svn commit: r289072 - in head/contrib: libc++/src llvm/lib/Transforms/Vectorize

[Konstantin Belousov]

On Sat, Oct 10, 2015 at 11:37:26AM +0000, Ed Maste wrote:
> On 10 October 2015 at 06:08, Konstantin Belousov <kostikbel at gmail.com> wrote:
> > On Fri, Oct 09, 2015 at 06:21:45PM +0000, Dimitry Andric wrote:
> >> Author: dim
> >> Date: Fri Oct  9 18:21:45 2015
> >> New Revision: 289072
> >> URL: https://svnweb.freebsd.org/changeset/base/289072
> >>
> >> Log:
> >>   Temporarily revert upstream llvm trunk r240144 (by Michael Zolotukhin):
> >
> > This might be indeed a clang bug, but probably not in the revision which
> > you reverted.  Or it might be a libc++ bug.
> 
> Yes, the bug is almost certainly not r240144; reverting it is a
> short-term workaround until the underlying issue is fixed.
> 
> > The story, from what I understand from the discussion at some other
> > place, is that the fault occurs on access to the common unaligned
> > symbol. Apparently, ELF does allow to specify alignment of the common
> > symbols, and static linker, when finally allocating .bss space for the
> > object, must obey the requirement. The symbol value for the common
> > symbol must be interpreted as the desired alignment.
> >
> > Could you look up the symbol reference in the .o files (again) and see
> > which alignment is requested ?  I think that it is probably clang which
> > should set it to be at least 16 bytes to generate ABI-compliant code.
> 
> The declaration is extern ostream cout; which correctly has 8 byte alignment.
The alignment of the declaration does not matter, most likely.
I am not sure what you mean by saying that 8 byte alignment is desirable,
while compiler generates accesses (movdqa) which assume 16-byte alignment.

> 
> In the implementation (contrib/libc++/src/iostream.cpp) it is defined
> as a char array which is where the 16-byte ABI alignment would arise,
> but explicitly specifies 8-byte alignment:
> _ALIGNAS_TYPE (ostream)  _LIBCPP_FUNC_VIS char cout[sizeof(ostream)];
> 
> Compiling with -emit-llvm --save-temps shows the desired 8-byte
> alignment for cout in the LLVM IR but 16-byte in the assembly file.
Could somebody show the readelf -s iostream.So and readelf -s libc++.so.X
output lines for the symbol, please ?