Why does integer overflow on x86 with GCC cause an infinite loop?

When the standard says it's undefined behavior, it means it. Anything can happen. "Anything" includes "usually integers wrap around, but on occasion weird stuff happens".

Yes, on x86 CPUs, integers usually wrap the way you expect. This is one of those exceptions. The compiler assumes you won't cause undefined behavior, and optimizes away the loop test. If you really want wraparound, pass -fwrapv to g++ or gcc when compiling; this gives you well-defined (twos-complement) overflow semantics, but can hurt performance.

It's simple: Undefined behaviour - especially with optimization (-O2) turned on - means anything can happen.

Your code behaves as (you) expected without the -O2 switch.

It's works quite fine with icl and tcc by the way, but you can't rely on stuff like that...

According to this, gcc optimization actually exploits signed integer overflow. This would mean that the "bug" is by design.

The important thing to note here is that C++ programs are written for the C++ abstract machine (which is usually emulated through hardware instructions). The fact that you are compiling for x86 is totally irrelevant to the fact that this has undefined behaviour.

The compiler is free to use the existence of undefined behaviour to improve its optimisations, (by removing a conditional from a loop, as in this example). There is no guaranteed, or even useful, mapping between C++ level constructs and x86 level machine code constructs apart from the requirement that the machine code will, when executed, produce the result demanded by the C++ abstract machine.

i += i;

// the overflow is undefined.

With -fwrapv it is correct. -fwrapv

Please people, undefined behaviour is exactly that, undefined. It means that anything could happen. In practice (as in this case), the compiler is free to assume it won't be called upon, and do whatever it pleases if that could make the code faster/smaller. What happens with code that should't run is anybody's guess. It will depend on the surrounding code (depending on that, the compiler could well generate different code), variables/constants used, compiler flags, ... Oh, and the compiler could get updated and write the same code differently, or you could get another compiler with a different view on code generation. Or just get a different machine, even another model in the same architecture line could very well have it's own undefined behaviour (look up undefined opcodes, some enterprising programmers found out that on some of those early machines sometimes did do useful stuff...). There is no "the compiler gives a definite behaviour on undefined behaviour". There are areas that are implementation-defined, and there you should be able to count on the compiler behaving consistently.

Even if a compiler were to specify that integer overflow must be considered a "non-critical" form of Undefined Behavior (as defined in Annex L), the result of an integer overflow should, absent a specific platform promise of more specific behavior, be at minimum regarded as a "partially-indeterminate value". Under such rules, adding 1073741824+1073741824 could arbitrarily be regarded as yielding 2147483648 or -2147483648 or any other value which was congruent to 2147483648 mod 4294967296, and values obtained by additions could arbitrarily be regarded as any value which was congruent to 0 mod 4294967296.

Rules allowing overflow to yield "partially-indeterminate values" would be sufficiently well-defined to abide by the letter and spirit of Annex L, but would not prevent a compiler from making the same generally-useful inferences as would be justified if overflows were unconstrained Undefined Behavior. It would prevent a compiler from making some phony "optimizations" whose primary effect in many cases is to require that programmers add extra clutter to the code whose sole purpose is to prevent such "optimizations"; whether that would be a good thing or not depends on one's point of view.