Re: [patch] libstdc++/66017 Avoid bad casts and fix alignment of _Rb_tree_node<long long>::_M_storage

[Martin Sebor <msebor at redhat dot com>]

On 05/22/2015 09:13 AM, Jakub Jelinek wrote:
> On Fri, May 22, 2015 at 03:59:47PM +0100, Jonathan Wakely wrote:
> > > > +      alignas(alignof(_Tp2)) unsigned char _M_storage[sizeof(_Tp)];
> > >
> > > Is alignof(_Tp2) always the same as alignof(_Tp2::_M_t) on all targets
> > > (I mean, won't some target align the structure more than its only field)?
> >
> > Hmm, maybe. I don't know.
> >
> > > Wouldn't it be safer to use alignof(_Tp2::_M_t) here?
> >
> > Yes.
> >
> > > Though, apparently that is a GNU extension, so you'd need to use __alignof__
> > > instead.
> >
> > Yes, that's what I did in an earlier version of the patch, so I'll go
> > back to that.
>
> Just grepped around, and e.g. on powerpc64le-linux -std=c++11 -malign-power -O2
> typedef double _Tp;
> struct _Tp2 { _Tp _M_t; };
> extern _Tp2 tp2e;
> int a = alignof(_Tp2);
> int b = __alignof__(_Tp2::_M_t);
> int c = alignof(_Tp);
> int d = __alignof__(tp2e._M_t);
> int e = alignof(_Tp2::_M_t);
>
> we have a = 8, b = 4, c = 8, d = 4, e = 4.

Sorry to divert the discussion from the main topic but the output
above doesn't seem quite correct for this target.

The alignment of double is required to be 8 by the POWER ABIs so
the values of B, D, and E don't match the requirements or the
documentation of -malign-power(*). IBM xlC with the default
-qalign=power setting prints all 8 on POWER running AIX, as it
does with -qalign=natural, and I couldn't get it to match the
GCC result with any combination of alignment options.

Martin

[*] The manual says GCC follows the ABI rules by default, and
that the -malign-power option instructs GCC to follow the ABI-
specified alignment rules.