[Bug other/60843] Documentation: 4.5 Integers/C99 6.3.1.3 ("reduce modulo 2^N")

["joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60843

--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Tue, 15 Apr 2014, kdevel at vogtner dot de wrote:

> For conversion to a type of width N, the value is reduced modulo 2^N to be
> within range of the type; no signal is raised."
> 
> Reduce A modulo M usually means find the least integer R in the range [0, M -
> 1] such that A is congruent R modulo M. But this is not what gcc compiled

I don't see the problem.  It explicitly says "reduced modulo 2^N to be 
within range of the type", and that unambiguously defines the result 
value, as there is exactly one result within the range of the type that is 
equal to the input, modulo 2^N.  The qualifier "to be within range of the 
type" says that [0, M - 1] is irrelevant, because that isn't the range of 
the type in question.

This is the normal form of modulo arithmetic for signed types, as used for 
example by -fwrapv (and defined in detail in the first (1994) edition of 
LIA-1; the second (2012) edition removes the modulo operations, but the 
underlying wrap_I operation is in LIA-2).