On 22/04/13 22:49, Paul Sokolovsky wrote: > 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). >
"Shift" does not have a "correct classification" or other "officially correct" mathematical definition. It is not one of the standard operations defined on integers. So it does what you define it to do in the context. Both the definitions above are perfectly good definitions for the workings of a "shift" operation on modular integers of 16-bit. Both have their uses, both are natural implementations in software or hardware. >> >> >> 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). I get the same "mask the shift value before shifting" effect with gcc on x86 and x86_64. But the compiler is calculating "(1 << 32)" as 0 rather than 1. This is an inconsistency in the compiler, and arguably it is /this/ target that is flawed, not the msp430 target. However, since (as we already know) such shifts are undefined, the compiler is free to produce different values in different circumstances. > >> >> >>> 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. > Still, the warning will only be triggered when the compiler can see the shift count at compile-time. ------------------------------------------------------------------------------ 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
