doc/asm: clarify that get_tls only works in runtime package

[PureWhiteWu]

What version of Go are you using (go version)?

$ go version
go version go1.12.7 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/purewhite/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/purewhite/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12.7/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12.7/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/gr/t7x82cps0w563940rhk76_400000gn/T/go-build992278676=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I'm studying go asm, and want to write a demo to get the *g.

According to the doc, I should be able to get *g using code like this:

#include "textflag.h"
#include "go_asm.h"

TEXT ·getg(SB),NOSPLIT,$0-8
    get_tls(CX)
    MOVQ    g(CX), AX
    MOVQ    AX, ret+0(FP)
    RET

What did you expect to see?

Successfully get current g's address.

What did you see instead?

$ go build .
# local/study/getg
./getg.s:5: unrecognized instruction "get_tls"
./getg.s:6: expected pseudo-register; found CX
./getg.s:7: expected pseudo-register; found AX
asm: assembly of ./getg.s failed

[agnivade]

It seems to me that macro moved to a runtime header file go_tls.h. You might have to do #include "../../src/runtime/go_tls.h".

But I will defer to some experts to confirm if my diagnosis is correct @ianlancetaylor @randall77

[PureWhiteWu]

It seems to me that macro moved to a runtime header file go_tls.h. You might have to do #include "../../src/runtime/go_tls.h".

But I will defer to some experts to confirm if my diagnosis is correct @ianlancetaylor @randall77

Hi, thanks for your reply.
After doing this, this program can be compiled, but if I want to get the m of g according to the doc, the problem still exists.

Code like this:

#include "textflag.h"
#include "go_asm.h"
#include "../../src/runtime/go_tls.h"

TEXT ·getm(SB),NOSPLIT,$0-8
    get_tls(CX)
    MOVQ    g(CX), AX
    MOVQ    g_m(AX), BX
    MOVQ    BX, ret+0(FP)
    RET

outputs:

$ go build .
# local/study/getm
./getm.s:8: expected pseudo-register; found AX
asm: assembly of ./getm.s failed

[cherrymui]

Only the runtime package's go_asm.h defines the offsets of the runtime data structures, like g_m. When you run go build in local/study/getg, go_asm.h will contain the offsets of the local/study/getg package, not the runtime package.

[ianlancetaylor]

The asm documentation should be clarified to explain that this code only works within the runtime package itself.

We do not want to support this outside of the runtime package.

[PureWhiteWu]

Hi, I've submitted a pr #33069 , please take a look.
Thanks!

[gopherbot]

Change https://golang.org/cl/185917 mentions this issue: doc/asm: document go_asm.h only works in the runtime package

[PureWhiteWu]

@ianlancetaylor Hi, I think this is a fix for the doc, and should be released asap. Therefore, maybe the milestone should be go1.13 instead?
Just an inquiry.

[ianlancetaylor]

The milestone doesn't mean that it has to wait for 1.14. It only means that it should be resolved before 1.14 comes out. I'm fine with getting a fix into 1.13, but I would like @robpike to take a look at the CL (if he hasn't already).