Hello, On Mon, 22 Apr 2013 21:57:16 +0200 David Brown <david.br...@hesbynett.no> wrote:
> On 22/04/13 19:32, Paul Sokolovsky wrote: > > Hello, > > > > On Mon, 22 Apr 2013 17:39:27 +0200 > > David Brown <da...@westcontrol.com> wrote: > > > >> Hi, > >> > >> When I compile the code you gave below (using gcc 4.6.3 from > >> 20120406), I get: > >> > >> warning: left shift count >= width of type [enabled by default] > >> > >> That is even without any sort of warning flags - and you should > >> normally enable lots of warnings. > >> > >> So here you have undefined code, and the compiler tells you of the > >> problem. You really cannot get better than that by trying to > >> invent your own ideas about how you want to define this undefined > >> behaviour. > > > > Yeah, but can get better by leveraging generally accepted ideas > > (math def of shift) and ideas put by more than just "you" into > > similar cases (other well known gcc ports, like x86) ;-). > > > > Defining > > (x << n) = (x * 2^(n % 16)) & 0xffff > > is just as good a mathematical definition as your desired > > (x << n) = (x * 2^n) & 0xffff > > (for 16-bit ints) > > > Neither definition is "normal" mathematics. Ok, let's recurse into getting terminology and foundation straight. So, I gather the most correct classification of shift is "arithmetic-logic bitwise operation". And it's defined in terms of iterative algorithm on the value represented as a vector of digits. The algorithm is of course obvious. So, I meant *that* mathematics - which deals with algorithms and stuff, not one dealt with in primary school. Prooflink that the former exists: http://en.wikipedia.org/wiki/Algorithm (though nowadays everyone laughs at wikipedia). > > > And note that gcc on the x86 has exactly the same effect - it's just > that with 32-bit ints, you get it with shifts of 32-bits or more. Exactly same - which? As I propose, yes. It's an easy try, so did you? I quoted http://stackoverflow.com/questions/7214263/unexpected-behavior-of-bitwise-shifting-using-gcc which describes x86 behavior (different compile/runtime, compile matches algo definition), which I verified with few gcc versions before sending the original mail. Granted, I didn't try other targets to see how common one vs another behavior, considering x86 good enough affinity (I'd guess they definitely had this discussion before). > > > > Thanks for all the flame, guys! ;-). > > > > You're welcome :-) It's important to think about these things and > get the correct. Yep, but given that there's on-by-default warning, as you mentioned, it's not *that* important. -- Best regards, Paul mailto:pmis...@gmail.com ------------------------------------------------------------------------------ Precog is a next-generation analytics platform capable of advanced analytics on semi-structured data. The platform includes APIs for building apps and a phenomenal toolset for data science. Developers can use our toolset for easy data analysis & visualization. Get a free account! http://www2.precog.com/precogplatform/slashdotnewsletter _______________________________________________ Mspgcc-users mailing list Mspgcc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mspgcc-users
