[llvm-dev] cmpxchg on floats

[Nicolai Hähnle via llvm-dev]

On Sun, Aug 23, 2020 at 12:27 AM Joerg Sonnenberger via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> On Sat, Aug 22, 2020 at 10:59:51AM +0200, Nicolai Hähnle wrote:
> > On Sat, Aug 22, 2020 at 2:52 AM Joerg Sonnenberger via llvm-dev
> > <llvm-dev at lists.llvm.org> wrote:
> > >
> > > On Fri, Aug 21, 2020 at 11:51:18PM +0200, Nicolai Hähnle wrote:
> > > > On Tue, Aug 18, 2020 at 1:27 AM Joerg Sonnenberger via llvm-dev
> > > > <llvm-dev at lists.llvm.org> wrote:
> > > > > On Fri, Aug 14, 2020 at 10:42:02AM -0700, JF Bastien via llvm-dev wrote:
> > > > > > We (C, C++, and LLVM) are generally moving towards supporting FP as a
> > > > > > first-class thing with all atomic operations †, including cmpxchg. It’s
> > > > > > indeed *usually* specified as a bitwise comparison, not a floating-point
> > > > > > one, although IIRC AMD has an FP cmpxchg. Similarly, some of the
> > > > > > operations are allowed to have separate FP state (say, atomic add won’t
> > > > > > necessarily affect the scalar FP execution’s exception state, might
> > > > > > have a different rounding mode, etc).
> > > > >
> > > > > We don't really FP cmpxchg in hardware to implement it, do we? It can be
> > > > > lowered as load, FP compare, if not equal cmpxchg load?
> > > >
> > > > Two points here:
> > > >
> > > > 1. Hardware with native fcmpxchg already exists.
> > > > 2. It's incorrect even if I replace your "if not equal" by "if equal"
> > > > (which I assume is what you meant).
> > > >
> > > > On the latter, assume your float in memory is initially -0.0, thread 1
> > > > does cmpxchg(-0.0, +0.0) and thread 2 does fcmpxchg(+0.0, 1.0). The
> > > > memory location is guaranteed to be 1.0 after both threads have run,
> > > > but this is no longer true with your replacement, because the
> > > > following ordering of operations is possible:
> > > >
> > > > - Thread 2 loads -0.0, compares to +0.0 => comparison is equal
> > > > - Thread 1 does cmpxchg, memory value is now changed to +0.0
> > > > - Thread 2 does cmpxchg(-0.0, 1.0) now, testing whether the memory
> > > > location is unchanged --> this fails, so the memory location stays
> > > > +0.0
> > >
> > > Thread 2 does the cmpxchg with the loaded value, not the value it is
> > > tested for. So thread 2 would be using +0.0 as well.
> >
> > Please re-read the sequence of events carefully. Thread 2 did read
> > -0.0, so that's the value it's using.
>
> Right and so it can fail as a weak cmpxchg does. It will pick up the
> correct value in the next iteration. This is not really that different
> from spurious failures of LL/SC etc.

Okay, I see what you mean. cmpxchg isn't weak by default though, so my
assumption is that a potential fcmpxchg would be the same. Can we
agree that your translation is correct for `fcmpxchg weak`, but not
for default `fcmpxchg`? You can make it correct by wrapping it in a
retry loop, at the cost of reducing performance even further compared
to a native fcmpxchg.

Cheers,
Nicolai


>
> Joerg
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