core: make `str::split_at_unchecked()` inline #143907

Kijewski · 2025-07-13T21:06:55Z

This PR adds #[inline] to the method str::split_at_unchecked(). This is done for two reasons:

The method is tiny, e.g. on AMD-64 (https://godbolt.org/z/ba68fdfxn):

movq    %rdi, %rax
subq    %rcx, %rdx
movq    %rsi, (%rdi)
addq    %rcx, %rsi
movq    %rcx, 8(%rdi)
movq    %rsi, 16(%rdi)
movq    %rdx, 24(%rdi)
retq

More importantly, inlining the method enables further automatic optimizations. E.g. if you split at index 3, then in the compiler (rustc, llvm or both) knows that this code cannot fail, and the panicking path is omitted in the generated code:

pub fn punctuation(i: &str) -> Result<(), ()> {
    const THREE_CHARS: &[[u8; 3]] = &[*b"<<=", *b">>=", *b"...", *b"..="];

    if let Some((head, _)) = i.split_at_checked(3)
        && THREE_CHARS.contains(&head.as_bytes().try_into().unwrap())
    {
        Ok(())
    } else {
        Err(())
    }
}

Without PR

https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=0234de8158f467eebd73286f20d6e27a

playground::punctuation:
        subq    $40, %rsp
        movq    %rsi, %rdx
        movq    %rdi, %rsi
        movb    $1, %al
        cmpq    $3, %rdx
        ja      .LBB2_2
        je      .LBB2_3
.LBB2_11:
        addq    $40, %rsp
        retq
.LBB2_2:
        cmpb    $-64, 3(%rsi)
        jl      .LBB2_11
.LBB2_3:
        leaq    8(%rsp), %rdi
        movl    $3, %ecx
        callq   *core::str::<impl str>::split_at_unchecked@GOTPCREL(%rip)
        movq    8(%rsp), %rcx
        movb    $1, %al
        testq   %rcx, %rcx
        je      .LBB2_11
        cmpq    $3, 16(%rsp)
        jne     .LBB2_12
        movzwl  (%rcx), %edx
        movzbl  2(%rcx), %ecx
        shll    $16, %ecx
        orl     %edx, %ecx
        cmpl    $4013115, %ecx
        jg      .LBB2_8
        cmpl    $3026478, %ecx
        je      .LBB2_10
        cmpl    $4009518, %ecx
        je      .LBB2_10
        jmp     .LBB2_11
.LBB2_8:
        cmpl    $4013630, %ecx
        je      .LBB2_10
        cmpl    $4013116, %ecx
        jne     .LBB2_11
.LBB2_10:
        xorl    %eax, %eax
        addq    $40, %rsp
        retq
.LBB2_12:
        leaq    .Lanon.d98a7fbb86d10a97c24516e267466134.2(%rip), %rdi
        leaq    .Lanon.d98a7fbb86d10a97c24516e267466134.1(%rip), %rcx
        leaq    .Lanon.d98a7fbb86d10a97c24516e267466134.6(%rip), %r8
        leaq    7(%rsp), %rdx
        movl    $43, %esi
        callq   *core::result::unwrap_failed@GOTPCREL(%rip)

With PR

https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=5d4058c79ce0f6cb1a434190427d2055

playground::punctuation:
        movb    $1, %al
        cmpq    $3, %rsi
        ja      .LBB0_2
        je      .LBB0_3
.LBB0_9:
        retq
.LBB0_2:
        cmpb    $-64, 3(%rdi)
        jl      .LBB0_9
.LBB0_3:
        movzwl  (%rdi), %eax
        movzbl  2(%rdi), %ecx
        shll    $16, %ecx
        orl     %eax, %ecx
        movb    $1, %al
        cmpl    $4013115, %ecx
        jg      .LBB0_6
        cmpl    $3026478, %ecx
        je      .LBB0_8
        cmpl    $4009518, %ecx
        je      .LBB0_8
        jmp     .LBB0_9
.LBB0_6:
        cmpl    $4013630, %ecx
        je      .LBB0_8
        cmpl    $4013116, %ecx
        jne     .LBB0_9
.LBB0_8:
        xorl    %eax, %eax
        retq

