signed division has undefined behaviour on overflow

[thestinger]

Division by zero leads to undefined behavior. Overflow also leads to undefined behavior; this is a rare case, but can occur, for example, by doing a 32-bit division of -2147483648 by -1.

http://llvm.org/docs/LangRef.html#sdiv-instruction

Note that this is undefined behaviour, not just an unspecified result like a shift by too many bits. We need to add a check for this.

[emberian]

@thestinger what needs to be done for this? Check for certain numbers when dividing and fail!() if it would trigger UB?

[thestinger]

@cmr: signed division should check x / y for x == INT_MIN && y == -1 in addition to y == 0.

[emberian]

So this check happens in trans, fail_if_zero in base.rs. It needs to add an extra comparison in the ty_int branch. What that comparison is, and how to effect it, I do not know.