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cmd/compile: SSA runtime performance regression on function with multiple idiv instructions #16061

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ALTree opened this issue Jun 14, 2016 · 2 comments
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cmd/compile: SSA runtime performance regression on function with multiple idiv instructions #16061

ALTree opened this issue Jun 14, 2016 · 2 comments
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FrozenDueToAge Performance
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Go1.8

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@ALTree
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ALTree commented Jun 14, 2016 •
edited 

$ gotip version
go version devel +53242e4 Tue Jun 14 05:17:57 2016 +0000 linux/amd64

code:

package foo

import "testing"

func maxRem(a int32) int32 {
    maxR := int32(0)
    m := a * a
    r1, r2 := a+1, a-1
    for n := int32(1); n <= a*a; n++ {
        r1 = (r1 * (a + 1)) % m
        r2 = (r2 * (a - 1)) % m
        if r := (r1 + r2) % m; maxR < r {
            maxR = r
        }
    }
    return maxR
}

var sink int32

func BenchmarkMaxRem(b *testing.B) {
    for n := 0; n < b.N; n++ {
        sink = maxRem(100)
    }
}

go1.6 vs tip:

name      old time/op  new time/op  delta
MaxRem-4   116µs ± 1%   134µs ± 2%  +15.48%  (p=0.000 n=8+8)

same function with int64 is also slower, by ~5%.
@ianlancetaylor ianlancetaylor added this to the Go1.8 milestone Jun 14, 2016
@randall77
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Looks like the SSA code does a bunch of spilling to the stack where the legacy compiler doesn't. The divide instructions require values in specific registers. Given two subsequent divide instructions, the result of the first needs to be moved somewhere else before the second is issued. SSA uses a stack slot for that, whereas the legacy compiler uses a register.
This will be a bit tricky to fix, but it is possible. 1.8 milestone sounds right.

@ALTree ALTree changed the title cmd/compile: SSA runtime performance regression on 32bit arithmetic cmd/compile: SSA runtime performance regression on function with multiple idiv instructions Jun 14, 2016
@ALTree ALTree mentioned this issue Jun 19, 2016
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@randall77 randall77 mentioned this issue Sep 23, 2016
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@gopherbot
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CL https://golang.org/cl/29732 mentions this issue.

@golang golang locked and limited conversation to collaborators Sep 27, 2017
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