runtime: use %gs:0x30 for TLS on macOS

[rsc]

Per private email discussion with Anthony Chivetta of Apple, Go should stop guessing pthread offsets and instead hard-code 0x30 off %gs as the g register. That location is already reserved for WINE and Win64, so as long as we're not trying to run Win64 code linked into macOS binaries, all will be well.

[randall77]

Q: 0x30 for amd64? What about 386, arm, arm64?

Is there any way we can get Apple to commit this offset for Go? I understand it is currently reserved, but if they are willing to give it out to us they might also give it out to some other Foo project. Then a binary with both Go and Foo in it wouldn't work.

[ianlancetaylor]

Apple has committed that we can use the 0x30 offset for amd64, on the assumption that we will never link Go and Windows code together while running on Darwin.

I sent an e-mail asking about the other platforms.

[gopherbot]

Change https://golang.org/cl/105975 mentions this issue: runtime: use fixed TLS offsets on darwin/amd64 and darwin/386

[gopherbot]

Change https://golang.org/cl/106156 mentions this issue: runtime: use fixed TLS offsets on darwin/amd64 and darwin/386

[toffaletti]

Any plans for this change to make it into 1.10.x?

[randall77]

No. Is there a reason you want it?