Date: Mon, 6 Dec 2021 20:39:32 -0500 From: Rich Felker <dalias@...c.org> To: David Edelsohn <dje.gcc@...il.com> 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 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? Rich
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