Re: [Qemu-trivial] [PATCH] Remove left shifts of negative signed integers

[Paolo Bonzini]

> > GCC correctly puts this warning under -Wextra, and promises not to ever
> > make use of this facet of undefined behavior.  The only correct patch
> > is the one that disables the warning for clang, and possibly adds
> > -fwrapv.  In GCC, -fwrapv correctly silences ubsan's left-shift
> > and signed-overflow warnings.  In Clang, this is reported at
> > https://llvm.org/bugs/show_bug.cgi?id=25552.  It's a heavy hammer
> > but it's the safest options as compiler evolve.
> 
> Where /exactly/ do you propose we canonicalize the clang invocation you
> would like to support?

In configure where we look for other warning flags.  Add -fwrapv and
-Wno-shift-negative-value so that they're enabled if the compilers
supports them.

Paolo