[LLVMdev] Integer divide by zero

[Joe Groff]

On Saturday, April 6, 2013, Jeff Bezanson wrote:

>
> Presumably the optimizer benefits from taking advantage of the
> undefined behavior, but to get a consistent result you need to check
> for both zero and this case, which is an awful lot of checks. Yes they
> will branch predict well, but this still can't be good, for code size
> if nothing else. How much performance can you really get by constant
> folding -9223372036854775808/-1 to an unspecified value?
>

Constant folding undefined expressions is sort of silly, but I appreciate
that it makes undefined behavior problems in frontends immediately apparent
with trivial cases before they creep up on you in more complicated
optimized code. After all, even if the backend makes practical concessions
to trivial cases, the underlying semantic problem is still there and will
bite you eventually. For high-level languages like Julia that want to
provide efficiency but also give defined behavior to all user-exposed
cases, I think adding an LLVM intrinsic to represent division that's
defined to trap on division by zero or overflow would be the best solution.
Since the trap is a side effect, it would stifle optimization of code that
used the intrinsic, but the intrinsic could still be lowered to a single
hardware instruction and avoid the branching and code bloat of manual
checks on hardware where division by zero natively traps.

-Joe
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