This PR adds `#[inline]` to the method `str::split_at_unchecked()`. This is done for two reasons: 1. The method is tiny, e.g. on AMD-64 (<https://godbolt.org/z/ba68fdfxn>): ```asm movq %rdi, %rax subq %rcx, %rdx movq %rsi, (%rdi) addq %rcx, %rsi movq %rcx, 8(%rdi) movq %rsi, 16(%rdi) movq %rdx, 24(%rdi) retq ``` 2. More importantly, inlining the method enables further automatic optimizations. E.g. if you split at index 3, then in the compiler (rustc, llvm or both) knows that this code cannot fail, and the panicking path is omitted in the generated code: ```rust pub fn punctuation(i: &str) -> Result<(), ()> { const THREE_CHARS: &[[u8; 3]] = &[*b"<<=", *b">>=", *b"...", *b"..="]; if let Some((head, _)) = i.split_at_checked(3) && THREE_CHARS.contains(&head.as_bytes().try_into().unwrap()) { Ok(()) } else { Err(()) } } ``` <details> <summary>Without PR</summary> <https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=0234de8158f467eebd73286f20d6e27a> ```asm playground::punctuation: subq $40, %rsp movq %rsi, %rdx movq %rdi, %rsi movb $1, %al cmpq $3, %rdx ja .LBB2_2 je .LBB2_3 .LBB2_11: addq $40, %rsp retq .LBB2_2: cmpb $-64, 3(%rsi) jl .LBB2_11 .LBB2_3: leaq 8(%rsp), %rdi movl $3, %ecx callq *core::str::<impl str>::split_at_unchecked@GOTPCREL(%rip) movq 8(%rsp), %rcx movb $1, %al testq %rcx, %rcx je .LBB2_11 cmpq $3, 16(%rsp) jne .LBB2_12 movzwl (%rcx), %edx movzbl 2(%rcx), %ecx shll $16, %ecx orl %edx, %ecx cmpl $4013115, %ecx jg .LBB2_8 cmpl $3026478, %ecx je .LBB2_10 cmpl $4009518, %ecx je .LBB2_10 jmp .LBB2_11 .LBB2_8: cmpl $4013630, %ecx je .LBB2_10 cmpl $4013116, %ecx jne .LBB2_11 .LBB2_10: xorl %eax, %eax addq $40, %rsp retq .LBB2_12: leaq .Lanon.d98a7fbb86d10a97c24516e267466134.2(%rip), %rdi leaq .Lanon.d98a7fbb86d10a97c24516e267466134.1(%rip), %rcx leaq .Lanon.d98a7fbb86d10a97c24516e267466134.6(%rip), %r8 leaq 7(%rsp), %rdx movl $43, %esi callq *core::result::unwrap_failed@GOTPCREL(%rip) ``` </details> <details> <summary>With PR</summary> <https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=5d4058c79ce0f6cb1a434190427d2055> ```asm playground::punctuation: movb $1, %al cmpq $3, %rsi ja .LBB0_2 je .LBB0_3 .LBB0_9: retq .LBB0_2: cmpb $-64, 3(%rdi) jl .LBB0_9 .LBB0_3: movzwl (%rdi), %eax movzbl 2(%rdi), %ecx shll $16, %ecx orl %eax, %ecx movb $1, %al cmpl $4013115, %ecx jg .LBB0_6 cmpl $3026478, %ecx je .LBB0_8 cmpl $4009518, %ecx je .LBB0_8 jmp .LBB0_9 .LBB0_6: cmpl $4013630, %ecx je .LBB0_8 cmpl $4013116, %ecx jne .LBB0_9 .LBB0_8: xorl %eax, %eax retq ``` </details>

rustbot · 2025-07-13T21:06:59Z

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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Mark-Simulacrum · 2025-07-15T00:24:22Z

