On Thu, Oct 31, 2019 at 1:51 PM Hans Åberg <haber...@telia.com> wrote:
> > > On 31 Oct 2019, at 21:40, David Blaikie <dblai...@gmail.com> wrote: > > > >> On Thu, Oct 31, 2019 at 12:00 PM Hans Åberg <haber...@telia.com> wrote: > >> > >> > On 31 Oct 2019, at 18:40, David Blaikie <dblai...@gmail.com> wrote: > >> > > >> >> Right, but that is something one would avoid when computing > arithmetical results. > >> > > >> > One would try to, yes - but that's sort of what the whole discussion > is resolving around: Is the code correct? I don't know. I wouldn't assume > it is (I'm not assuming it isn't either) - but without a reduced test case > that gets to the root of the difference in behavior, we don't know if the > code is correct. > >> > >> Nor whether it is a compiler bug. > > > > Indeed - but you can imagine that, on average (just due to there being > way more code compiled by the compiler, than the code of the compiler > itself) the bug is in external code, not the compiler. > > GMP is not the average program, though. > > > Such that it's not practical for the compiler developers to do all the > leg work of investigating 3rd party code bugs to determine if it's a bug in > the compiler. It doesn't scale/we wouldn't have any time to work on the > compiler & most of the time we'd be finding user bugs, not compiler bugs. > > The GMP developers feel exactly the same, dropping Clang support. It is > mostly a problem for MacOS users that do not have access to GCC. > Yep, that's certainly their call - there's a cost to maintaining compatibility with each compiler/toolchain/platform, etc. If you have a personal interest in GMP on MacOS, then perhaps the cost falls to you, if you're willing to pay it, to investigate this sort of thing & help support this particular library+compiler combination, if it's worth your time to do so. > > Apologies for the snark in the title of this article, but it covers some > of the ideas: > https://blog.codinghorror.com/the-first-rule-of-programming-its-always-your-fault/ > & other articles around discuss similar ideas. > > This article is pretty naive: Yes, it is a good starting point to check > ones own code first, but eventually one learns to identify compiler bugs as > well. It is very time consuming, though. > Certainly - which is why it's not practical for compiler engineers to be spending all that time on everyone's bugs, right? > > Yes, there are compiler bugs - but you've sort of got to continue under > the assumption that that's not the issue until you've got some fairly > compelling evidence of one (very narrow test case where you can look at all > the code & visually inspect/discuss/reason about its standards conformance > - currently "all of GMP" is too big to apply that level of scrutiny). > > GMP is indeed very complex, not only from a programming point of view, but > also the underlying algorithms. > Yep - which makes it all the harder for me or someone else on the LLVM project to likely be able to find any potential compiler bugs in it. - Dave
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