On C/C++ undefined behaviours

[Walter Bright]

bearophile wrote:
> Three good blog posts about undefined behaviour in C and C++: 
> http://blog.regehr.org/archives/213 http://blog.regehr.org/archives/226 
> http://blog.regehr.org/archives/232
> 
> In those posts (and elsewhere) the expert author gives several good bites to
> the ass of most compiler writers.
> 
> Among other things in those three posts he talks about two programs as:
> 
> import std.c.stdio: printf; void main() { printf("%d\n", -int.min); }
> 
> import std.stdio: writeln; void main() { enum int N = (1L).sizeof * 8; auto
> max = (1L << (N - 1)) - 1; writeln(max); }
> 
> I believe that D can't be considered a step forward in system language
> programming until it gives a much more serious consideration for
> integer-related overflows (and integer-related undefined behaviour).
> 
> The good thing is that Java is a living example that even if you remove most
> integer-related undefined behaviours your Java code is still able to run as
> fast as C and sometimes faster (on normal desktops).

You're conflating two different things here - undefined behavior and behavior on 
overflow. The Java spec says that integer overflow is ignored, for example.

In C++, overflow behavior is undefined because C++ still supports 
ones-complement arithmetic. Java and D specify integer arithmetic to be 2's 
complement. Java defined left shift to, not surprisingly, match what the x86 CPU 
does. This, of course, will conveniently not result in any penalty on the x86 
for conforming to the spec.