Reason to use different address space in gollvm

[Kavindu Gimhan Zoysa]

Hi all,

Can you please explain the reason to use different address space when GC is enabled as shown in the below line?

https://go.googlesource.com/gollvm/+/refs/heads/master/driver/CompileGo.cpp#559

Thank you,

Kavindu

[Than McIntosh]

See https://llvm.org/docs/Statepoints.html for more on this. 

Thanks, Than

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[Kavindu Gimhan Zoysa]

Hi,

Regarding address spaces, I saw that there is a pass to remove address space casts. But if I generate IR using `llvm-goc -dump-ir -enable-gc=1 test.go` for the following go file, I still can see the addrspacecast instruction. What is the reason to see that addrspacecast? I feel that pass is not running. Can you help me to understand this problem?

package main

func main() {

   _ = stackIt2()

}

//go:noinline

func stackIt2() *int {

   y := 2

   res := y * 2

   return &res

}

Thank you,

Kavindu

[Than McIntosh]

Hi again,

Sorry for the delay, I have been offline for a while.

You wrote:
>Regarding address spaces, I saw that there is a pass to remove address space casts. But if I generate IR using `llvm-goc -dump-ir -enable-gc=1 test.go` for the following go file, I still can see the addrspacecast instruction. What is the reason to see that addrspacecast? I feel that pass is not running. Can you help me to understand this problem?

A couple of things on this.

First, the "-dump-ir" option to llvm-goc shows the LLVM IR after the front end and bridge are done with creating it -- at this point no LLVM passes will have been run, so you would not expect to see any address space casts removed.  Probably what you want here is "-emit-llvm", which will show LLVM IR after all the various module and function passes have run (before the switch to machine-dependent IR). Example:

 $ llvm-goc -S -emit-llvm -enable-gc=1 test.go
 $ cat test.ll
 ...
 $

A second thing is that the cast-removal pass you mention isn't removing every cast, only casts on initialized global variables fed with constants. If you look at this source code

https://go.googlesource.com/gollvm/+/38bada572789a19cf881201c923e6b56ed32ae53/passes/RemoveAddrSpace.cpp#95

you can see that it's visiting only initialized globals. If you want to examples of cast removal, try running your compile on a larger and more interesting test case, like

  https://golang.org/test/method5.go

In general if you are unsure about whether a pass is running and/or what the effects of the pass are, there are developer options you can try (note: available only in a pure debug build) that can show you more. Example:

 # Dump LLVM IR before a pass
 $ ./bin/llvm-goc -O2 -c -emit-llvm -S -enable-gc=1 -mllvm -print-before=remove-addrspacecast test.go 1> before.dump.txt 2>&1

 # Dump LLVM IR after a pass
 $ ./bin/llvm-goc -O2 -c -emit-llvm -S -enable-gc=1 -mllvm -print-after=remove-addrspacecast test.go 1> after.dump.txt 2>&1

Hope this helps.

Thanks, Than

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[Kavindu Gimhan Zoysa]

Hi Than,

Thank you a lot for sharing this valuable information. It helps me a lot to understand this.

Thank you,

Kavindu