cmd/objdump: confusing disassembly of arm64 BL instruction

[kevinGC]

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

$ go version
go version go1.18.2 linux/arm64

This also happens in bazel with go version 1.17.6.

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
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/home/parallels/.cache/go-build"
GOENV="/home/parallels/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/parallels/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/parallels/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_arm64"
GOVCS=""
GOVERSION="go1.18.2"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/parallels/gvisor/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build4026508879=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Ran go tool objdump on a binary.

What did you expect to see?

There are many lines that look like:

  iptables.go:320       0x211214                97f9b198                CALL -413288(PC)

However, the offset displayed should be -1653152. Objdump appears to be incorrectly parsing the offset from the instruction 0x97f9b198.

What did you see instead?

-413288. This is because the 2 trailing 0 bits described by the arm documentation appear not to be accounted for.

[cherrymui]

Thanks for the report. In Go assembly (at least as input for the Go assembler), the n(PC) relative addressing counts the number of instructions, instead of the byte offset. So to some extent this makes sense. But I agree that this is somewhat confusing. Perhaps we should just compute the target's absolute address, instead of relative offset.

[kevinGC]

Ah that makes sense now -- counting instruction offset is the same as (byte_offset >> 2). In that case I don't have a strong opinion anymore, but I agree that an absolute address would disambiguate.

[mengzhuo]

cc @golang/arm