Go 1.2

Cooperative preemption via stackguard0. Funcdata/pcdata and pointer maps; precise stack scanning built but shipped opt-in (precisestack). New in-memory symbol table and pclntab. Three-index slices. Per-P allocator caches and persistentalloc.

Each area lists the stage every thread reached in this release (with what was left open afterwards), then changes that are not part of a thread. Thread names link to the full thread on its topic page.

Memory management

From Memory management: GC and allocator (Go 1.0–1.17)

Allocator refinement for per-P caches (whole-span transfer, inlined malloc fast path), persistentalloc, lazy page table and flexible 64-bit mapping; precise stack scan tried and reverted.

• Stop-the-world mark-sweep, then the parallel collector — Done: the collector runs on g0 instead of a goroutine stack (71f061043d, de316388a7), and may not be triggered from g0 (dfdd1ba028); GC and the finalizer goroutine are synchronised for the preemptive scheduler (72c4ee1a9d); setGCPercent race fixed (94dc963b55). Left open: nothing noted beyond stack precision.
• Stack scanning: conservative, precise by frames, stack barriers, hybrid barrier, async safe points — Done: stack-by-frames scanning enabled (0e6007e4f9) and then disabled (c3dadca977). Left open: cost of gentraceback and incomplete precision.
• Precise heap scanning: type information, the heap bitmap and GC programs — Done: type info for interface values (21ea5103a4), array/slice fix (c6293d2106), cnewarray (5782f4117d). Left open: nothing noted.
• Size classes, tiny allocator and mcache/mcentral — Done: whole-span transfers, inlined classification and MCache_Alloc, simplified MCache (23ad563119, 5a89b35bca, 5166013f75, c4cfef075e, c075d82cca), persistentalloc for small metadata (47e0a3d7b1, 86da989ee5). Left open: nothing noted.
• Scavenger: returning memory to the OS — Done: Windows SysUnused (4e76abbc60), scavenger details printed on FreeOSMemory (9f46efce52). Left open: nothing noted.
• Heap address space: contiguous arena to sparse arenas — Done: lazy page table (671814b904, 8bbb08533d, ccd1d07cc4), flexible 64-bit mapping (a8ad859c30), persistentalloc use (99922aba8b). Left open: nothing noted.
• Finalizers and span specials — Done: GC and finalizer goroutine synchronised (72c4ee1a9d), SetFinalizer(func(interface{})) (6350e45892), no finalizers on g0 (6401e0f83f), frame in runfinq hidden (eb04df75cd). Left open: nothing noted.
• Runtime allocators for runtime metadata and the port to Go — Done: persistentalloc, FixAlloc use (47e0a3d7b1, 86da989ee5, 99922aba8b, 4f514e8691), mheap hidden from GC (2a6520c2d3), all sys memory accounted (a33ef8d11b, see stats). Left open: nothing noted.
• Memory statistics and accounting — Done: user-visible GCSys, OtherSys, all sys memory accounted (a33ef8d11b), cheaper stats (5d637b83a9, c075d82cca). Left open: nothing noted.

Other changes

• Leaks and dead code internal — one-time Linux memory leak (fd41926347), dead type-metadata allocation removed (0a904a3f2e), markfreed messages (139cc96a57).
• Heap-sizing fixes internal — MSpan.limit for large spans (d6f89d735e), coalescing bug in heap (9ba551bb87), 64-bit atomic cas64 (fb63e4fefb, f70a19f085), GC-time scan fixes (2f5825d427, 50ba6e13b4, c51152f438, df3e6ce4b1), and not marking OS memory as executable on Windows (28f74608b5, 3d513faa6f).
• Rename FlagNoPointers to FlagNoScan internal — Renamed the malloc flag because it better describes objects that have pointers but that the GC should not scan. (d0dd420a24)

Scheduler and concurrency

From Scheduler and concurrency runtime (Go 1.0–1.17)

Cooperative preemption of long-running goroutines (10ms), GODEBUG/schedtrace, stack and thread limits, Windows and Solaris pollers; threads: preemption, netpoll, diagnostics.

