It's worse than you might expect, try it with unsigned int ;) $ cc -O3 foo.c $ ./a.out 5e574b68 $ cat foo.c #include <stdio.h> i

It's worse than you might expect, try it with unsigned int ;) $ cc -O3 foo.c $ ./a.out 5e574b68 $ cat foo.c #include <stdio.h> int main(int argc, char ** argv) { unsigned int a = 1; unsigned int b = 0; printf("%x\n", a/b); } Yes overflow wraps on unsigned as defined, but that example is divide-by-zero which is UB for unsigned too and the compiler is free to optimize there, which LLVM does.