Need for __builtin_issignaling()

Need for __builtin_issignaling()

[FX]

Hi,

In order to finalize our support for Fortran IEEE modules, we need to have access to a reliable issignaling() macro, that would work across all floating-point types. Some targets (glibc ones, notably) have it, but not all, far from it. Instead of implementing our own, probably buggy, probably not very portable version… I think it would be a great idea for a GCC builtin: __builtin_issignaling()

As far as I can see, this macro was proposed in the “ Optional support for Signaling NaNs” WG14 document (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1011.htm#7.12.3.x%20the%20issignaling%20macro). Many other proposed macros from this document are available as GCC builtins, so I wonder: why not __builtin_issignaling()?

Is it particularly hard to do? I came across a post in the list archives from Joseph, who said it would be good to have. I’d be willing to try and put something together, unless you think it’s a big project. Any pointers as to how to start would be appreciated.

Best,
FX

Re: Need for __builtin_issignaling()

[Joseph Myers]

On Wed, 29 Dec 2021, FX via Gcc wrote:

> Is it particularly hard to do? I came across a post in the list archives 
> from Joseph, who said it would be good to have. I’d be willing to try 
> and put something together, unless you think it’s a big project. Any 
> pointers as to how to start would be appreciated.

Start by looking at Tamar Christina's patch for bugs 77925, 77926, 66462, 
which got reverted because of problems it caused.  In particular, see my 
comment 18 in bug 66462 pointing to some of the reports of issues, and 
look at the relevant discussion in June 2017.  I think the following 
comment 19 is incorrect (when it refers to June and November, those are 
June 2017 and November 2016, so the November version is an *older* one, 
but that comments is under the apprehension that it was a newer one).

There's no need to use that patch as a starting point, but it may well be 
helpful to do so, or at least to get ideas from it.  It didn't add 
__builtin_issignaling, but did add implementations of other related 
built-in functions based on bit-manipulation, and __builtin_issignaling 
would need to be implemented based on such bit-manipulation.

Apart from avoiding the bugs in that patch, for __builtin_issignaling 
there isn't any other implementation approach to fall back on, and there 
isn't any defined external-linkage function to fall back on either.  So 
it's strongly desirable to have a built-in function that works (is 
expanded inline) for *all* floating-point formats supported by GCC, not 
just some.  (For formats not supporting signaling NaNs, it can trivially 
return 0 after evaluating the argument for its side effects.)  Note that 
supporting all formats includes working for formats where integer 
arithmetic on a same-size integer isn't supported (TFmode on 32-bit 
architectures where there isn't TImode integer arithmetic, in particular), 
so you need to be careful about working correctly in that case.

-- 
Joseph S. Myers
joseph@codesourcery.com