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

[Konstantin Belousov]

On Sat, Oct 10, 2015 at 12:24:33PM +0000, Ed Maste wrote:
> On 10 October 2015 at 12:03, Konstantin Belousov <kostikbel at gmail.com> wrote:
> >> 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.
> 
> I mean that this object is naturally expected to have 8-byte
> alignment.  In previous versions of libc++.so shipped with FreeBSD it
> did, and can't be changed now (cout comes from the binary itself, via
> a copy relocation).
No, the natural alignment for char[] is 1, while ABI requires an array
longer than 15 bytes to have 16-bytes 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 ?
> 
> readelf -s iostream.So:
>    120: 0000000000000150   160 OBJECT  GLOBAL DEFAULT    5 _ZNSt3__14coutE
> 
> libc++.so.1.full
>   1280: 00000000002bb4a0   160 OBJECT  GLOBAL DEFAULT   25 _ZNSt3__14coutE
And there the object has right alignment.