cmd/link: bad relocations on darwin

[dvyukov]

parent: 19024:e2f19c96b7e2 tip, darwin/amd64

$ hg clpatch 55100044
$ ./make.bash
$ go run -race pkg/runtime/race/testdata/cgo_test_main.go

crashes with random failures depending on revision number.
The root cause is that runtime.racefuncenter calls to a random address instead of
__tsan_func_enter.

If 
    CALL    __tsan_func_enter(SB)
is replaced with:
    MOVQ    $__tsan_func_enter(SB), AX
    CALL    AX
then it works correctly.

The difference that I see between these versions is:
machoreloc1 rs->name=__tsan_read r->type=120
machoreloc1 rs->name=__tsan_read r->type=247

120 refers to D_ADDR and works, 247 refers to D_PCREL and does not work.
Not sure where to dig further.

[dvyukov]

Comment 1:

Russ, can you help with this? It seems to be your code.

[rsc]

Comment 2:

My suggestion is to keep using that workaround for now.
We can look at this for Go 1.4, but I don't want to make linker changes now, and you've
got a way to make it work.

Labels changed: added release-go1.4, removed release-go1.3.

Owner changed to ---.

Status changed to Accepted.

[rsc]

Comment 3:

Labels changed: added release-go1.5, removed release-go1.4.

[rsc]

It looks to me like later refactoring of the code has removed even the possibility of writing the alternate form, so there's not much left to do here. I don't know why the PC-relative instruction failed, but it has to do with relocations and linking and probably magic indirections, so it's not surprising there is a detail wrong somewhere.