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Date: Thu, 29 Jun 2017 22:11:23 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] powerpc64le: Add single instruction math functions
On Thu, Jun 29, 2017 at 09:07:16PM -0400, David Edelsohn wrote:
> On Thu, Jun 29, 2017 at 1:59 PM, Rich Felker <dalias@...c.org> wrote:
> > On Thu, Jun 29, 2017 at 01:00:51PM -0400, David Edelsohn wrote:
> >> >> A colleague of mine reminded me that ELFv2 ABI specifies POWER8 as the
> >> >> minimum hardware (not little-endian).
> >> >
> >> > This is a gratuitous requirement and has nothing to do with the
> >> > meaning of ELFv2 we're using (and likewise not with the gcc
> >> > --with-abi=elfv2).
> >>
> >> 2.1.1. Processor Architecture
> >>
> >> This ABI is predicated on, at a minimum, Power ISA version 2.7 and
> >> contains additional implementation characteristics.
> >
> > Yes, I understand that it's there but this "requirement" is orthogonal
> > to the actual interface boundaries the ABI defines. ARM's EABI has a
> > similar gratuitous baseline of v4t; the compiler can satisfy all the
> > interface boundary requirements even on v4 and probably lower if it
> > wants to, but nobody has implemented that.
> >
> >> >> The implementation of ELFv2 can
> >> >> operate on earlier hardware, but binaries may not be forward
> >> >> compatible because of VSX. Because of the calling convention of VSX
> >> >> registers in ELFv2, the stack may be corrupted if an application built
> >> >> without VSX support is linked with a library that does support VSX.
> >> >> One cannot mix and match musl libc built for POWER4 or PPC970 and musl
> >> >> libc built for POWER7.
> >> >
> >> > I don't think this is accurate. If it is then it's a serious bug we
> >> > need to fix, and it should have been discussed at the time the port
> >> > was added...
> >>
> >> This is not an implementation detail in the library, it is the calling
> >> convention in the compilers.
> >>
> >> >
> >> > Can you provide a citation for the usage of VSX registers in the
> >> > calling convention, and how you think that affects the stack?
> >>
> >> Table 2.22 Vector Register Roles in Section 2.2.1.1 Register Roles.
> >> The definition of volatile and non-volatile registers for vector
> >> registers affects the amount of stack allocated and the saving of
> >> non-volatile registers.
> >
> > Are you saying the caller has to allocate space that the callee might
> > use to preseve call-saved registers it wants to use, and that the
> > amount saved depends on ISA level? If so I'll look for the associated
> > logic in GCC and see what it's doing. If that's the case it should be
> > reserving space regardless of whether it's built for an ISA level with
> > the registers or not.
>
> I thought that there was a change in the volatile status of the VSR
> registers. I'm don't remember if this affected the stack frame or
> stdarg. There was some corner case.
>
> How can the toolchain save space for registers that it doesn't know about?
Normally (on clean ABIs), the caller is not responsible for setting up
room on the stack for the callee to backup call-saved registers it may
want to use. Rather, the callee does this, and therefore it can choose
the exact amount of space for the actual registers it wants to spill.
If the caller is responsible for allocating the stack space into which
the callee might need to spill, this both wastes lots of stack space
(when the callee doesn't actually need to spill much or anything) and
introduces ABI breakage whenever new call-saved registers are added.
I suspect the PPC64 ELFv2 ABI is doing all this right. On page 44 I
found:
"For the purpose of function calls, the right half of VSX registers,
corresponding to the classic floating-point registers (that is, vsr0 -
vsr31), is volatile."
Which suggests they put some attention into not breaking things when
the caller and callee are working with different ISA levels.
Rich
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