cmd/go: build the cgo artifacts for each package in parallel

[petermattis]

The lack of parallelization for compilations within a single package doesn't matter too much when building go files, but does affect cgo compilation. I have a package with about 32k lines of (generated) C++. Compiling it serially takes ~10.5s. A small hack to cmd/go/build.go to compile C/C++/Obj-C files in parallel brings this down to ~3.7s.

The patch I hacked together to parallelize cgo compilation work is ~50 lines of deltas and is mostly contained in builder.cgo().

PS The compilation times above were measured using go 1.4.1 on Mac OS X, but this code hasn't changed at head and similar compilation times can be seen there.

[robpike]

Seems reasonable to me. Why not send the patch?

[petermattis]

Done: https://go-review.googlesource.com/#/c/4931/

[gopherbot]

CL https://golang.org/cl/4931 mentions this issue.

[petermattis]

I'm curious if there is any plan to parallelize cgo compilation for go1.7. The finer-grained work scheduling for go tool actions mentioned in https://go-review.googlesource.com/#/c/4931/ would be great if it ever happens, though it doesn't look like there is any movement on that front.

[bcmills]

Contrast #21605. Are there any similar ordering issues for other languages?

[ianlancetaylor]

Currently we only support C, C++, and Fortran, and C and C++ have no ordering issues.

I'm not aware of ordering issues for any other language.