all: remove 'extern register M *m' from runtime The runtime has historically held two dedicated values g (current goroutine) and m (current thread) in 'extern register' slots (TLS on x86, real registers backed by TLS on ARM). This CL removes the extern register m; code now uses g->m. On ARM, this frees up the register that formerly held m (R9). This is important for NaCl, because NaCl ARM code cannot use R9 at all. The Go 1 macrobenchmarks (those with per-op times >= 10 µs) are unaffected: BenchmarkBinaryTree17 5491374955 5471024381 -0.37% BenchmarkFannkuch11 4357101311 4275174828 -1.88% BenchmarkGobDecode 11029957 11364184 +3.03% BenchmarkGobEncode 6852205 6784822 -0.98% BenchmarkGzip 650795967 650152275 -0.10% BenchmarkGunzip 140962363 141041670 +0.06% BenchmarkHTTPClientServer 71581 73081 +2.10% BenchmarkJSONEncode 31928079 31913356 -0.05% BenchmarkJSONDecode 117470065 113689916 -3.22% BenchmarkMandelbrot200 6008923 5998712 -0.17% BenchmarkGoParse 6310917 6327487 +0.26% BenchmarkRegexpMatchMedium_1K 114568 114763 +0.17% BenchmarkRegexpMatchHard_1K 168977 169244 +0.16% BenchmarkRevcomp 935294971 914060918 -2.27% BenchmarkTemplate 145917123 148186096 +1.55% Minux previous reported larger variations, but these were caused by run-to-run noise, not repeatable slowdowns. Actual code changes by Minux. I only did the docs and the benchmarking. TODO: - remove TLS slot from linkers and runtime/cgo - check runtime for comments about m