Re: [musl] [PATCH] ppc64: check for AltiVec in setjmp/longjmp

[Rich Felker <dalias@libc.org>]

On Wed, Dec 08, 2021 at 08:37:13AM -0500, Rich Felker wrote:
> On Wed, Dec 08, 2021 at 10:43:05AM +0200, Stijn Tintel wrote:
> > On 7/12/2021 02:59, Rich Felker wrote:
> > > On Tue, Dec 07, 2021 at 01:37:12AM +0100, Florian Weimer wrote:
> > >> * Stijn Tintel:
> > >>
> > >>> diff --git a/src/setjmp/powerpc64/setjmp.s b/src/setjmp/powerpc64/setjmp.s
> > >>> index 37683fda..32853693 100644
> > >>> --- a/src/setjmp/powerpc64/setjmp.s
> > >>> +++ b/src/setjmp/powerpc64/setjmp.s
> > >>> @@ -69,7 +69,17 @@ __setjmp_toc:
> > >>>  	stfd 30, 38*8(3)
> > >>>  	stfd 31, 39*8(3)
> > >>>  
> > >>> -	# 5) store vector registers v20-v31
> > >>> +	# 5) store vector registers v20-v31 if hardware supports AltiVec
> > >>> +	mflr 0
> > >>> +	bl 1f
> > >>> +	.hidden __hwcap
> > >>> +	.long __hwcap-.
> > >>> +1:	mflr 4
> > >> This de-balances the return stack and probably has quite severe
> > >> performance impact.  The ISA manual says to use
> > >>
> > >>   bcl 20,31,$+4
> > >>
> > >> and you'll have to store the __hwcap offset somewhere else.
> > > To begin with, let's change the .s files to .S files and put the whole
> > > branch logic inside #ifndef __ALTIVEC__ so that it does not impact
> > > normal builds with an ISA level where Altivec can be assumed to be
> > > present.
> > >
> > > I'm not sufficiently familiar with the PowerPC ISA to know how bcl
> > > works, but if there's a less expensive solution along those lines
> > > that's compatible with all ISA levels, by all means let's use it. The
> > > same could be done for powerpc-sf (32-bit) and its SPE branches, too.
> > >
> > > Also the add and lwz can be used into lwzx (indexed load).
> > >
> > The code for ppc64 uses ld after add, not lwz. This is required to make
> > it work on both big and little endian systems. We therefore cannot use
> > lwzx, but have to use ldx.
> 
> OK, I don't understand why endianness would matter, but I do see a
> problem here: ld expects to load a 64-bit value, but the value is only
> 32-bit (.long). Unless I'm missing something, we need to either make
> it 64-bit (.llong, and with proper alignment) or use a sign-extending
> 32-bit load. The latter would assume a model where the whole program
> (for static linking) or libc.so (for dynamic) fits in ±2GB. This is
> clearly valid for dynamic but dubious for static (although maybe GCC
> already assumes this with how it loads the GOT address and DSO-local
> globals?).

OK, I see now -- I was mixing up the load of __hwcap (which is
necessarily 64-bit) and the load of the relative address constant
(which could be done either way). The comment about lwzx not being
appropriate was just because __hwcap is 64-bit not 32-bit, and that's
fixed by using ldx. But I'm still unclear on whether we should use a
full 64-bit relative address constant or a 32-bit one like we've been
using (which assumes everything in ±2GB).

Rich