[llvm-dev] cmpxchg on floats

Nicolai Hähnle via llvm-dev llvm-dev at lists.llvm.org
Fri Aug 21 14:51:18 PDT 2020

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


> Joerg
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