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Re: [x86-64 psABI]: Extend x86-64 psABI to support AVX-512
- From: Rich Felker <dalias at aerifal dot cx>
- To: OndÅej BÃlka <neleai at seznam dot cz>
- Cc: Jakub Jelinek <jakub at redhat dot com>, Richard Biener <richard dot guenther at gmail dot com>, "H.J. Lu" <hjl dot tools at gmail dot com>, GNU C Library <libc-alpha at sourceware dot org>, GCC Development <gcc at gcc dot gnu dot org>, Binutils <binutils at sourceware dot org>, "Girkar, Milind" <milind dot girkar at intel dot com>, "Kreitzer, David L" <david dot l dot kreitzer at intel dot com>
- Date: Thu, 25 Jul 2013 12:50:53 -0400
- Subject: Re: [x86-64 psABI]: Extend x86-64 psABI to support AVX-512
- References: <CAMe9rOrvMxSLj3LcYBs71tVdw6C0vJFKD2HxvnoHc13UamftwA at mail dot gmail dot com> <ddab98c2-bb3b-4d02-b403-e7d5690cfe00 at email dot android dot com> <CAMe9rOpxErCVtE-PDZ3Yb9mL+4E+XQ-are9Df4YBbEioj+MmZA at mail dot gmail dot com> <b9c5d467-834a-4b57-b48c-ac4bb450c9e5 at email dot android dot com> <20130725030655 dot GL14138 at laptop dot redhat dot com> <20130725065538 dot GA18427 at domone dot kolej dot mff dot cuni dot cz>
On Thu, Jul 25, 2013 at 08:55:38AM +0200, OndÅej BÃlka wrote:
> On Thu, Jul 25, 2013 at 05:06:55AM +0200, Jakub Jelinek wrote:
> > On Wed, Jul 24, 2013 at 07:36:31PM +0200, Richard Biener wrote:
> > > >Make them callee saved means we need to change ld.so to
> > > >preserve them and we need to change unwind library to
> > > >support them. It is certainly doable.
> > >
> > > IMHO it was a mistake to not have any callee saved xmm register in the
> > > original abi - we should fix this at this opportunity. Loops with
> > > function calls are not that uncommon.
> >
> > I've raised that earlier already. One issue with that beyond having to
> > teach unwinders about this (dynamic linker if you mean only for the lazy PLT
> > resolving is only a matter of whether the dynamic linker itself has been
> > built with a compiler that would clobber those registers anywhere) is that
> > as history shows, the vector registers keep growing over time.
> > So if we reserve now either 8 or all 16 zmm16 to zmm31 registers as call
> > saved, do we save them as 512 bit registers, or say 1024 bit already?
>
> We shouldn't save them all as we would often need to unnecessarily save
> register in leaf function. I am fine with 8. In practice 4 should be
> enough for most use cases.
You can't add call-saved registers without breaking the ABI, because
they need to be saved in the jmp_buf, which does not have space for
them.
Also, unless you add them at the same time the registers are added to
the machine (so there's no existing code using those registers),
you'll have ABI problems like this: function using the new call-saved
registers calls qsort, which calls application code, which assumes the
registers are call-clobbered and clobbers them; after qsort returns,
the original caller's state is gone.
Adding call-saved registers to an existing psABI is just fundamentally
misguided.
Rich