Question

In the case of the overflow flag, it would seem that access to this flag would be a great boon to cross-architecture programming. It would provide a safe alternative to relying on undefined behaviour to check for signed integer overflow such as:

if(a < a + 100) //detect overflow

I do understand that there are safe alternatives such as:

if(a > (INT_MAX - 100)) //detected overflow

However, it would seem that access to the status register or the individual flags within it is missing from both the C and C++ languages. Why was this feature not included or what language design decisions were made that prohibited this feature from being included?

Answer 1

• Because C++ is designed as a portable language, i.e. one that compiles on many CPUs (e.g. x86, ARM, LSI-11/2, with devices like Game Boys, Mobile Phones, Freezers, Airplanes, Human Manipulation Chips and Laser Swords).
  • the available flags across CPUs may largely differ
  • even within the same CPU, flags may differ (take x86 scalar vs. vector instructions)
  • some CPUs may not even have the flag you desire at all
• The question has to be answered: Should the compiler always deliver/enable that flag when it can’t determine whether it is used at all?, which does not conform the pay only for what you use unwritten but holy law of both C and C++
• Because compilers would have to be forbidden to optimize and e.g. reorder code to keep those flags valid

Example for the latter:

int x = 7;
x += z;
int y = 2;
y += z;

The optimizer may transform this to that pseudo assembly code:

alloc_stack_frame 2*sizeof(int)
load_int 7, $0
load_int 2, $1
add z, $0
add z, $1

which in turn would be more similar to

int x = 7;
int y = 2;
x += z;
y += z; 

Now if you query registers inbetween

int x = 7;
x += z;
if (check_overflow($0)) {...}
int y = 2;
y += z;

then after optimizing and dissasembling you might end with this:

int x = 7;
int y = 2;
x += z;
y += z;
if (check_overflow($0)) {...}

which is then incorrect.

More examples could be constructed, like what happens with a constant-folding-compile-time-overflow.

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Sidenotes: I remember an old Borland C++ compiler having a small API to read the current CPU registers. However, the argumentation above about optimization still applies.

On another sidenote: To check for overflow:

// desired expression: int z = x + y
would_overflow = x > MAX-y;

more concrete

auto would_overflow = x > std::numeric_limits<int>::max()-y;

or better, less concrete:

auto would_overflow = x > std::numeric_limits<decltype(x+y)>::max()-y;