labeled loop vs for statement inlining

[Ariel Mashraki]

Hello,

Running `go build -gcflags -m` on the given code below will produce:

   main.go:3: can inline f1

   main.go:24: inlining call to f1

Can someone please explain why doesn't the f2 function get inlined ?

Thanks

package main

func f1() int {

       i := 0

loop:

       if i > 10 {

               return i

       }

       i++

       goto loop

}

func f2() int {

       i := 0

       for {

               if i > 10 {

                       return i

               }

               i++

       }

}

func main() {

       f1()

       f2()

}

[Chris Manghane]

I can't explain exactly because my explanation is likely very flawed, but the logic you are looking for is in https://github.com/golang/go/blob/320ddcf8344beb1c322f3a7f0a251eea5e442a10/src/cmd/compile/internal/gc/inl.go#L186. Basically, labeled loops are not considered "hairy" by the compiler and can be inlined.

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[Peter Mogensen]

On 2016-08-29 16:59, 'Chris Manghane' via golang-nuts wrote:
> I can't explain exactly because my explanation is likely very flawed,
> but the logic you are looking for is
> in https://github.com/golang/go/blob/320ddcf8344beb1c322f3a7f0a251eea5e442a10/src/cmd/compile/internal/gc/inl.go#L186.
> Basically, labeled loops are not considered "hairy" by the compiler and
> can be inlined.

On the other hand, f1() uses goto to jump back in previous execution
path. *shudder*

... hopefully not something to be encouraged.

/Peter

[Sam Whited]

On Mon, Aug 29, 2016 at 9:40 AM, Ariel Mashraki
<ariel.m...@ironsrc.com> wrote:
> Can someone please explain why doesn't the f2 function get inlined ?

If you build with m=2 (go build -gcflags -m=2) it will spit out a reason:

> cannot inline f2: unhandled op for

(closures with "for" in them apparently can't be inlined)

—Sam



--
Sam Whited
pub 4096R/54083AE104EA7AD3

[Ariel Mashraki]

I didn't know it has more levels. thanks 

[Sokolov Yura]

But still question is interesting: why decision is based on source code and not on instructions size? Both functions likely to produce same assembler code.

[Dave Cheney]

Inlining is conservative. As well as the size of the code being inlined, current 40 units (unit has not scale), some operations are not inlined, as they are considered hairy. switch used to be considered hairy, 1.7 fixed that, for is still hairy.