• The G-M-P scheduler: from global-lock M:N to per-P run queues, spinning Ms and work stealing — Done: fairness 4bb491b12e (global runqueue no longer starved when goroutines keep re-readying each other); CPU underutilization fixes 15a1c3d1e4/32fef9908a (newproc/ready were “deliberately sloppy” about waking Ms); starttheworld race dbcfed93e7; sysmon no longer parks while goroutines run 156e8b306d/658d19a53f; singleproc removed d5ab784611; stack size limit 757e0de89f and thread count limit 665feeedcb; LockOSThread fix 4961483e7d; windows priority boost disabled a574822f80. Left open: timers/locks still via notes.
• Preemption: cooperative stack-check preemption (1.2) to signal-based asynchronous preemption (1.14) — Done: user-visible preempt function (354ec51666, not yet used), preemption for GC (1e112cd59f), preempt goroutines running more than 10ms (bc31bcccd3), enabled by default a20784bdaf after temporary disable/re-enable (08e064135d, b913cf84dc, 1da96a3039), debugging knob removed 10ebb84d48, old gcwaiting checks removed f195ae94ca; notetsleepg and notetsleep returning false on timeout for preemptive sysmon e97d677b4e, e932c2035f; sysmon polls network if idle >10ms 6857264457. Left open: loops without function calls cannot be preempted.
• Network poller: from the netFD select loop to the runtime-integrated epoll/kqueue/IOCP poller — Done: Windows poller moved into the runtime 6ea7bf253c with GetQueuedCompletionStatusEx batching 65834685d3; Solaris (event ports, level-triggered) 50df136483^1.3; netpoll refactors 8486d96a27, 2ea859a779^1.3 (level-triggered IO); FreeBSD/ARM e82614e5be; deadlines for already-issued IO aaab946943; netFD fdMutex 23e15f7253; fewer allocations per IO 04b1cfa946; netpollready called once per event 82ef961af5.
• Timers: timerproc goroutine → per-bucket heaps → timers on Ps — Done: 4-ary heap fcf6a7e5ce, monotonic clock on linux/386, amd64 86c976ffd0^1.3, tsleep overflow 3548ab5ebb, fewer notewakeups in timerproc d1b66439f9, network timers crash 9707f269c1.
• Channels and select — Done: close/send race detection in select 62747bde6c; chan blocking profile incl. select and close c92287686d; closechan stack split 10d1e55103.
• Semaphores, runtime locks and Notes — Done: notetsleepg e97d677b4e; notetsleep timeout result e932c2035f.
• sync.Mutex, RWMutex, WaitGroup, Cond, Once — Done: Cond without allocation 5a20b4a6a9; WaitGroup race fixes 0f4897ae1a, 07cb48c31f.
• sync.Map, atomic.Value and sync/atomic — Done: user-visible Swap functions 0e15b03f93, 883530c019, 66c58cea67.
• Goroutine and M lifecycle: caches, ids, LockOSThread, Goexit, cgo extra Ms, deadlock detection — Done: deadlock in cgo programs 34c67eb24e; Goexit during init 14062efb16; false deadlock 1da1030b5d; traceback of running goroutines via freezetheworld 01f1e3da48.
• Stop-the-world, safe points and GC–scheduler interaction — Done: preempt goroutines for GC 1e112cd59f; stoptheworld race dbcfed93e7.
• Scheduler diagnostics: schedtrace, GODEBUG, execution tracer, block/mutex profiles — Done: user-visible GODEBUG 4b536a550f, schedtrace/scheddetail 4f2e382c9f, creator in running traceback c33d490020, block profile for channels c92287686d.
• GOMAXPROCS, thread and resource limits — Done: user-visible stack and thread limits 757e0de89f, 665feeedcb.

Other changes

• netFD fdMutex and net allocations internal — 23e15f7253, 04b1cfa946.
• Misc runtime internal — GODEBUG-gated e69012ce2a, goroutine stack accounting 187b9c695f, fix traceback with cgo 326ae8d14e, 33bd9694cd.
• Small scheduler/poller fixes after the preemptive scheduler internal — fee1d1cda0 (G status after syscall), b65271d008, ae5991695c, 91d35ad1b8 (Windows netpoll blocked-call deadlock), a0935cc979, 8f746af65d, 5290e55164 (persistentalloc in netpoll), 38abb09a2e, cc0a005c9d, 0286b4738e, e33e476e07 (disable CPU profiling around fork), d91219e458, 3d6bce411c, 1308194204.
• Plumbing internal — 081129e286, b36f2db12a, ab1270bcfc, 3453a2204b, f8a850b250, c7d5c438a2, a05237f20a, 8b789e1738, ce9ddd0eee, 6d68fc8eea, 16d6b6c771, 551ada4742, 828a4b9376, c4579635cf.
• Traceback of goroutines in syscalls internal — Traceback of goroutines in syscalls uses gcpc/gcsp, which are more stable than gp->sched because sched is mutated by entersyscall. (f73972fa33, 9c0500b466)

Data structures and type metadata

From Runtime data structures and type metadata (Go 1.0–1.17)

Hash lookup shortcuts, hashmap internals get typed GC info instead of bespoke GC code, itabs from persistentalloc, faster x86 memmove, Slice3, and ARM softfloat/preemption fixes.

