Add associated constant `BITS` to all integer types #76492

m-ou-se · 2020-09-08T19:40:33Z

Recently I've regularly come across this snippet (in a few different crates, including core and std):

std::mem::size_of<usize>() * 8

I think it's time for a usize::BITS.

leonardo-m · 2020-09-08T19:54:49Z

This is useful, good idea.
But call it NBITS? Or N_BITS? Or BIT_LEN? (In Python numbers have a bit_length() runtime method).

m-ou-se · 2020-09-08T20:06:29Z

I tried to follow the convention that was already used in this repository:

The name BITS is used in a few places for the number of bits in an integer (although only internally). N_BITS occurs zero times. The prefix N_ in general also occurs zero times (unless you count the rust-analyzer submodule, where it is used once).

camelid · 2020-09-08T21:17:00Z

Bikeshed: BITSIZE? But it doesn't matter too much either way :)

Mark-Simulacrum · 2020-09-08T21:30:36Z

Assigning to @dtolnay for T-libs hat "unstable API" review.

leonardo-m · 2020-09-08T22:01:15Z

The prefix N_ in general also occurs zero times

Nice, this shows how much different are coding habits ;-) And why I don't like to think as name choice as bikeshedding. Constant/function names of libraries must be chosen carefully if you want those libraries to be ergonomic :)

bors · 2020-09-14T00:52:14Z

☔ The latest upstream changes (presumably #76678) made this pull request unmergeable. Please resolve the merge conflicts.

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dtolnay

This looks good. Please resolve the merge conflict in library/alloc/src/lib.rs and library/alloc/tests/string.rs and file a tracking issue, and r=me on the PR.

dtolnay · 2020-09-19T04:37:28Z

@bors delegate=m-ou-se

bors · 2020-09-19T04:37:29Z

✌️ @m-ou-se can now approve this pull request

m-ou-se · 2020-09-19T06:16:28Z

@bors r=dtolnay rollup

bors · 2020-09-19T06:16:30Z

📌 Commit 1bfe5ef has been approved by dtolnay

…r=dtolnay Add associated constant `BITS` to all integer types Recently I've regularly come across this snippet (in a few different crates, including `core` and `std`): ```rust std::mem::size_of<usize>() * 8 ``` I think it's time for a `usize::BITS`.

Rollup of 14 pull requests Successful merges: - rust-lang#73963 (deny(unsafe_op_in_unsafe_fn) in libstd/path.rs) - rust-lang#75099 (lint/ty: move fns to avoid abstraction violation) - rust-lang#75502 (Use implicit (not explicit) rules for promotability by default in `const fn`) - rust-lang#75580 (Add test for checking duplicated branch or-patterns) - rust-lang#76310 (Add `[T; N]: TryFrom<Vec<T>>` (insta-stable)) - rust-lang#76400 (Clean up vec benches bench_in_place style) - rust-lang#76434 (do not inline black_box when building for Miri) - rust-lang#76492 (Add associated constant `BITS` to all integer types) - rust-lang#76525 (Add as_str() to string::Drain.) - rust-lang#76636 (assert ScalarMaybeUninit size) - rust-lang#76749 (give *even better* suggestion when matching a const range) - rust-lang#76757 (don't convert types to the same type with try_into (clippy::useless_conversion)) - rust-lang#76796 (Give a better error message when x.py uses the wrong stage for CI) - rust-lang#76798 (Build fixes for RISC-V 32-bit Linux support) Failed merges: r? `@ghost`

leonardo-m · 2020-09-21T07:42:45Z

size_of returns an usize, while BITS is a u32, so to replace one with the other I need to introduce a cast. Please consider making BITS a usize.

m-ou-se · 2020-09-21T07:52:54Z

Everything that counts bits of an integer type does so as a u32. E.g. leading_zeros, count_ones, the argument to rotate_left and wrapping_shr, etc.

myrrlyn · 2020-09-28T21:07:16Z

As another maintainer of a BITS constant, I'm very eager to see this occur.

Like leonardo-m, I also would prefer a usize rather than a u32, but given that we cannot change the existing u32 values to usize, I agree that consistency with what exists is more important than an inconsistent convenience. as usize is not overly burdensome to write.

petrochenkov · 2020-09-29T07:09:34Z

Wait a second, what happened?
We had the BITS constants and they had the type usize - #23832.

petrochenkov · 2020-09-29T07:15:51Z

Ah, here's what happened:

FIXME(#11621): Should be deprecated once CTFE is implemented in favour of calling the mem::size_of function.

