrot, or

Update: (1) is out.  There's a warning in cputlb.c vs a pointer that's known to be 
aligned, and it still fires.  I have filed a bug:

  https://bugs.llvm.org/show_bug.cgi?id=51076

> (2) write our own routines, compatible with libatomic, using cmpxchg8b directly.  which 
> requires no (extra) locking, and so is compatible with the tcg jit output, or
>
> (3) file a bug with clang, and document "use clang-11 and not clang-12".

So, Cole, with respect to (3), is this just general regression testing that discovered 
this (in which case, yay) or is there some other reason clang is required?

Assuming that (3) isn't really viable long term, I guess (2) is the only viable 
option.

Thoughts?


r~