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cmd/compile: suboptimal compilation of 64-bit OMOD for 386/arm #15010

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mdempsky opened this issue Mar 29, 2016 · 3 comments
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cmd/compile: suboptimal compilation of 64-bit OMOD for 386/arm #15010

mdempsky opened this issue Mar 29, 2016 · 3 comments
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@mdempsky
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Compiling:

package p

func f(t int64) int64 {
        return t%1e9
}

for 386 or arm compiles as

func f(t int64) int64 {
        return t - runtime.int64div(t, 1e9) * 1e9
}

even though we're not able to do anything to optimize the * 1e9 multiplication. We might as well just call runtime.int64mod.
@minux
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minux commented Mar 29, 2016 via email

@mdempsky
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@minux Looking at https://github.com/golang/go/blob/master/src/runtime/vlrt.go, it seems like there should be. int64div and int64mod both just call a common dodiv function that calculates (quotient, remainder), and then they return the appropriate value. So they seem like they should be roughly identical in cost.

The int64div form also costs 48 extra bytes of 386 instructions and 52 extra bytes of arm instructions compared to just calling int64mod.

@bradfitz bradfitz added this to the Unplanned milestone Apr 7, 2016
@ALTree
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ALTree commented Dec 18, 2017

Both go1.9 and go1.10 generate a int64mod call on 386 and arm, so I'm assuming this is fixed.

@ALTree ALTree closed this as completed Dec 18, 2017
@golang golang locked and limited conversation to collaborators Dec 18, 2018
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