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[rustc_span][perf] Hoist lookup sorted by words out of the loop. #114395

Merged
merged 1 commit into from
Aug 3, 2023
Merged

[rustc_span][perf] Hoist lookup sorted by words out of the loop. #114395

merged 1 commit into from
Aug 3, 2023

Commits on Aug 3, 2023

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

    @lqd commented on rust-lang#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.
    @ttsugriy
    ttsugriy committed Aug 3, 2023
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