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Date: Thu, 16 Jan 2020 14:33:43 -0500
From: Rich Felker <>
Subject: Re: Considering x86-64 fenv.s to C

On Fri, Jan 17, 2020 at 05:56:06AM +1100, Damian McGuckin wrote:
> On Thu, 16 Jan 2020, Rich Felker wrote:
> >As an aside, rather than implementing x86-specific C versions of
> >these, I'd like to end up with a general framework where the arch
> >just exposes a header (fenv_arch.h?) defining some primitives, and
> >the actual fenv function logic is all shared C. I'm not sure what
> >the right generalization is though. It looks like all archs have a
> >get/set exception-flags (status word) operation, but the rest
> >varies a bit.
> I will try and put together a summary of what things do and we can
> go from there. It will need input from somebody with way more
> expertise on ARM and RISC-V (and X87) than I.
> >Would you be interested in assessing what kind of abstraction makes
> >sense here?
> I think the abstraction might need to be consensus. Some of it is
> not even 'fenv' related. For example, some routines only ever return
> zero, while for other FPUs, the same routine can return a (-1) if
> the exception mask
> had an component (bit) which was not a valid exception.

That's one point of having the high level logic in C: not having to
repeat it in each arch where it's potentially subtly wrong. The C code
should check the validity of the mask before attempting to use the
arch primitive to set it.


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