[ummonk]

No, optimizing compilers don't translate overflow to platform-specific behavior for signed integers - since it's UB they'll freely make arithmetic or logic assumptions that can result in behavior that can't really be humanly predicted without examining the generated machine code.

[uecker]

They are free to but not required. You can pick a different compiler, or you can configure your compiler to something else, if it provides such options.

I always found it surprising that people did not reject clang for aggressively optimizing based on UB, but instead complained about the language while still using clang with -O3.

[ummonk]

Programmers don’t have much choice, since most compilers don’t really provide an option / optimization level that results in sane behavior for common UB footguns while providing reasonable levels of performance optimization.

The one exception I know of is CompCert but it comes with a non-free license.

I definitely do think the language committee should have constrained UB more to prevent standards-compliant compilers from generating code that completely breaks the expectations of even experienced programmers. Instead the language committees went the opposite route, removing C89/90 wording from subsequent standards that would have limited what compilers can do for UB.

[uecker]

The C89/C90 wording change story is a myth. And I am not sure I understand your point about CompCert. The correctness proof of CompCert covers programs that have no UB. And programmers do have some choice and also some voice. But I do not see them pushing for changes a lot.

[ummonk]

The C standard changed its note about "permissible" undefined behavior (in C89/90) to "possible" undefined behavior (from C99 onwards). This is easily verifiable and not a myth.

As to CompCert, I'm not sure if they still make guarantees about type punning, but the older paper had said this:

"The semantics of Clight is formally defined in big-step operational style. The semantics is deterministic and makes precise a number of behaviors left unspecified or undefined in the ISO C standard, such as the sizes of data types, the results of signed arithmetic operations in case of overflow, and the evaluation order. Other undefined C behaviors are consistently turned into “going wrong” behaviors, such as dereferencing the null pointer or accessing arrays out of bounds. Memory is modeled as a collection of disjoint blocks, each block being accessed through byte offsets; pointer values are pairs of a block identifier and a byte offset. This way, pointer arithmetic is modeled accurately, even in the presence of casts between incompatible pointer types"

[pjmlp]

The choice is going for other languages because they don't believe WG14, or WG21 will ever sort this out, as many are doing nowadays.

[uecker]

This is my point, programmers apparently fail to understand that they would need to push for changes at the compiler level. The committee is supposed to standardize what exist, it has no real power to change anything against the will of the compiler vendors.

[tredre3]

FYI all major C compilers have flags to enforce the usual two's-complement rollover, even if you enable all optimizations. I always enable at least fwrapv, even when I know that the underlying CPU has well defined overflow behavior (gcc knows this so the flag presumably becomes a no-op, but I've never validated that thought).

gcc has -fwrapv and -f[no-]strict-overflow, clang copied both, and MSVC has had a plethora of flags over the years (UndefIntOverflow, for example) so your guess is as good as mine which one still works as expected.