Slides for P3104R3
Bit permutations

Document number:: P3730R0
Date:: 2025-06-04
Audience:: LEWG
Project:: ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
Reply-to:: Jan Schultke <janschultke@gmail.com>
Source:: github.com/Eisenwave/cpp-proposals/blob/master/src/bit-permutations-slides.cow

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Bit permutations
P3104R3

Jan Schultke | Slides for P3104R3 — Bit permutations | Sofia 2025 | Slide 1

History

<bit> functions added by P0553R4: Bit operations (C++20)
- simple utilities like has_single_bit
- instruction wrappers like rotl, popcount, countl_zero, …
<stdbit.h> functions added by N3022: Modern Bit Utilities (C23)
P3104R0 continues work on <bit>
- first seen in LEWGI at Tokyo 2024
- removal of some functions requested to increase consensus
- proposal minified twice since then; four function templates remain

Jan Schultke | Slides for P3104R3 — Bit permutations | Sofia 2025 | Slide 2

Synopsis

template

unsigned-integer

constexpr

bit_repeat

(

int

length

)

not noexcept

;

template

unsigned-integer

constexpr

bit_reverse

(

)

noexcept

;

template

unsigned-integer

constexpr

bit_compress

(

)

noexcept

;

template

unsigned-integer

constexpr

bit_expand

(

)

noexcept

;

unsigned-integer means "Constraints: T is an unsigned integer type".

Jan Schultke | Slides for P3104R3 — Bit permutations | Sofia 2025 | Slide 3

`std::bit_repeat`

template

unsigned-integer

constexpr

bit_repeat

(

int

length

)

not noexcept

;

repeats/broadcasts bit pattern with length in x
hardware support depends on length (> 0)

bit_repeat(0b****'****'****'0110u, 4)
                               │
                ┌────┬────┬────┤
                │    │    │    │
        == 0b0110'0110'0110'0110u

Jan Schultke | Slides for P3104R3 — Bit permutations | Sofia 2025 | Slide 4

`std::bit_reverse`

template

unsigned-integer

constexpr

bit_reverse

(

)

noexcept

;

reverses the order of bits in x
supported by rbit (ARM), bswap (x86), …

bit_reverse(0b1000'1111'0000'1010u)
               ┌───────────────┘│
              ┌─────────────────┘
              ││        …
         == 0b0101'0000'1111'0001u

Jan Schultke | Slides for P3104R3 — Bit permutations | Sofia 2025 | Slide 5

`std::bit_compress`

template

unsigned-integer

constexpr

bit_compress

(

)

noexcept

;

packs bits of x where the "mask" m has a one-bit, contiguously
supported by bext (ARM), pext (x86)

unsigned m = 0b0001'0010'0000'1000u;
bit_compress(0b***1'**1*'****'0***u, m)
                  │   │       └─┐
                  │   └────────┐│
                  └───────────┐││
         == 0b0000'0000'0000'0110u

Jan Schultke | Slides for P3104R3 — Bit permutations | Sofia 2025 | Slide 6

`std::bit_expand`

template

unsigned-integer

constexpr

bit_expand

(

)

noexcept

;

opposite of std::bit_compress
supported by bdep (ARM), pdep (x86)

unsigned m = 0b0001'0010'0000'1000u;
bit_expand(  0b****'****'****'*110u, m)
                  ┌────────────┘││
                  │   ┌─────────┘│
                  │   │       ┌──┘
          == 0b0001'0010'0000'0000u

Jan Schultke | Slides for P3104R3 — Bit permutations | Sofia 2025 | Slide 7




              _____________________________
            < vote SF for bit permootations >
              -----------------------------
                     \   ^__^ 
                      \  (oo)\_______
                         (__)\       )\/\\
                             ||----w |
                             ||     ||

Jan Schultke | Slides for P3104R3 — Bit permutations | Sofia 2025 | Slide 8

Slides for P3104R3Bit permutations

Bit permutationsP3104R3

History

Synopsis

std::bit_repeat

std::bit_reverse

std::bit_compress

std::bit_expand

Slides for P3104R3
Bit permutations

Bit permutations
P3104R3

`std::bit_repeat`

`std::bit_reverse`

`std::bit_compress`

`std::bit_expand`