I64.min_value() / -1 returns unexpected results

[SeanTAllen]

When running:

actor Main
    new create(env: Env) =>
    let x = I64.min_value() / -1
    env.out.print(x.string())

I would expect the value to wrap back around to I64.min_value because

min value = -9223372036854775808
max value = 9223372036854775807

and -9223372036854775808 / -1 should be 9223372036854775808
which given the I would expect to be be -9223372036854775808 as it is when I do when I run:

actor Main
    new create(env: Env) =>
    let x = I64.max_value() + 1
    env.out.print(x.string())

[SeanTAllen]

Apparently the first triggers undefined behavior in LLVM that I think we should discuss:

http://llvm.org/docs/LangRef.html#id109

A discussion of how Julia handled is here:

JuliaLang/julia#8188

[plietar]

Looking at the implementation:

ponyc/src/libponyc/codegen/genoperator.c

Line 194 in a22e910

LLVMAddIncoming(phi, &zero, &nonzero_block, 1);

we intentionally avoid UB and set the result to 0, same as the x/0 case.

GCC returns INT64_MIN, whereas Clang treats this as UB and optimizes everything out, even if-fwrapv is set. https://godbolt.org/g/tpTrZZ

Rust panics, even if overflows checks are disabled. Using the explicit "wrapping division" returns INT64_MIN. https://godbolt.org/g/Gr2mNC

The best choice I think would be to just change that line in genoperator.c to return INTXX_MIN.

[jemc]

It seems like @mfelsche found that this behaviour is not giving the expected zero on arm and armhf? #2865 (comment)

[mfelsche]

... and on windows it seems.

[mfelsche]

So the inconsistency is only valid for I128 on platforms arm, armhf and on whatever platform appveyor is running on. There is already a comment on this in genoperator.c.
The reason seems to be that the code in make_divmod for detecting overflows is not working on platforms without native 128 bit support. I hope we can get the behaviour to be consistent across platforms somehow.

[aturley]

This looks like something that needs to have the same behavior across all platforms. We should figure out what behavior is needed here.