Small tweaks in ToOwned::clone_into

[cuviper]

• <[T]>::clone_into is slightly more optimized.
• CStr::clone_into is new, letting it reuse its allocation.
• OsStr::clone_into now forwards to the underlying slice/Vec.

[rust-highfive]

r? @dtolnay

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

[cuviper]

To justify the slice optimization, here's output from compiler explorer. They are nearly the same, but note the smaller prologue and epilogue in the new split_at version, apparently indicating less register pressure if nothing else.

example::clone_into_orig:
        push    r15
        push    r14
        push    r12
        push    rbx
        push    rax
        mov     r14, rdx
        mov     r12, rsi
        mov     r15, rdi
        mov     rbx, qword ptr [rdx + 16]
        cmp     rbx, rsi
        jb      .LBB1_2
        mov     qword ptr [r14 + 16], r12
        mov     rbx, r12
.LBB1_2:
        test    rbx, rbx
        je      .LBB1_4
        mov     rdi, qword ptr [r14]
        lea     rdx, [4*rbx]
        mov     rsi, r15
        call    qword ptr [rip + memcpy@GOTPCREL]
.LBB1_4:
        lea     rsi, [r15 + 4*rbx]
        sub     r12, rbx
        mov     rdi, r14
        mov     rdx, r12
        add     rsp, 8
        pop     rbx
        pop     r12
        pop     r14
        pop     r15
        jmp     alloc::vec::Vec<T>::extend_from_slice

example::clone_into_split:
        push    r15
        push    r14
        push    rbx
        mov     r14, rdx
        mov     rbx, rsi
        mov     rsi, rdi
        mov     rdx, qword ptr [rdx + 16]
        cmp     rdx, rbx
        jb      .LBB2_2
        mov     qword ptr [r14 + 16], rbx
        mov     rdx, rbx
.LBB2_2:
        lea     r15, [rsi + 4*rdx]
        sub     rbx, rdx
        test    rdx, rdx
        je      .LBB2_4
        mov     rdi, qword ptr [r14]
        shl     rdx, 2
        call    qword ptr [rip + memcpy@GOTPCREL]
.LBB2_4:
        mov     rdi, r14
        mov     rsi, r15
        mov     rdx, rbx
        pop     rbx
        pop     r14
        pop     r15
        jmp     alloc::vec::Vec<T>::extend_from_slice

[Centril]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit 28791f6 with merge c462c8500772945a0f68a265dc1f8decf6b42e99...

[bors]

☀️ Try build successful - checks-azure
Build commit: c462c8500772945a0f68a265dc1f8decf6b42e99 (c462c8500772945a0f68a265dc1f8decf6b42e99)

[rust-timer]

Queued c462c8500772945a0f68a265dc1f8decf6b42e99 with parent 1057dc9, future comparison URL.

[rust-timer]

Finished benchmarking try commit c462c8500772945a0f68a265dc1f8decf6b42e99, comparison URL.

[cuviper]

Seems neutral to compiler performance. I think clone_into and clone_from are not used very often.

[dtolnay]

Nice!

[dtolnay]

@bors r+

[bors]

📌 Commit 28791f6 has been approved by dtolnay

[Dylan-DPC-zz]

Failed in rollup #70237 (comment)

@bors r-

[dtolnay]

The new test in c_str.rs fails on Windows x86_64-msvc-1:

---- ffi::c_str::tests::test_c_str_clone_into stdout ----
thread '<unnamed>' panicked at 'assertion failed: `(left == right)`
  left: `0x210c5e05800`,
 right: `0x210c5e05830`', src\libstd\ffi\c_str.rs:1527:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    ffi::c_str::tests::test_c_str_clone_into

test result: FAILED. 743 passed; 1 failed; 2 ignored; 0 measured; 0 filtered out

[cuviper]

Hmm, maybe that test is assuming too much about the allocator. I could try it with equal lengths, but I'll confirm on Windows before I resubmit.

[dtolnay]

It could just be there is #[cfg(windows)] codepath that is missing a clone_into function.

[cuviper]

OK, I was able to confirm that failure on Windows, and now it passes when the strings are equal length.

[dtolnay]

@bors r+

[bors]

📌 Commit f854070 has been approved by dtolnay

[bors]

⌛ Testing commit f854070 with merge 83177ff73c809ab897cbc450cf19aee95d48f94a...

[Dylan-DPC-zz]

@bors retry (yield)

[bors]

⌛ Testing commit f854070 with merge 42abbd8...