On Sat, Oct 10, 2015 at 12:24:33PM +0000, Ed Maste wrote: > On 10 October 2015 at 12:03, Konstantin Belousov <kostik...@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. _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