@bors r+

bors · 2025-07-15T00:24:25Z

📌 Commit 6760cd2 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

…ked, r=Mark-Simulacrum core: make `str::split_at_unchecked()` inline This PR adds `#[inline]` to the method `str::split_at_unchecked()`. This is done for two reasons: 1. The method is tiny, e.g. on AMD-64 (<https://godbolt.org/z/ba68fdfxn>): ```asm movq %rdi, %rax subq %rcx, %rdx movq %rsi, (%rdi) addq %rcx, %rsi movq %rcx, 8(%rdi) movq %rsi, 16(%rdi) movq %rdx, 24(%rdi) retq ``` 2. More importantly, inlining the method enables further automatic optimizations. E.g. if you split at index 3, then in the compiler (rustc, llvm or both) knows that this code cannot fail, and the panicking path is omitted in the generated code: ```rust pub fn punctuation(i: &str) -> Result<(), ()> { const THREE_CHARS: &[[u8; 3]] = &[*b"<<=", *b">>=", *b"...", *b"..="]; if let Some((head, _)) = i.split_at_checked(3) && THREE_CHARS.contains(&head.as_bytes().try_into().unwrap()) { Ok(()) } else { Err(()) } } ``` <details> <summary>Without PR</summary> <https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=0234de8158f467eebd73286f20d6e27a> ```asm playground::punctuation: subq $40, %rsp movq %rsi, %rdx movq %rdi, %rsi movb $1, %al cmpq $3, %rdx ja .LBB2_2 je .LBB2_3 .LBB2_11: addq $40, %rsp retq .LBB2_2: cmpb $-64, 3(%rsi) jl .LBB2_11 .LBB2_3: leaq 8(%rsp), %rdi movl $3, %ecx callq *core::str::<impl str>::split_at_unchecked@GOTPCREL(%rip) movq 8(%rsp), %rcx movb $1, %al testq %rcx, %rcx je .LBB2_11 cmpq $3, 16(%rsp) jne .LBB2_12 movzwl (%rcx), %edx movzbl 2(%rcx), %ecx shll $16, %ecx orl %edx, %ecx cmpl $4013115, %ecx jg .LBB2_8 cmpl $3026478, %ecx je .LBB2_10 cmpl $4009518, %ecx je .LBB2_10 jmp .LBB2_11 .LBB2_8: cmpl $4013630, %ecx je .LBB2_10 cmpl $4013116, %ecx jne .LBB2_11 .LBB2_10: xorl %eax, %eax addq $40, %rsp retq .LBB2_12: leaq .Lanon.d98a7fbb86d10a97c24516e267466134.2(%rip), %rdi leaq .Lanon.d98a7fbb86d10a97c24516e267466134.1(%rip), %rcx leaq .Lanon.d98a7fbb86d10a97c24516e267466134.6(%rip), %r8 leaq 7(%rsp), %rdx movl $43, %esi callq *core::result::unwrap_failed@GOTPCREL(%rip) ``` </details> <details> <summary>With PR</summary> <https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=5d4058c79ce0f6cb1a434190427d2055> ```asm playground::punctuation: movb $1, %al cmpq $3, %rsi ja .LBB0_2 je .LBB0_3 .LBB0_9: retq .LBB0_2: cmpb $-64, 3(%rdi) jl .LBB0_9 .LBB0_3: movzwl (%rdi), %eax movzbl 2(%rdi), %ecx shll $16, %ecx orl %eax, %ecx movb $1, %al cmpl $4013115, %ecx jg .LBB0_6 cmpl $3026478, %ecx je .LBB0_8 cmpl $4009518, %ecx je .LBB0_8 jmp .LBB0_9 .LBB0_6: cmpl $4013630, %ecx je .LBB0_8 cmpl $4013116, %ecx jne .LBB0_9 .LBB0_8: xorl %eax, %eax retq ``` </details>

