Re: Ada front-end depends on signed overflow

[Paul Schlie <schlie at comcast dot net>]

> From: Andrew Pinski <pinskia@physics.uc.edu>
>>> No they should be using -ftrapv instead which traps on overflow and then
>>> make sure they are not trapping when testing.
>> 
>> - why? what language or who's code/target ever expects such a behavior?
> Everyone's who writes C/C++ should know that overflow of signed is undefined.
> 
> Now in Java it is defined, which is the reason why -fwrapv exists in the
> place since GCC has a "Java" compiler.
> 
> I think you need to go back in the archives and read the disscusions about
> when -fwrapv was added and see why it is not turned on by default for C.
> http://gcc.gnu.org/ml/gcc-patches/2003-05/msg00850.html
> http://gcc.gnu.org/ml/gcc-patches/2003-03/msg02126.html
> http://gcc.gnu.org/ml/gcc-patches/2003-03/msg01727.html

Thank again, upon fully reviewing the threads I still conclude:

- C/C++ defines integer overflow as undefined because it's a target
  specific behavior, just as dereferencing a NULL is (although a large
  majority of targets factually do wrap overflow, and don't terminally
  trap NULL dereferences; so GCC's got it backwards in both cases).

- So technically as such semantics are undefined, attempting to track
  and identify such ambiguities is helpful; however the compiler should
  always optimize based on the true semantics of the target, which is
  what the undefined semantics truly enable (as pretending a target's
  semantics are different than the optimization assumptions, or forcing
  post-fact run-time trapping semantics, are both useless and potentially
  worse, inefficient and/or erroneous otherwise).