Re: [PATCH 00/13] Go closures, libffi, and the static chain

[Dominik Vogt <vogt@linux.vnet.ibm.com>]

On Fri, Oct 10, 2014 at 01:42:40PM -0700, Richard Henderson wrote:
> The background here is my thread from last week[1], and Ian's reply[2],
> wherein he rightly points out that not needing to play games with
> mmap in order to implement closures for Go is a strong reason to
> continue using custom code within libgo.
> 
> While that thread did have a go at implementing that custom code for
> aarch64, I still think that replicating libffi's calling convention
> knowledge for every interesting target is a mistake.
> 
> So instead I thought about how I'd add some support for Go directly
> into libffi.
...
> But the comment immediately before __go_set_closure itself says
> that it would be better to use the static chain register.
...
> Before I go too much farther down this road, I wanted to get some
> feedback.  FWIW, a complete tree can be found at [4].
...
> [4] git://github.com/rth7680/gcc.git rth/go-closure

1)

On s390x, the static chain register cannot be used for passing the
Go closure pointer to a function:  According to the Abi, the
dynamic linker is allowed to destroy the contents of r0 (static
chain register) eventually causing a crash if libgo is linked
dynamically.  The assumption that the static chain register can be
used to pass information to a function is wrong for s390x.

2)

With this branch, the reflection tests on amd64 crash:

  $ cd <gcc source tree>/build
  # build gcc
  $ cd <targetdir>/libgo
  $ make reflect/check

  -->

-- snip --
Aborted

reflect.call
	../../../libgo/runtime/go-reflect-call.c:216
reflect.call.N13_reflect.Value
	GCCDIR/build-go-closure/x86_64-unknown-linux-gnu/libgo/gotest30365/test/value.go:579
reflect.Call.N13_reflect.Value
	GCCDIR/build-go-closure/x86_64-unknown-linux-gnu/libgo/gotest30365/test/value.go:412
reflect_test.TestCallWithStruct
	GCCDIR/build-go-closure/x86_64-unknown-linux-gnu/libgo/gotest30365/test/all_test.go:1490
testing.tRunner
	../../../libgo/go/testing/testing.go:422

goroutine 16 [chan receive]:
testing.RunTests
	../../../libgo/go/testing/testing.go:505
testing.Main
	../../../libgo/go/testing/testing.go:435
main.main
	GCCDIR/build-go-closure/x86_64-unknown-linux-gnu/libgo/gotest30365/test/_testmain.go:124
created by main
	../../../libgo/runtime/go-main.c:42

goroutine 18 [finalizer wait]:
created by runtime_createfing
	../../../libgo/runtime/mgc0.c:2572

goroutine 53 [sleep]:
reflect_test.selectWatcher
	GCCDIR/build-go-closure/x86_64-unknown-linux-gnu/libgo/gotest30365/test/all_test.go:1377
created by reflect_test.$nested2
	GCCDIR/build-go-closure/x86_64-unknown-linux-gnu/libgo/gotest30365/test/all_test.go:1107
FAIL: reflect
make: *** [reflect/check] Error 1
-- snip --

Ciao

Dominik ^_^  ^_^

-- 

Dominik Vogt
IBM Germany