Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #60629

Re: Negative shift counts

Show all headers | View raw

Noob <root@127.0.0.1> writes:

> I am aware that negative shift counts have undefined behavior.
>
>   int i = 40 << -2; // UB
>
> It seems to me that a natural way to extend shift counts to
> negative values would be to just flip the shift direction.
>
> The above statement would behave "as if" we'd written
>
>   int i = 40 >> 2;
>
>
> I suppose negative shift counts were unsupported in the original
> language definition (?) in the 70s and remained unsupported in the
> first edition of K&R C? Perhaps this was done to save space when
> RAM was tight, and such a "feature" was not considered to bring
> any value?

The original view of C was that it should not offer very much over and
above what the machine offers.  The structuring (blocks, ifs, loops
etc.) is no more than thin syntax over tests and jumps.  Functions in C
can't be nested because most machines offer little more than stack
pointer relative addressing.

A higher-level shift would require (on some systems) code to do checks
every time, except in those probably rare cases when the compiler can
optimise 40 << -2 into 40 >> 2.  That's the sort of universal cost that
C tries to avoid.

> Then ANSI saw no benefit to discuss defining the behavior during
> the standardization process in the 80s? Because no implementation
> offered this as an extension?
>
> I'm perplexed, because it seems to be a trivial extension, with
> low complexity, for a small gain in expressiveness.

It certainly simple (low complexity) but it's not zero-cost, and the
cost has to be set against with the ease with which an inline function
can be written to do the job.

<snip>
-- 
Ben.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Negative shift counts Noob <root@127.0.0.1> - 2015-04-21 11:33 +0200
  Re: Negative shift counts David Brown <david.brown@hesbynett.no> - 2015-04-21 12:22 +0200
    Re: Negative shift counts Noob <root@127.0.0.1> - 2015-04-21 13:12 +0200
  Re: Negative shift counts Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-04-21 12:02 +0100
  Re: Negative shift counts glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-04-29 20:08 +0000

csiph-web