• Hash map core: from the first hashmap.c to the 8-entry bucket design — Done: Hmap cut to 48 bytes (4042b77776), hash-lookup shortcuts for same-length strings (a696ae56db), type information recorded for the internal structures and all hashtable-specific GC code removed (fb376021be, preceded by 0df438c683 telling GC not to scan them), fix for iterator/growth/NaN-key interaction (869368a528), shorter lookup stack frames (a3f842a4c1). Left open: accessors still took key/value by value via C varargs.
• Map growth, iteration order, overflow buckets and GC interaction — Done: typed GC data replaces bespoke GC code (fb376021be), iterator/NaN/growth bug (869368a528). Left open: nothing noted.
• Map fast paths and compiler/runtime specialization — Done: lookups optimized with same-length string checks (a696ae56db, BenchmarkSameLengthMap 43→4 ns).
• Interface values, itabs and type assertions — Done: itabs from persistentalloc (b36f2db12a), SetFinalizer accepts any compatible f (5822e7848a), wrappers for blank-interface methods skipped (f316a7ea87).
• reflect redesign: from string parsing to Type/Value structs — Done: user-visible Value.Slice3, SetCap (4d8aefde47); Interface of non-addressable no longer allocates (94179d61ab, InterfaceBig 90→14 ns), Convert indir-bit fix (0e73497a4b), DeepEqual visit key (f7b5a01999, 780f5b714d), call arguments exposed to GC (0ab8f2d287).
• memmove, memclr, duff routines and pointer-write atomicity — Done: fast x86 memmove (6021449236), ARM memmove parameter corruption fix (f8fd77baa9).
• internal/bytealg and string/byte comparison primitives — Done: Compare for bytes/strings (b3946dc119).
• Strings: concatenation, conversion and printing helpers — Done: correct types for maxstring/concatstring (98a80b95b4, 249f807c39).
• Soft-float, 64-bit division and complex arithmetic helpers — Done: no preemption in softfloat, argument/frame records (e03dd07981, 8166b2da19, 6a13897bc6, cba880e04a), 386 uint64 division (70138a2108), timediv avoiding _vasop (c7d5c438a2).

Other changes

• Hashtable/reflect constants and docs internal — BUCKETSIZE/MAXKEYSIZE/MAXVALSIZE unexported from reflect (da50221e8e), FieldByName shortcoming documented (7fb3d8e45e).
• ARM/Plan 9 build fixes in data files internal — 64db2ec915, 14e3540430, 184b02ea9f, 33bd9694cd, 9ddfb64365 (argument size recorded in assembly functions), 249f807c39, 98a80b95b4, f4f2cf16b0.
• Debug enums internal — e69012ce2a uses enums so dead code is eliminated.
• Complex types have no pointers internal — 4949dcb211.

Diagnostics and knobs

From Diagnostics and knobs (Go 1.0–1.17)

Block profiler gains select/close support; race instrumentation fixes; "created by" in tracebacks.

• Race detector integration — Done: Read/Write syscalls annotated with the memory they touch (fc80764792); instrumentation of for loops, arrays, selectors, type switches fixed (591d58a3bb, 3be794cdc2, 6c4943cb51, a15074c4dc, e7657de717); forkAndExecInChild excluded since instrumentation may allocate and preempt (63e0ddc7bf); no preemption of race calls (d017f578d0); unused step param removed (e2d95c1f24). Left open: nothing noted.
• CPU profiler and runtime/pprof writers (legacy format to compressed protobuf) — Done: user-visible block profiler supports select, channel close and shows the top frame in debug mode (c92287686d); SetBlockProfileRate corner cases fixed (3bd0b0a80d); memory profiler uses persistentalloc (8cf7044983); “created by” printed for running goroutines (c33d490020). Left open: nothing noted.
• GODEBUG, runtime/debug, debug log and traceback knobs — Done: thread count limit (665feeedcb); m stack shown on crash (ab38e2a498). Left open: nothing noted.
• Traceback correctness and inlining (diagnostic output) — Done: traceback/GC boundary (e58f798c0c), arg printing (eac6bee7c1), Windows (0a3bd045f5^1.3, 1.3), up-to-10 args (c40480ddd9^1.3). Left open: nothing noted.
• GDB runtime support — Done: explicit init of helpers (927b7ac327, 1.2).

