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runtime: always set up TLS on all G-register architectures? #35853
Currently, on G-register architectures (i.e. all non-x86), we set up TLS only when cgo is used, by libc. The reason is probably that the G register is reserved in Go code and so it should always be valid. However, there are more and more cases where we call external functions even in a non-cgo program, for example, VDSO, and libc calls on darwin. These external functions could potentially clobbers the G register temporarily. If a signal is received during the execution of an external function, we cannot directly use the G register's content as the G. This has caused a number of problems. Currently we play various tricks and workarounds. See e.g. #32912, #34391, #35800.
Maybe we should consider initializing TLS ourselves in non-cgo programs, and always save/restore G in TLS across external calls? This way, we unify the cgo and non-cgo code paths, and can remove various workarounds and maybe potential problems.