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Re: Optimisations and undefined behaviour
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: gcc-help at gcc dot gnu dot org
- Date: Sun, 8 Nov 2015 18:49:46 -0600
- Subject: Re: Optimisations and undefined behaviour
- Authentication-results: sourceware.org; auth=none
- References: <563BC190 dot 7080406 at hesbynett dot no> <563C7EB6 dot 9050401 at redhat dot com> <563C9DD3 dot 9030407 at hesbynett dot no> <563F9E4C dot 5000504 at redhat dot com> <563FA2DF dot 4050008 at redhat dot com> <alpine dot DEB dot 2 dot 20 dot 1511082042570 dot 2160 at laptop-mg dot saclay dot inria dot fr> <20151109002734 dot GD5808 at zira dot vinc17 dot org>
On Mon, Nov 09, 2015 at 01:27:34AM +0100, Vincent Lefevre wrote:
> > Why not simply:
> > unsigned y = x;
> > return y*y*y;
> > ? This is an example where defined behavior is so easy to get...
>
> But what if the result of y*y*y (an unsigned int) does not fit in
> an int?
That is implementation-defined. not undefined (6.3.1.3); GCC documents
it like this:
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.
Segher