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 jos...@codesourcery.com