On Mon, 23 Sep 2013 20:48:23 +0100 Andrew Haley <aph@xxxxxxxxxx> wrote: > On 09/23/2013 08:38 PM, Dave Allured - NOAA Affiliate wrote: > > I believe the CPU overflow flag is updated after most integer > > arithmetic instructions. Does GCC have any facility for checking > > this flag after each integer operation? This would be a runtime > > check, of course, not a compile time check. > > It wouldn't help with optimized code. GCC reorganizes code, and it > assumes that overflow doesn't happen. GCC inserts some arithmetic > instructions while optimizing and deletes others. So, even if an > overflow happens in your code, it doesn't necessarily happen at > runtime. Could you unpack that a bit? Regardless of optimization, the CPU, not the compiler, executes the ADD or MUL operation, or whatever, and sets or does not set the overflow bit accordingly, right? Why can't the compiler generate code that senses that, and raises a runtime error? It's no different than answering divide-by-zero. I've written a lot of SAFE_CAST macros that check the return of sizeof or strlen(3) before casting it to an int and assigning the result to something that *must* be an int. That code is terribly inefficient, clumsy to read, noise on the screen, really. But made necessary IMO because the compiler conceals what the processor reports. --jkl
