What to do: clang-4.0 fastdebug assertion failure in os_linux_x86:os::verify_stack_alignment()

[Christian Thalinger]

> On Jul 11, 2018, at 10:56 AM, Martin Buchholz <martinrb at google.com> wrote:
> 
> 
> 
> On Wed, Jul 11, 2018 at 7:34 AM, Christian Thalinger <cthalinger at twitter.com <mailto:cthalinger at twitter.com>> wrote:
> 
> > - why does stack alignment even matter? Isn't it the alignment of c++
> > objects on the stack that matter?
> 
> C++ compilers can (and actually do) emit instructions that need alignment, like movaps.  I’ve seen many crashes in the past with movaps and an unaligned stack coming from JIT or stub code.
> 
> Sure, actual instructions need alignment.  But os::verify_stack_alignment isn't doing a good job of finding misaligned instructions, while causing trouble for clang builds.  Christian, what do you suggest we do to fix the failing assertion in os::verify_stack_alignment ?  
> 
> I don't know what movaps does, but perhaps os::verify_stack_alignment could simply use that instruction via inline asm?

movaps is used in the prologue to spill to the stack.  If the stack isn’t 16-byte aligned it crashes.  os::verify_stack_alignment just makes it easier to find these bugs.

I’m not exactly sure what to do.  I think the best solution is to avoid that os::verify_stack_alignment is being inlined.