Other changes

• Unused declarations removed internal (15baf6b4ac).
• Traceback/GC boundary adjusted internal (e58f798c0c), crash-on-m-stack display (ab38e2a498), explicit runtime-gdb init (927b7ac327), thread limit (665feeedcb).
• Race runtime update user-visible — Updated the race runtime to r183644, which handles memory range accesses properly. (cc99e6e949)

OS layer, stacks and panics

From OS interface, stacks and panics (Go 1.0–1.4)

Preemptive scheduling via stackguard0, rewindmorestack prologue, 8 kB segments and the 1 GB stack limit, -shared/external linking on ARM, dragonfly, lazy extra M, wrapper-aware recover.

• Segmented stacks to contiguous (copied) stacks — Done: user-visible 4 kB to 8 kB default segment (408238e20b); stack-size limit 757e0de89f; f0d73fbc7c uses gp->sched.sp for the overflow check; 6fa3c89b77 records proper goroutine state during split (see prologue thread); ebc5513be6 double-checks the calling goroutine in newstack; fd23958f49 fixes memory leaks where defer argp/pc pointed into freed segments. Left open: the commit message of 408238e20b calls 8 kB a stopgap pending new stacks.
• Stack-split prologue, nosplit budget and C-on-Go-stack detection — Done: 6fa3c89b77 rewindmorestack; a83748596c, 9ddfb64365 record argument size in assembly functions and use it for frames; 58f12ffd79, 13507e0697 morestack/lessstack in tracebacks; e84d9e1fb3 no stack splits in syscall status (new notetsleepg, entersyscall/exitsyscall arranged not to call split functions) with the follow-ups d83688165a, ddc01d5b06, 21315c3428; 12e46e42ec call trampolines not NOSPLIT; 9cd570680b/034d5fcc30 reimplement then restore the old reflect.call (the no-split version was dropped for cgo); 5a54696d78, 0273dc131e, e838334beb, ed467db6d8 replace numeric textflags by symbolic NOSPLIT|DUPOK|NOPROF. Left open: nosplit headroom is tight.
• Goroutine preemption (stackguard0, 10 ms and GC preemption) — Done: user-visible preemption of tight loops that call functions: f5becf4233, 354ec51666, d67e7e3acf, 1e112cd59f, bc31bcccd3, ef12bbfc9d, 4eb17ecd1f. Left open: loops with no function calls are not preempted (the mechanism needs a prologue check).
• Panic, defer, recover and Goexit — Done: 7276c02b41 panicwrap; ef12bbfc9d; fa4984d535 shows the runtime.panic frame in tracebacks. Left open: panicwrap accounting wrong (replaced in 1.4).
• Tracebacks, crash output and GOTRACEBACK — Done: fa4984d535, cc84176108, 13507e0697, 58f12ffd79, 326ae8d14e, 16d6b6c771 (PCDATA reader exported), 32b770b2c0 (jump to badmcall so traceback reports the caller of mcall). Left open: nothing noted.
• Symbol tables, pclntab and runtime.Caller/FuncForPC — Done: persistentalloc 83445fdcc3; PCDATA reader exported 16d6b6c771; ed467db6d8. Left open: nothing noted.
• Signals: handling, os/signal, foreign threads — Done: 2f1ead7095, ae73b90369, e555172592 (NetBSD/ARM sigreturn). Left open: nothing noted.
• Threads, locks and the OS scheduler interface — Done: a574822f80 (Windows priority boost); 45cff65502 switch to OS stack in Windows osyield/usleep; dfdd1ba028 no GC on g0; 6401e0f83f no finalizers on g0; 34c67eb24e lazy extra M. Left open: nothing noted.
• cgo runtime support and non-Go threads — Done: user-visible 34c67eb24e; dba623b1c7, f011282578 cgocallback_gofunc work; 2560f8fe22, cb30917387 NOSPLIT callbacks; cgo on dragonfly 2b44b36487; 397ba2cb4a. Left open: nothing noted.
• TLS, g/m registers and shared-library linking — Done: user-visible 45233734e2 external linking on ARM and -shared; d8fd8d89ea; 3ec0427a07, 254dc5fdbe ARMv5T target; e0db7fae87^1.1 was 1.1. Left open: “this CL only addresses the low level linker work”.
• Windows runtime port — Done: user-visible a574822f80; 6ea7bf253c, 65834685d3; 05a5de30f0; 4216203bcf. Left open: nothing noted.
• Platform ports: OS and architecture matrix — Done: user-visible dragonfly ac00524beb, 465ba6b78c, 2b44b36487; 6252b41981 removes OABI check; 649a2a9be5; 555da73c56; ff86d222be fixes syscall numbers on freebsd/openbsd 386. Left open: “still some breakages” on freebsd/arm.
• Integrated network poller: OS back-ends — Done: c5732c8526, c4cdd35e6e, e82614e5be, 6ea7bf253c, 65834685d3. Left open: nothing noted.

