Re: kyua SIGBUS'ing with clang 3.7.0 (was FreeBSD_HEAD-tests - Build #1540 - Unstable)

On Fri, Oct 9, 2015 at 11:37 AM, Dimitry Andric <dim_at_freebsd.org> wrote:

>
> I have committed a (hopefully temporary) workaround in r289072.  If you
> don't want to do a full buildworld, first rebuild clang and install it,
> then rebuild libc++ and install it.
>

After some back-and-forth about the cause, I am now reasonably convinced
> that it is due to a bug in clang 3.7.0, as I noted in the commit message:
>
> Some further explanation: libc++'s iostream.cpp contains the definitions
> of std::cout, std::cerr and so on.  These global objects are effectively
> declared with an alignment of 8 bytes.  When an executable is linked
> against libc++.so, it can sometimes get a copy of the global object,
> which is then at the same alignment.
>
> However, with clang 3.7.0, the initialization of these global objects
> will incorrectly use SSE instructions (e.g. movdqa), whenever the
> optimization level is high enough, and SSE is enabled, such as on amd64.
> When any of these objects is not aligned to 16 bytes, this will result
> in a SIGBUS during iostream initialization.  In contrast, clang 3.6.x
> and earlier took the 8 byte alignment into consideration, and avoided
> SSE for those particular operations.
>
> After bisecting of upstream changes, I found that the above revision
> [llvm r240144] caused the change of this behavior, so I am reverting it
> now as a workaround, while a discussion and test case is being prepared
> for upstream.
>
> -Dimitry
>
>

Thank you for analyzing and fixing the problem.  You have excellent
detective
skills and worked hard to fix this!

--
Craig