Re: [PATCH v2] add powerpc64 port

[Rich Felker <dalias@libc.org>]

On Thu, Apr 14, 2016 at 03:01:38AM -0500, Bobby Bingham wrote:
> On Thu, Apr 14, 2016 at 01:05:07AM +0200, Szabolcs Nagy wrote:
> > * Bobby Bingham <koorogi@koorogi.info> [2016-04-04 00:26:11 -0500]:
> > > +++ b/arch/powerpc64/bits/setjmp.h
> > > @@ -0,0 +1 @@
> > > +typedef unsigned long long __jmp_buf[66];
> > 
> > hm glibc seems to use long[64] with 16byte alignment,
> > is the size diff because of alignment?
> 
> Yes.  Though apparently the glibc setjmp asm has code to detect a
> misaligned jmp_buf, but its handling of that case ends up overflowing
> the jmp_buf.
> 
> I can make some changes to get our jmp_buf down to 65, but the only ways
> to get it down to 64 are either with 16 byte alignment, or to have setjmp
> spill vector registers to the stack first so it can copy them from there
> to the jmp_buf through a gpr.
> 
> How important is it to match glibc here?

I think you could avoid the need for alignment or increased buffer
size by positioning the vector registers at
jmp_buf_end-vector_save_size rounded _down_ to alignment, then
positioning the grps around them (so, putting the last gpr at the end
rather than before the vectors if the buffer as a whole is
misaligned).

But it might be preferable to have the alignment match ABI too. Is
there any way it can be achieved with just things guaranteed to exist
by the psABI (is __int128 required by the psABI?) or does it require
C11 and/or GNUC attributes to get 16-byte alignment?

Rich