Other changes

• Small 1.2 changes internal — Plan 9 usleep NOSPLIT (8b85a3d480), removal of badcallback functions (a3e0002e6a), jmpdefer argument size (a97a91de06), intgo for findnull (82f5ca1ef0), ARM formatting and port builds (92254d4463, 48c0d8b6e2, a70cbf1329, e5cbebc1ad, cefdb9c286), style (83d4cd758c).
• Profiling signal accounting user-visible — cc4e6aad8e stops losing CPU profile signals (GC, stack splits, syscalls were previously absent); 4207897dcc profiles on g0; 439f9397fc avoids inconsistent goroutine state in the profiler.
• runtime.nanotime on BSDs / timediv internal — 98cc58e2c7, d83688165a.
• Defer leak internal — fd23958f49.
• IndexByte in runtime internal — e2a1bd68b3, 598c78967f, b3946dc119 (bytes.Compare), ee66972dce.
• append inlined internal — 045dbeaf05 frontend append; 8ce8adbe7a.
• Static data internal — e017e0cb24 marks statics no-pointers.
• Symbolic textflags internal — see prologue thread.
• Windows defs and comment fixes internal — Updated defs_windows files and fixed comments since g->gobuf became g->sched. (9bc0eaf60e, 528534c1d4)

Compiler frontend

From Compiler frontend (Go 1.1–1.9)

Three-index slices (b4e92cee97), precise stack collection built but disabled (30ecb4cd05), large stack variables moved to heap, many inlining-export and message fixes.

• Parser: from yacc to a hand-written recursive-descent parser — Done: b4e92cee97 and 8cd6341cf8 add full slice expressions x[i:j:k] to go.y (12 files; commit says “experimental … may not be included in the release”, then allowed in), see language thread; d00bd1d1f4 C-style-if message. Left open: nothing noted.
• Binary export data and the object-file split — Done: 78f5b616fc, 62891fb811, 20ebee2c31, c1fc8d5296, a85cfbd433 (tag builtin error/byte/rune so they are not exported), 8d6bc666fb (qualified embedded fields with owner package), a9e119ac70. Left open: nothing noted.
• Inlining — Done: 49da9a8e44, ae5e791ed2.
• Escape analysis and stack allocation — Done: 1f4d58ad5d, 7cfa8310c7.
• Closures, method values and variable capture — Done: 0d0d57ccfe (_ initializers in closures), 280c8b90e2 unique method names for function-scoped types.
• Language changes and rules as implemented in the compiler — Done: user-visible b4e92cee97, 8cd6341cf8; 880d869764 init order for top-level blank vars; 66c8935f73 initialization loop detection; 933d7129c0; 903c2fda18 (_ = nil).
• Write barriers, precise stacks and liveness (compiler side) — Done: 30ecb4cd05 precisestack off by default; f91e682cca shorter bitmaps. Left open: call-site pointer bitmaps “not ready in time”.
• Race detector and sanitizer instrumentation in the frontend — Done: e440354c40, 3be794cdc2, 269b2f2d4d.
• Diagnostics and error messages — Done: 703b897f78, d00bd1d1f4, 2254785c3e, 8d530f2472.

Other changes

• Stack/array fixes internal — b15a64e35d reset haspointers computation, bf9a00bc8f overflow in array bounds calculation, 428ea6865c equality class, 66e8471391 missing package is fatal, 27d17255db, cd2eb48722, e9453e0545, 7d734d9252 (C undefined behaviour), 04c40c97c3 (no algs for internal map types), 3ac5d54cf9, 6f5af9c0b1, 1d4ed0c86b, 7e270cf6c4, 7dba510c7b, e1ac15743b, 85195e2ccf, a85cfbd433, 8d530f2472, 5be1821a31, abe384f68a.

