Make `abs`, `wrapping_abs`, `overflowing_abs` const functions by tspiteri · Pull Request #63786 · rust-lang/rust

tspiteri · 2019-08-21T13:41:35Z

This makes abs, wrapping_abs and overflowing_abs const functions like #58044 makes wrapping_neg and overflowing_neg const functions.

abs is made const by returning (self ^ -1) - -1 = !self + 1 = -self for negative numbers and (self ^ 0) - 0 = self for non-negative numbers. The subexpression self >> ($BITS - 1) evaluates to -1 for negative numbers and 0 otherwise. The subtraction overflows when self is min_value(), as we would be subtracting max_value() - -1; this is when abs should overflow.

wrapping_abs and overflowing_abs make use of wrapping_sub and overflowing_sub instead of the subtraction operator.

rust-highfive · 2019-08-21T13:41:38Z

r? @cramertj

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

cramertj · 2019-08-22T22:14:40Z

r? @alexcrichton

alexcrichton · 2019-08-22T22:22:51Z

@rfcbot fcp merge

rfcbot · 2019-08-22T22:24:55Z

Team member @alexcrichton has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

Centril · 2019-08-30T22:35:51Z

Ping @withoutboats @Kimundi @sfackler in case y'all missed this. :)

rfcbot · 2019-08-30T22:59:21Z

🔔 This is now entering its final comment period, as per the review above. 🔔

rfcbot · 2019-09-09T23:08:54Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

alexcrichton · 2019-09-10T14:19:57Z

@bors: r+

bors · 2019-09-10T14:19:58Z

📌 Commit adee559 has been approved by alexcrichton

Make `abs`, `wrapping_abs`, `overflowing_abs` const functions This makes `abs`, `wrapping_abs` and `overflowing_abs` const functions like rust-lang#58044 makes `wrapping_neg` and `overflowing_neg` const functions. `abs` is made const by returning `(self ^ -1) - -1` = `!self + 1` = `-self` for negative numbers and `(self ^ 0) - 0` = `self` for non-negative numbers. The subexpression `self >> ($BITS - 1)` evaluates to `-1` for negative numbers and `0` otherwise. The subtraction overflows when `self` is `min_value()`, as we would be subtracting `max_value() - -1`; this is when `abs` should overflow. `wrapping_abs` and `overflowing_abs` make use of `wrapping_sub` and `overflowing_sub` instead of the subtraction operator.

@ghost

Rollup of 8 pull requests Successful merges: - #63786 (Make `abs`, `wrapping_abs`, `overflowing_abs` const functions) - #63989 (Add Yaah to clippy toolstain notification list) - #64256 (test/c-variadic: Fix patterns on powerpc64) - #64292 (lowering: extend temporary lifetimes around await) - #64311 (lldb: avoid mixing "Hit breakpoint" message with other output.) - #64330 (Clarify E0507 to note Fn/FnMut relationship to borrowing) - #64331 (Changed instant is earlier to instant is later) - #64344 (rustc_mir: buffer -Zdump-mir output instead of pestering the kernel constantly.) Failed merges: r? @ghost

RalfJung · 2019-09-10T20:27:57Z

-                    self
-                }
+            pub const fn wrapping_abs(self) -> Self {
+                (self ^ (self >> ($BITS - 1))).wrapping_sub(self >> ($BITS - 1))


I am not a big fan of making code less readable just to please our const qualification. :/

Cc @oli-obk @eddyb

We've had variables for a while, please use them.

Has it been verified that this optimizes to identical LLVM IR?

@eddyb Since this has already passed final comment period and been merged, shall I open a new PR to use variables?

@nikic I can't read LLVM IR; what I did do before submitting the PR was to use llvm-mca, and although the assembly is not identical, the performance measure from llvm-mca was the same, or if I remember well, in some tests the reverse throughput improved by something tiny like 0.1, but not regressed. I only used llvm-mca with i686 or x86-64 instructions, and I don't think I tested all types comprehensively.

@nikic Hmm, using variables as @eddyb suggested seems to hit two birds with one stone. If I insert a variable for the sign, I get one implementation and an alias: https://godbolt.org/z/m-seT0

Does that mean that the performance will not regress?

@tspiteri Oops, I simply made a typo in my test and that's why the result is different, duh :) It actually does produce the same IR, both with variable and without. So forget everything I said, this change is fine from the optimization perspective.

@nikic While abs and wrapping_abs seem to be fine performance-wise, overflowing_abs from this PR actually does suffer from the problem discussed, so I will not forget everything you said! :) It's easily fixed by making it return (self.wrapping_abs(), self == Self::min_value()).

And also, the code with the sign factored out is more readable. ;)

That said, I still think we should rather wait for CTFE to support conditionals than compromise code readability, and even the let sign version is still arcane magic. But that call is up to T-libs.

No, we should definitely make as many of these small ops into const as soon as we can.

tspiteri and others added 2 commits August 21, 2019 14:10

make abs, wrapping_abs, and overflowing_abs const functions

39260d9

test const abs, wrapping_abs, and overflowing_abs

adee559

rust-highfive assigned cramertj Aug 21, 2019

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 21, 2019

Centril added relnotes Marks issues that should be documented in the release notes of the next release. T-libs-api [DEPRECATED; DO NOT USE] labels Aug 21, 2019

Centril added this to the 1.39 milestone Aug 21, 2019

Centril added the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Aug 21, 2019

rust-highfive assigned alexcrichton and unassigned cramertj Aug 22, 2019

rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Aug 22, 2019

Centril removed the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Aug 30, 2019

rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Aug 30, 2019

JohnCSimon closed this Sep 7, 2019

JohnCSimon reopened this Sep 7, 2019

rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Sep 9, 2019

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 Sep 10, 2019

Centril mentioned this pull request Sep 10, 2019

Rollup of 8 pull requests #64354

Merged

bors merged commit adee559 into rust-lang:master Sep 10, 2019

RalfJung reviewed Sep 10, 2019

View reviewed changes

tspiteri deleted the const-abs branch September 10, 2019 21:01

tspiteri mentioned this pull request Sep 11, 2019

use sign variable in abs and wrapping_abs methods #64386

Merged

RalfJung mentioned this pull request Sep 16, 2019

Stabilize Vec::new and String::new as const fns #64028

Merged

Centril added the const-hack label Sep 17, 2019

RalfJung mentioned this pull request Oct 5, 2019

constify some checked arithmetics #65107

Closed

TimDiekmann mentioned this pull request Nov 4, 2019

Tracking Issue for more const int functions #53718

Closed

59 tasks

jumbatm mentioned this pull request Dec 27, 2019

Clean up const-hack PRs now that const if / match exist. #67657

Merged

RalfJung removed the const-hack label Aug 7, 2020

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tspiteri commented Aug 21, 2019 •

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rfcbot commented Aug 22, 2019 •

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