Make some std::intrinsics `const fn`s by faern · Pull Request #51171 · rust-lang/rust

faern · 2018-05-29T14:57:06Z

Making some rustc intrinsics (ctpop, cttz, ctlz and bswap) const fns.

This is a pre-step to being able to make swap_bytes, to_be and from_be constant functions. That in itself could be ergonomic and useful. But even better is that it would allow Ipv4Addr::new etc becoming const fns as well. Which might be really useful since I find it quite common to want to define them as constants.

r? @oli-obk

rust-highfive · 2018-05-29T14:57:16Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @oli-obk (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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leonardo-m · 2018-05-29T19:29:48Z

Eventually I'd like log/sin/cos too const.

oli-obk

These are inherently unstable, so there's nothing wrong with making these const except if they were somehow exposed by macros.

oli-obk · 2018-05-29T19:50:15Z

@eddyb I know you dislike these kinds of matches. We could probably do all the ops on u128 and modify the result depending on the type's bitwidth, but that feels fragile

let extra = 128 - bits;
ctpop(x) = x.count_ones()
ctlz(x) = x.leading_zeros() - extra
cttz(x) = (x << extra).trailing_zeros() - extra
bswap(x) = (x << extra).swap_bytes()

I agree the code can probably be written a bit nicer. Looking into it. Will push as soon as I have the time.

oli-obk · 2018-05-29T19:53:25Z

@leonardo-m I suggest you open a PR for those intrinsics then ;)

eddyb · 2018-05-29T21:34:46Z

Eventually I'd like log/sin/cos too const.

We can't do that because we don't have an soft-float (rustc_apfloat) implementation of them.

oli-obk · 2018-05-31T08:31:22Z

@bors r+

bors · 2018-05-31T08:31:25Z

📌 Commit 3fefa96 has been approved by oli-obk

bors · 2018-05-31T08:55:06Z

🔒 Merge conflict

eddyb · 2018-05-31T09:23:06Z

If anything, I'd call these bits, not bytes.

Ok. I agree. Matches better with the field same in Scalar as well. Pushed a rename.

faern · 2018-05-31T17:35:28Z

Fixed the merge conflict by rebasing.

oli-obk · 2018-05-31T18:50:04Z

@bors r+

bors · 2018-05-31T18:50:05Z

📌 Commit 97a0d46 has been approved by oli-obk

bors · 2018-06-01T03:25:00Z

⌛ Testing commit 97a0d46 with merge 63cd4a3...

@oli-obk

Make some std::intrinsics `const fn`s Making some rustc intrinsics (`ctpop`, `cttz`, `ctlz` and `bswap`) `const fn`s. This is a pre-step to being able to make `swap_bytes`, `to_be` and `from_be` constant functions. That in itself could be ergonomic and useful. But even better is that it would allow `Ipv4Addr::new` etc becoming `const fn`s as well. Which might be really useful since I find it quite common to want to define them as constants. r? @oli-obk

bors · 2018-06-01T05:36:55Z

☀️ Test successful - status-appveyor, status-travis
Approved by: oli-obk
Pushing 63cd4a3 to master...

const fn integer operations A follow up to rust-lang#51171Fixesrust-lang#51267 Makes a lot of the integer methods (`swap_bytes`, `count_ones` etc) `const fn`s. See rust-lang#51267 for a discussion about why this is wanted and the solution used.

rust-highfive assigned oli-obk May 29, 2018

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 29, 2018

oli-obk reviewed May 29, 2018
View reviewed changes

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 31, 2018

bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels May 31, 2018

eddyb reviewed May 31, 2018
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faern added 3 commits May 31, 2018 19:18

Make ctpop, cttz, ctlz and bswap const fns
376b640

Add test for intrinsics::bswap
12c330c

Rewrite numeric_intrinsic without macro
0d7d5e9

faern force-pushed the const-bswap-ctpop-cttz-ctlz branch from 3fefa96 to 0d7d5e9Compare May 31, 2018 17:19

Rename bytes -> bits
8939fe6

Rename num -> bits and num -> out_val
97a0d46

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 31, 2018

bors merged commit 97a0d46 into rust-lang:masterJun 1, 2018

hanna-kruppe mentioned this pull request Jun 1, 2018
[tracking issue] const fn: count_zeros, count_ones, leading_zeros, trailing_zeros #51267
Closed
3 tasks

This was referenced Jun 1, 2018
Tracking issue for IP constructors #44582
Closed
const fn integer operations #51299
Merged

oli-obk mentioned this pull request Aug 20, 2018
[tracking issue] const fn std::mem::needs_drop #51929
Closed

faern deleted the const-bswap-ctpop-cttz-ctlz branch January 31, 2019 19:50

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