Re: mspgcc "undefines" too-many-bits shifts in weird way
Grant Edwards <[email protected]>
| Newsgroups | gmane.comp.hardware.texas-instruments.msp430.gcc.user |
|---|---|
| Message-ID | <[email protected]> |
On 2013-04-21, Paul Sokolovsky <[email protected]> wrote: > It's all nice and good. But there's difference between "undefined", > "any value" and "weird". No, there isn't. "Undefined" means _exactly_ that: you might get any value -- no matter how "wierd" you think it. > Because "too many bits" shifts may be undefined in C standard, but > shifts by arbitrary number of bits are very well defined in > arithmetic - and by very definition of (unsigned) shift, any value > shifted by more bits than available in its representation is 0. That's not what the C standard says. > That's logical, that's what users know, that's what they expect from > compiler, They're wrong to expect that. If the standard says the result is undefined, then expecting anything in particular is wrong. > So, msp430-gcc just masks out higher bits of shift count, and in this > case leaves original value intact. Which turns term ((1 << BITS) - > 1), which is common to do BITS-modular arithmetic, and would be > expected to just optimize out in case of a full type, into an > expression killer with infinite loops, etc. ensuing. If you write incorrect code, you oughtn't be surprised when it behaves incorrectly. -- Grant Edwards grant.b.edwards Yow! Will this never-ending at series of PLEASURABLE gmail.com EVENTS never cease? ------------------------------------------------------------------------------ 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