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cmd/compile: loop condition loop-invariant expression not lifted out of the loop #47919

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prattmic opened this issue Aug 23, 2021 · 1 comment
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cmd/compile: loop condition loop-invariant expression not lifted out of the loop #47919

prattmic opened this issue Aug 23, 2021 · 1 comment
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@prattmic
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package main

//go:noinline
func nop() {
}

//go:noinline
func loop(n, d int) {
	for i := 0; i < n*d; i++ {
		nop()
	}
}

func main() {
	loop(32, 16)
}

https://play.golang.org/p/RxXnFIJqtcj

loop compiles to:

00000 (29) TEXT "".loop(SB), ABIInternal
00001 (29) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
00002 (29) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
00003 (29) FUNCDATA $5, "".loop.arginfo1(SB)
00004 (29) MOVQ AX, "".n(SP)
00005 (29) MOVQ BX, "".d+8(SP)
00006 (29) XORL CX, CX
00007 (30) JMP 15
00008 (30) MOVQ CX, "".i-8(SP)
00009 (+31) PCDATA $1, $0
00010 (+31) CALL "".nop(SB)
00011 (+30) MOVQ "".i-8(SP), AX
00012 (30) LEAQ 1(AX), CX
00013 (30) MOVQ "".n(SP), AX
00014 (30) MOVQ "".d+8(SP), BX
00015 (+30) IMULQ BX, AX
00016 (30) CMPQ CX, AX
00017 (30) JLT 8
00018 (33) PCDATA $1, $-1
00019 (33) RET

PCs 13-15 (IMULQ and associated loads) could be hoisted out of the loop to avoid re-computing the product on every iteration.

The original motivation of this issue is from an application I was profiling with a hot path something like:

func process(s []float64, d int) {
  for i := 0; i < len(s)/d; i++ {
    ... = asmVectorOps(s[i*d:(i+1)*d])
  }
}

The custom assembly used most of the time, but 5% of cycles still went to the division of len(s)/d on every iteration. I thought this was an optimization breaking down because it thought len(s) might change, but it turns out not to depend on slices at all.

cc @randall77 @dr2chase @mdempsky

Potentially related to or duplicate of #15808.
@prattmic prattmic added this to the Backlog milestone Aug 23, 2021
@prattmic
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After reading #15808 more closely, I'd say this is a duplicate.

@golang golang locked and limited conversation to collaborators Aug 23, 2022
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