Rollup of 16 pull requests Successful merges: - #142301 (tests: Fix duplicated-path-in-error fail with musl) - #142936 (rustdoc-json: Structured attributes) - #143355 (wrapping shift: remove first bitmask and table) - #143630 (Drop `./x suggest`) - #143738 (Move several float tests to floats/mod.rs) - #143752 (Don't panic if WASI_SDK_PATH not set when detecting compiler) - #143820 (Fixed a core crate compilation failure when enabling the `optimize_for_size` feature on some targets) - #143837 (Adjust `run_make_support::symbols` helpers) - #143878 (Port `#[pointee]` to the new attribute parsing infrastructure) - #143907 (core: make `str::split_at_unchecked()` inline) - #143910 (Add experimental `backtrace-trace-only` std feature) - #143927 (Preserve constness in trait objects up to hir ty lowering) - #143935 (rustc_type_ir/walk: move docstring to `TypeWalker` itself) - #143938 (Update books) - #143941 (update `cfg_select!` documentation) - #143948 (Update mdbook to 0.4.52) Failed merges: - #143926 (Remove deprecated fields in bootstrap) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 13 pull requests Successful merges: - #142301 (tests: Fix duplicated-path-in-error fail with musl) - #143630 (Drop `./x suggest`) - #143736 (Give all bytes of TypeId provenance) - #143752 (Don't panic if WASI_SDK_PATH not set when detecting compiler) - #143837 (Adjust `run_make_support::symbols` helpers) - #143878 (Port `#[pointee]` to the new attribute parsing infrastructure) - #143905 (Recover and suggest to use `;` to construct array type) - #143907 (core: make `str::split_at_unchecked()` inline) - #143910 (Add experimental `backtrace-trace-only` std feature) - #143927 (Preserve constness in trait objects up to hir ty lowering) - #143935 (rustc_type_ir/walk: move docstring to `TypeWalker` itself) - #143938 (Update books) - #143941 (update `cfg_select!` documentation) Failed merges: - #143926 (Remove deprecated fields in bootstrap) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #143907 - Kijewski:pr-inline-split_at_unchecked, r=Mark-Simulacrum core: make `str::split_at_unchecked()` inline This PR adds `#[inline]` to the method `str::split_at_unchecked()`. This is done for two reasons: 1. The method is tiny, e.g. on AMD-64 (<https://godbolt.org/z/ba68fdfxn>): ```asm movq %rdi, %rax subq %rcx, %rdx movq %rsi, (%rdi) addq %rcx, %rsi movq %rcx, 8(%rdi) movq %rsi, 16(%rdi) movq %rdx, 24(%rdi) retq ``` 2. More importantly, inlining the method enables further automatic optimizations. E.g. if you split at index 3, then in the compiler (rustc, llvm or both) knows that this code cannot fail, and the panicking path is omitted in the generated code: ```rust pub fn punctuation(i: &str) -> Result<(), ()> { const THREE_CHARS: &[[u8; 3]] = &[*b"<<=", *b">>=", *b"...", *b"..="]; if let Some((head, _)) = i.split_at_checked(3) && THREE_CHARS.contains(&head.as_bytes().try_into().unwrap()) { Ok(()) } else { Err(()) } } ``` <details> <summary>Without PR</summary> <https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=0234de8158f467eebd73286f20d6e27a> ```asm playground::punctuation: subq $40, %rsp movq %rsi, %rdx movq %rdi, %rsi movb $1, %al cmpq $3, %rdx ja .LBB2_2 je .LBB2_3 .LBB2_11: addq $40, %rsp retq .LBB2_2: cmpb $-64, 3(%rsi) jl .LBB2_11 .LBB2_3: leaq 8(%rsp), %rdi movl $3, %ecx callq *core::str::<impl str>::split_at_unchecked@GOTPCREL(%rip) movq 8(%rsp), %rcx movb $1, %al testq %rcx, %rcx je .LBB2_11 cmpq $3, 16(%rsp) jne .LBB2_12 movzwl (%rcx), %edx movzbl 2(%rcx), %ecx shll $16, %ecx orl %edx, %ecx cmpl $4013115, %ecx jg .LBB2_8 cmpl $3026478, %ecx je .LBB2_10 cmpl $4009518, %ecx je .LBB2_10 jmp .LBB2_11 .LBB2_8: cmpl $4013630, %ecx je .LBB2_10 cmpl $4013116, %ecx jne .LBB2_11 .LBB2_10: xorl %eax, %eax addq $40, %rsp retq .LBB2_12: leaq .Lanon.d98a7fbb86d10a97c24516e267466134.2(%rip), %rdi leaq .Lanon.d98a7fbb86d10a97c24516e267466134.1(%rip), %rcx leaq .Lanon.d98a7fbb86d10a97c24516e267466134.6(%rip), %r8 leaq 7(%rsp), %rdx movl $43, %esi callq *core::result::unwrap_failed@GOTPCREL(%rip) ``` </details> <details> <summary>With PR</summary> <https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=5d4058c79ce0f6cb1a434190427d2055> ```asm playground::punctuation: movb $1, %al cmpq $3, %rsi ja .LBB0_2 je .LBB0_3 .LBB0_9: retq .LBB0_2: cmpb $-64, 3(%rdi) jl .LBB0_9 .LBB0_3: movzwl (%rdi), %eax movzbl 2(%rdi), %ecx shll $16, %ecx orl %eax, %ecx movb $1, %al cmpl $4013115, %ecx jg .LBB0_6 cmpl $3026478, %ecx je .LBB0_8 cmpl $4009518, %ecx je .LBB0_8 jmp .LBB0_9 .LBB0_6: cmpl $4013630, %ecx je .LBB0_8 cmpl $4013116, %ecx jne .LBB0_9 .LBB0_8: xorl %eax, %eax retq ``` </details>

rustbot assigned Mark-Simulacrum Jul 13, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 13, 2025

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 Jul 15, 2025

jhpratt mentioned this pull request Jul 15, 2025

Rollup of 16 pull requests #143951

Closed

samueltardieu mentioned this pull request Jul 15, 2025

Rollup of 13 pull requests #143958

Merged

bors merged commit 460a627 into rust-lang:master Jul 15, 2025
11 checks passed

rustbot added this to the 1.90.0 milestone Jul 15, 2025

Kijewski deleted the pr-inline-split_at_unchecked branch July 15, 2025 15:05

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core: make `str::split_at_unchecked()` inline #143907

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