make.bash: CC_FOR_${GOOS}_${GOARCH} not documented

[rittneje]

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

$ go version
1.17.7

Does this issue reproduce with the latest release?

Yes.

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

Relevant env vars (darwin):

CC=gcc
CC_FOR_TARGET=clang
CXX_FOR_TARGET=clang++
GOOS=darwin
GOARCH=amd64
GOHOSTOS=linux
GOHOSTARCH=amd64

Relevant env vars (windows):

CC=gcc
CC_FOR_TARGET=gcc
CXX_FOR_TARGET=g++
GOOS=windows
GOARCH=amd64
GOHOSTOS=linux
GOHOSTARCH=amd64

What did you do?

Ran make.bash.

What did you expect to see?

I expected it to produce a compiler that runs on darwin with clang as the default CC, and a compiler that runs on windows with gcc as the default CC.

What did you see instead?

For GOOS=darwin it tries to use CC_FOR_TARGET, and then claims that CC isn't set.

go tool dist: cannot invoke C compiler "clang": exec: "clang": executable file not found in $PATH

Go needs a system C compiler for use with cgo.
To set a C compiler, set CC=the-compiler.
To disable cgo, set CGO_ENABLED=0.

For GOOS=windows it tries to pass Windows-only flags to the Linux C compiler.

# runtime/cgo
gcc: error: unrecognized command-line option '-mthreads'; did you mean '-pthread'?
go tool dist: FAILED: /opt/go/pkg/tool/linux_amd64/go_bootstrap install -gcflags=all= -ldflags=all=-s -w std cmd: exit status 2

If I try with CGO_ENABLED=0, then the resulting compiler has CGO_ENABLED=0 by default when run on the target, which is undesirable. (#51305)

The intent of passing CC_FOR_TARGET and CXX_FOR_TARGET is to burn these default values into the resulting compiler, not to actually use them while building the compiler. For example, the pre-canned macos toolchain knows to use clang by default instead of gcc, and we want to replicate this.

[ianlancetaylor]

The CC_FOR_TARGET environment variable should be set to a C compiler that runs on the host machine (the machine on which you are running make.bash) and generates code that runs on the target machine (the machine described by the GOOS/GOARCH settings).

You are trying to set it do something different, but that's not what it's for. To do what you want, set an environment variable like CC_FOR_DARWIN_AMD64

[rittneje]

You are trying to set it do something different, but that's not what it's for. To do what you want, set an environment variable like CC_FOR_DARWIN_AMD64

Oh, yes, that is the one I was thinking of. It is currently missing from the make.bash documentation, but is mentioned in the cgo documentation. Can it be added? https://github.com/golang/go/blob/master/src/make.bash

[ianlancetaylor]

Sure, want to send a patch?

[gopherbot]

Change https://go.dev/cl/388176 mentions this issue: make.bash: add document of CC_FOR_${GOOS}_${GOARCH}