On Sun, 27 Mar 2016 05:13 pm, Paul Rubin wrote: > Steven D'Aprano <st...@pearwood.info> writes: >> For example, would you consider that this isolated C code is >> "meaningless"? >> int i = n + 1; > > It's meaningful as long as n is in a certain range of values so there's > no overflow. > >> But according to the standard, it's "meaningless", since it might >> overflow, and signed int overflow is Undefined Behaviour. > > No it's not meaningless if it "might" overflow, it's meaningless if it > -does- overflow,
No! That's exactly wrong! Paul, thank you for inadvertently proving the point I am trying to get across. People, even experienced C coders, simply don't understand what the C standard says and what C compilers can and will do. If the C compiler cannot prove that n is strictly less than MAXINT (or is that spelled INT_MAX?), the *entire program* (or at least the bits reachable from this line, in both directions) is Undefined, and the compiler has no obligations at all. You probably don't believe me because this sounds crazy, something that no sane person would design a programming language to behave. Well, yeah, exactly. It does allow C a lot of powerful optimizations, but only at the cost of making it impossible to reason about the behaviour of code that is Undefined. No real compiler is going to intentionally erase your hard disk, but in non-toy code, it can introduce serious bugs even though you explicitly wrote code to avoid the buggy case. But don't believe me. What do I know about C, I don't even know whether to spell the biggest int MAXINT or INT_MAX or MAX_INT. Instead, believe these guys: http://blog.regehr.org/archives/213 http://blog.regehr.org/archives/226 http://blog.regehr.org/archives/232 http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html https://blogs.msdn.microsoft.com/oldnewthing/20140627-00/?p=633/ https://randomascii.wordpress.com/2014/05/19/undefined-behavior-can-format-your-drive/ I've emphasised all the bad things that undefined behaviour causes, but the above (written by C programmers who presumably like C) are much more even-handed, describing the good things that compilers can get out of this. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