Anyway, I also agree that they should be usize because they are almost always used in usize context.

m-ou-se · 2020-09-29T07:49:15Z

I don't agree usize is a good type for this. usize is meant for counting bytes in memory. It has exactly enough space to address all bytes in the address space. There's nothing like that for bits. If you want to count bits in all of the address space, you need something with three bits more, as there are eight times more bits than bytes. More importantly, this counts bits in the integer types, which is not related to the size of the address space. It does not need more space when switching from a 32 bit to a 64 bit system, unlike .len() etc.

All the existing functions that work with the number of bits in an integer (shifting, leading zeros, counting ones, etc.) work with u32. Patterns like u32::BITS - some_u32.leading_zeros() are very common.

I don't think the argument that it's mostly used as usize is true. This PR changed 24 places where mem::size_of * 8 was used to ::BITS. Only in seven cases is it casted to a usize. In quite a few cases it was combined with a function like leading_zeros() which also gives a u32. In a some cases, a as u32 was even removed.

Some examples from popular crates:

INT_NBITS and FRAC_NBITS in the fixed crate are u32: https://docs.rs/fixed/1.2.0/fixed/traits/trait.Fixed.html#associatedconstant.INT_NBITS
BITS in compiler-builtins is also a u32: https://github.com/rust-lang/compiler-builtins/blob/eff506cd49b637f1ab5931625a33cef7e91fbbf6/src/int/mod.rs#L39-L40
BITS in bitvec is also not a usize, it's a u8: https://docs.rs/bitvec/0.19.2/bitvec/mem/trait.BitMemory.html#associatedconstant.BITS

leonardo-m · 2020-09-29T09:28:17Z

BITS in bitvec is also not a usize, it's a u8

An advantage of it being u8 is that you can convert it to both u32 and usize without "as" and without unwraps.

`int_bits_const` feature [1] adds `i32::BITS`, etc. We already have `BitInteger` trait, which has `BitInteger::BITS`, and using it triggers `unstable_name_collision` compatibility lint [2] unless `int_bits_const` is explicitly enabled, gets stabilized or gets rejected and removed from the compiler. [1]: rust-lang/rust#76492 [2]: rust-lang/rust#48919

This ports rust-lang/rust@1e2dba1e, part of rust-lang/rust#76492. `usize::BITS` was stabilized in Rust 1.53.0.

m-ou-se changed the title ~~Int bits~~ Add associated constant BITS to all integer types Sep 8, 2020

jyn514 added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels Sep 8, 2020

Mark-Simulacrum assigned dtolnay Sep 8, 2020

dtolnay approved these changes Sep 19, 2020

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dtolnay 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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 19, 2020

m-ou-se added 3 commits September 19, 2020 06:50

Add BITS associated constant to all integer types.

3f68ae4

Add example/test to <int types>::BITS.

5c30a16

Use T::BITS instead of size_of::<T> * 8.

1e2dba1

m-ou-se force-pushed the int-bits branch from 3194211 to 1e2dba1 Compare September 19, 2020 04:59

m-ou-se mentioned this pull request Sep 19, 2020

Tracking Issue for feature(int_bits_const): <integer>::BITS #76904

Closed

Add tracking issue number for int_bits_const.

1bfe5ef

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 Sep 19, 2020

RalfJung mentioned this pull request Sep 19, 2020

Rollup of 13 pull requests #76907

Closed

RalfJung mentioned this pull request Sep 19, 2020

Rollup of 14 pull requests #76912

Merged

bors merged commit fef3324 into rust-lang:master Sep 19, 2020

rustbot added this to the 1.48.0 milestone Sep 19, 2020

JohnTitor mentioned this pull request Oct 27, 2020

compiler-builtins emits a lof of warnings about conflicts method name rust-lang/compiler-builtins#391

Closed

leonardo-m mentioned this pull request Dec 5, 2020

Compress RWU from at least 32 bits to 4 bits #79727

Merged

m-ou-se deleted the int-bits branch January 6, 2021 18:21

olback mentioned this pull request Dec 2, 2021

Impl rand_core::RngCore & support all integers in Rng stm32-rs/stm32h7xx-hal#284

Merged

clint-white added a commit to clint-white/binary-heap-plus-rs that referenced this pull request Sep 23, 2022

Use usize::BITS instead of 8 * std::mem::size_of::<usize>()

5c40e51

This ports rust-lang/rust@1e2dba1e, part of rust-lang/rust#76492. `usize::BITS` was stabilized in Rust 1.53.0.

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Add associated constant `BITS` to all integer types #76492

Add associated constant `BITS` to all integer types #76492

m-ou-se commented Sep 8, 2020

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leonardo-m commented Sep 29, 2020

Add associated constant BITS to all integer types #76492

Add associated constant BITS to all integer types #76492

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Add associated constant `BITS` to all integer types #76492

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