Add nowrap_{add|sub|mul|neg} intrinsics

[scottmcm]

LLVM cannot deduce nsw/nuw for things like Range::next today, so add these so that my next PR can use them to improve what Range tells LLVM.

(This adds the ones that rustc_codegen_llvm::builder::Builder and rustc_llvm::ffi currently support, which is why it has nowrap_neg but not nowrap_shl.)

tag guess: T-compiler

[rust-highfive]

r? @varkor

(rust_highfive has picked a reviewer for you, use r? to override)

[scottmcm]

Yes, this one is useless, since a non-overflowing negation of an unsigned type is always zero, but it was easier to support than to not support.

[oli-obk]

@bors r+

I see no reason not to do this, and if it ends up not helping we can always remove them

[bors]

📌 Commit 9135ec5 has been approved by oli-obk

[nagisa]

@bors r-

There’s no necessity to land intrinsics before a PR with an actual use case can be made (such as e.g. Range optimisations).

Please implement actual changes against Range and provide benchmarks before landing these intrinsics. All you’ll need are #[cfg(stage0)] and #[cfg(not(stage0))].

[nagisa]

Also,

we can always remove them

is a very slippery slope, simply because it is way easier to forget and not remove something that ended up not being used (or not useful for some immediate use case). It is way easier to land everything together or not land stuff at all :)

[scottmcm]

Closing while I do that.

(Aside: cool to see that bors knows that an r- means the author probably needs to do something)

[gnzlbg]

Here is an use case for these intrinsics: rust-lang/rfcs#2508

Basically, there is no easy way to generate LLVM-IR with nsw nuw flags via rustc today. This makes it very hard to even evaluate whether such operations should be exposed by higher-level APIs, or whether these flags have an impact at all on the generated code.

So I'd argue that these intrinsics are at least useful for hacking on rustc/LLVM, debugging performance issues, etc.

Also, I'd like to be able to play with intrinsics that also add both, the nsw and nuw flags, as well. That is, for add:

• add nuw
• add nsw
• add nuw nsw