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Message-ID: <1453375937.3681.79.camel@t-online.de>
Date: Thu, 21 Jan 2016 20:32:17 +0900
From: Oleg Endo <oleg.endo@...nline.de>
To: Rich Felker <dalias@...c.org>
Cc: gcc@....gnu.org, musl@...ts.openwall.com
Subject: Re: SH runtime switchable atomics - proposed design
On Wed, 2016-01-20 at 20:22 -0500, Rich Felker wrote:
> On Thu, Jan 21, 2016 at 08:08:18AM +0900, Oleg Endo wrote:
> > On Tue, 2016-01-19 at 15:28 -0500, Rich Felker wrote:
> > > I've been working on the new version of runtime-selected SH
> > > atomics
> > > for musl, and I think what I've got might be appropriate for
> > > GCC's
> > > generated atomics too. I know Oleg was not very excited about
> > > doing
> > > this on the gcc side from a cost/benefit perspective
> >
> > I am just not keen on making this the default atomic model for SH.
> > If you have a system built around this atomic model and want to add
> > it
> > to GCC, please send in patches. Just a few comments below...
>
> OK, thanks for clarifying. I don't have a patch yet but I might do
> one
> later. Sato-san's work on adding direct cas.l support showed me how
> this part of the gcc code seems to work, so it shouldn't be too hard
> to hook it up, but there are ABI design considerations still if we
> decide to go this way.
>
> > > Inputs:
> > > - R0: Memory address to operate on
> > > - R1: Address of implementation function, loaded from a global
> > > - R2: Comparison value
> > > - R3: Value to set on success
> > >
> > > Outputs:
> > > - R3: Old value read, ==R2 iff cas succeeded.
> >
> > > Preserved: R0, R2.
> > >
> > > Clobbered: R1, PR, T.
> >
> > The T bit is obviously the result of the cas operation. So you
> > could
> > use it as an output directly instead of the implicit R3 == R2
> > condition.
>
> I didn't want to impose a requirement that all backends leave the
> result in the T bit. At the C source level, I think most software
> uses
> old==expected as the test for success; this is the API
> __sync_val_compare_and_swap provides, and what people used to x86
> would naturally do anyway.
>
> > > This call (performed from __asm__ for musl, but gcc would do it
> > > as SH
> > > "SFUNC") is highly compact/convenient for inlining because it
> > > avoids
> > > clobbering any of the argument registers that are likely to
> > > already
> > > be
> > > in use by the caller, and it preserves the important values that
> > > are
> > > likely to be reused after the cas operation.
> > >
> > > For J2 and future J4, the function pointer just points to:
> > >
> > > rts
> > > cas.l r2,r3,@r0
> > >
> >
> > > and the only costs vs an inline cas.l are loading the address of
> > > the
> > > function (done in the caller; involves GOT access) and clobbering
> > > R1
> > > and PR.
> > >
> > > This is still a draft design and the version in musl is subject
> > > to
> > > change at any time since it's not a public API/ABI, but I think
> > > it
> > > could turn into something useful to have on the gcc side with a
> > > -matomic-model=libfunc option or similar. Other ABI
> > > considerations
> > > for
> > > gcc use would be where to store the function pointer and how to
> > > initialize it. To be reasonably efficient with FDPIC the caller
> > > needs
> > > to be responsible for loading the function pointer (and it needs
> > > to
> > > always point to code, not a function descriptor) so that the
> > > callee
> > > does not need a GOT pointer passed in.
> >
> > Obviously the ABI has been constructed around the J-core's cas.l
> > instruction.
>
> Yes, but that was a choice I made after a first draft that was no
> more
> optimal for the other backends and less optimal for J-core. And the
> only real choices that were based on the instruction's properties
> were
> using r0 for the address input and swapping the old value into r3
> rather than producing it in a different register. Other than these
> minor details ABI was guided more by avoiding clobbers/reloads of
> potentially valuable data in the caller.
>
> One possible change I just thought of: with one extra instruction in
> the J-core version we could have the result come out in r1 and
> preserve r3. Similar changes to the other versions are probably easy.
>
> > Do you have plans to add other atomic operations (like
> > arithmetic)?
>
> No, at least not in musl. From musl's perspective cas is the main one
> that's used anyway. But even in general I don't think there's a
> significant advantage to doing 'direct' arithmetic ops without a cas
> loop even when you can (with llsc, gusa, or imask model). With gusa
> and imask the only time you benefit from not implementing them in
> terms of cas is on the _highly_ unlucky/unlikely occasion where an
> interrupt occurs between the old-value read before cas and the cas.
> For llsc there's more potential advantage because actual smp
> contention is possible, but sh4a is probably not a very interesting
> target anymore.
>
> > If not, then I'd suggest to name the atomic model
> > "libfunc-musl-cas".
>
> I'm not sure how the "musl" naming here makes sense unless you're
> thinking of having it just call into musl's definitions, which is
> certainly a possible design but not what I had in mind. I was
> thinking
> of adapting the design to gcc and providing something similar via
> libgcc.a.
>
I think it will be easier to discuss this with a patch at hand. Right
now I can't really imagine what exactly you want to put where.
The exact asm code of the sequences/functions and the ABI is up to you.
You will know what works best for your system.
Of course we can add the resulting necessary support functions to
libgcc (which get compiled-in only if the compiler is configured to use
the new atomic model).
Cheers,
Oleg
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