[rustc_span][perf] Hoist lookup sorted by words out of the loop.

[ttsugriy]

@lqd commented on #114351 asking if sort_by_words(lookup) is computed repeatedly. I was assuming that rustc should have no difficulties to hoist it automatically outside of the loop to avoid repeated pure computation, but according to
https://godbolt.org/z/frs8Kj1rq it seems like I was wrong:
original version seems to have 2 calls per loop iteration

.LBB16_3:
        mov     rbx, qword ptr [r13]
        mov     r14, qword ptr [r13 + 8]
        lea     rdi, [rsp + 40]
        mov     rsi, rbx
        mov     rdx, r14
        call    example::sort_by_words
        lea     rdi, [rsp + 64]
        mov     rsi, qword ptr [rsp + 8]
        mov     rdx, qword ptr [rsp + 16]
        call    example::sort_by_words
        mov     rdi, qword ptr [rsp + 40]
        mov     rdx, qword ptr [rsp + 56]
        mov     rsi, qword ptr [rsp + 64]
        cmp     rdx, qword ptr [rsp + 80]
        mov     qword ptr [rsp + 32], rdi
        mov     qword ptr [rsp + 24], rsi
        jne     .LBB16_5
        call    qword ptr [rip + bcmp@GOTPCREL]
        test    eax, eax
        sete    al
        mov     dword ptr [rsp + 4], eax
        mov     rsi, qword ptr [rsp + 72]
        test    rsi, rsi
        jne     .LBB16_8
        jmp     .LBB16_9

but the manually hoisted version just 1:

.LBB16_3:
        mov     r13, qword ptr [r15]
        mov     r14, qword ptr [r15 + 8]
        lea     rdi, [rsp + 64]
        mov     rsi, r13
        mov     rdx, r14
        call    example::sort_by_words
        mov     rdi, qword ptr [rsp + 64]
        mov     rdx, qword ptr [rsp + 16]
        cmp     qword ptr [rsp + 80], rdx
        mov     qword ptr [rsp + 32], rdi
        jne     .LBB16_5
        mov     rsi, qword ptr [rsp + 8]
        call    qword ptr [rip + bcmp@GOTPCREL]
        test    eax, eax
        sete    bpl
        mov     rsi, qword ptr [rsp + 72]
        test    rsi, rsi
        jne     .LBB16_8
        jmp     .LBB16_9

This code is probably not very hot, but there is no reason to leave such a low hanging fruit.

[rustbot]

r? @eholk

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[lqd]

Great, thank you!

As we said on the other PR, this code is quite cold, and IIUC only used in diagnostics: there shouldn't be any perf impact, so let's roll it up.

r? lqd @bors r+ rollup

[bors]

📌 Commit 6ae2677 has been approved by lqd

It is now in the queue for this repository.