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Message-ID: <CAGWvnymu7X4fZkhkcxtsNZDo1H++VdUQMVD8PwweLrh901XZvQ@mail.gmail.com>
Date: Mon, 6 Dec 2021 20:44:47 -0500
From: David Edelsohn <dje.gcc@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com, Florian Weimer <fweimer@...hat.com>,
Stijn Tintel <stijn@...ux-ipv6.be>
Subject: Re: [PATCH] ppc64: check for AltiVec in setjmp/longjmp
On Mon, Dec 6, 2021 at 8:39 PM Rich Felker <dalias@...c.org> wrote:
>
> On Mon, Dec 06, 2021 at 08:15:48PM -0500, David Edelsohn wrote:
> > On Mon, Dec 6, 2021 at 7:59 PM Rich Felker <dalias@...c.org> 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.
> >
> > bl = branch and link
> > bcl = branch conditional and link
> >
> > link means place the next instruction address in the link register.
> > Normally a branch and link would be used for a matching "return"
> > instruction, but in this case it is being used to compute a position
> > independent code address. As Florian correctly points out, the "bl"
> > will corrupt the link stack in the processor used to predict return
> > addresses and the recommended sequence is the one that he suggests.
> >
> > bcl 20,31,addr
> >
> > which means branch always and, because the condition register bits are
> > irrelevant, a special value that instructs the processor to not push
> > the address onto the link stack so that the "calls" and "returns"
> > remain matched.
>
> Thanks. Am I correct in understanding then that we don't need $+4, but
> can instead use the 1f just as now, with inline .long __hwcap-. -- in
> other words that "bcl 20,31," is a drop-in replacement for "bl"
> without the link stack impact?
It should work, but it's slightly preferred to use $+4 because one
explicitly wants the address of the next instruction and labels of the
form "1f" are not supported by all assemblers.
Thanks, David
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