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cmd/compile: reuse temporaries introduced by compiler #21557

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randall77 opened this issue Aug 22, 2017 · 3 comments

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commented Aug 22, 2017

package main

type T struct {
	p *int
	i int
}

func f(m map[T]int) {
	m[T{nil, 1}] = 1
	m[T{nil, 2}] = 2
}

If you look at the disassembly of f, we use an autotmp to pass the key to mapassign. Each call to mapassign uses its own temporary. We should reuse these temporaries - they are live only for the duration of the mapassign call.

This situation gets particularly painful for large map initializations in init() functions. There could be thousands of mapassign calls, and each one gets its own temporary. That means really large stack frames and correspondingly large live pointer maps. In fact the live pointer map size is quadratic in source size, as it is proportional to both the number of temporaries and the number of safe points.

There are other places we use temporaries like this, mostly introduced in order.go. We should reuse temps for as many as we can.

Note: I use the weird key in the example to avoid the _fast version for int32, int64, and string. _fast versions don't use an autotmp.

Related: #19751

@josharian @mdempsky @martisch

@randall77 randall77 added this to the Go1.10 milestone Aug 22, 2017

@randall77 randall77 added the ToolSpeed label Aug 22, 2017

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commented Aug 23, 2017

I tried this when I was working on #19751 and found that it didn't help all that much; maybe I did it wrong though. It was also a pain to implement, due to how walk and sinit ping-pong back and forth--there was no good place to put state, but again, I'd be delighted to have been wrong about this.

@mdempsky mdempsky modified the milestones: Go1.10, Go1.11 Nov 29, 2017

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commented Nov 29, 2017

Still desirable, but too late for 1.10.

@bradfitz bradfitz modified the milestones: Go1.11, Go1.12 Jun 19, 2018

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commented Nov 17, 2018

This was fixed with CL 140301.

@randall77 randall77 closed this Nov 17, 2018

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