mrb_int_mul_overflow is missing

[Asmod4n]

Currently multiplications aren't checked for overflowing, there is __builtin_mul_overflow in GCC/Clang etc. I just don't know how to implement it in compilers which lack it.

[zzak]

Is it available in C99?

[Asmod4n]

It would be mostly the same as https://github.com/mruby/mruby/blob/master/include/mruby/numeric.h#L40, so i guess its not in C99 too.

[zzak]

I'm not familiar enough with the builtin overflows, maybe matz can advise

[cremno]

I'll add it. The difficult part here is the fallback for compilers without these built-ins. It needs to be portable and fast. Of course, only to some degree. With 16/32 b mrb_int it's actually quite simple: 32/64 b multiplication + FIXABLE() (which btw. should be renamed to mrb_fixable or even mrb_fixable_p). But that's not an option with the 64b one…