At least GCC is I believe fairly buggy in that regard since isnan is optimised out, thus you cannot check if say an incoming value read from the network is a NaN. There's some lengthy discussion about it on the big tracker iirc..

I was misremembering, was thinking about this recent convo on LLVM mailinglist: https://lists.llvm.org/pipermail/llvm-dev/2021-September/152...

I don't think you are misremembering; -ffast-math causes __builtin_isnan and __builtin_isinf to unconditionally return zero, for both gcc and clang:

https://godbolt.org/z/1qjxvW3KK

I found this out the hard way several years ago and still have my own implementations of these functions laying about in case I want to use -ffast-math.

He means simple stuff like ensuring a function like

    float
    unit_price (float volume, int count) {
       return volume / count;
    }

is only called where count > 0

But couldn’t the compiler optimize out the check because you’re guaranteeing to never divide by 0, so logically count would never be 0. Akin to undefined behavior almost.

This test can not be optimized to always true:

    if (count > 0)
        total += unit_price(volume, count);

Are you confusing it with the opposite scenario:

    total += unit_price(volume, count);
    if (count == 0)
        printf("oops\n");

That test might be optimized out because divide by zero is undefined behavior.

I probably am confusing them. I don’t program in C or C++, so I’m not aware of all the “gotchas”, and can easily confuse them (as shown here). Not to mention that compilers have been getting more aggressive in optimizing undefined behavior.

What the compiler assumes depends on the compilation options.

Unfortunately most compilers have too permissive default options.

C and C++ programs should always be compiled, at least during development, with strict options, e.g.:

-fsanitize=undefined -fsanitize-undefined-trap-on-error

or equivalent options, which eliminate all the horror stories with unexpected behavior caused by integer divide-by-zero or overflow, pointer overflow, out-of-bounds access and many others.

Because unlike integer operations the FP operations are standardized, there is no need for compiler options, the behavior for exceptions can be controlled from the program with the functions defined in <fenv.h> in C or <cfenv> in C++.