Middle end and SSA optimizer

From Middle end and SSA optimizer (Go 1.0–1.9)

Small: escape-driven stack make, redundant bounds checks dropped in inlined append and slicing (threads: escape analysis, order/walk).

• Escape analysis (esc.c → esc.go) — Done: 2c4b029b75 (make([]T, const) uses the escape result), 148fac79a3 ordering fix. Left open: nothing noted.
• Order, walk and temporary lifetimes (order.c, walk.c) — Done: inlined append loses a useless bounds check (4b0eb19a05), redundant slice bound check removed (19e2922688), panicslice unlikely (7c8be15b8a), constants not leaked to backends (d7c99cdf9f). Left open: nothing noted.

Other changes

• Misc internal — fc3bec386e, ae5e791ed2 (save local var list before inlining), c929ac5f7e, a7d8b35aac, 0218dfe7eb, 51266761fd, 36f8480965.

Backends and assemblers

From Backends and assemblers (Go 1.0–1.6)

Groundwork for precise GC: funcdata/pcdata, pointer maps from cc and gc, zeroing of pointers on entry, proginfo, portable optimizer code and temporary merging. Threads: pointer maps/liveness, register allocator.

• Per-architecture compilers 6g/8g/5g/9g and the shared gc code generator — Done: trampolines moved to portable code (1f51d27922, 31be5deae4), proginfo in every back end (see optimizer thread). Left open: nothing noted.
• Register allocation and the peephole optimizer (reg.c, peep.c) — Done: proginfo in all three (24c8035fbe, ac0df6ce89, a07218385c, 9a0a59f171); portable optimization logic and flow graph (b3b87143f2, dbf96addfb); temporary-merging pass using greedy linear scan (Poletto-Sarkar), cutting the number of functions with too many vars in godoc from 83 to 32 (fa72679f07, 72636eb506); O(n) reset loop removed from copyprop (6d47de2f40); faster compilation by memset (06e686def6); MOVBS/MOVHS pseudo-instructions on ARM to make short-integer semantics explicit (3670337097, 357f733695). Left open: the back ends only track the first 128 variables per function (commit text, fa72679f07).
• Expression and statement code quality in 6g/8g/5g (division, bounds checks, loops, slices, interfaces) — Done: copy inlined as memmove (ff416a3f19), nil-check on &x and removal of redundant &x.Field checks (999a36f9af, aa0439ba65), 6c/8c avoid long multiplication (2c0b00744f), full 64-bit address in block copy (f268c295a3), very wide offsets (9c21ce54dd), no constant propagation of non-constant LEA (3c3ce8e7fb), clearfat de-interleaved (85a7c090c4), defer caller line numbers via a real NOP (7e97d39879; XCHG AX,AX / AND.EQ R0,R0,R0).
• Stack frames, pointer maps and liveness analysis in the compiler — Done: funcdata/pcdata plumbing (48769bf546, 47b89a37d9, 8c741c97f7, de6da4a8bd), C and Go argument/locals bitmaps (9b1f1833de, 73c93a404c, 2db5e4bb0b), interface encoding (abc516e420, 21ea5103a4), zero pointers on entry (7910cd68b5, 3b4d792606, d3b04f46b5, 390656affd), nosplit pointer maps from the linker (037a1a9f31), . Left open: maps conservative; liveness analysis not written (commit text: “intermediate step”).
• Stack frame layout, prologues, stack split and frame pointers — Done: pointer-map-aware zeroing of frames and stkptrsize (d3b04f46b5, 390656affd).
• The Plan 9 C compilers (cc, 5c/6c/8c) in the Go toolchain — Done: cc rejects unions with pointers (dbee8ad0f9), isolated textflag/dataflag (2449909d81), wide-char fix (5b15b44434), full 64-bit block copy (f268c295a3), pointer maps (9b1f1833de, 2db5e4bb0b), %Q (237e7c134c), AFUNCDATA PC fix (de6da4a8bd), fix undefined behaviour (8d5ec52e6c).
• Assemblers: 5a/6a/8a/9a and the Go cmd/asm — Done: flag parsing (8124a02cb2), PCDATA/FUNCDATA (47b89a37d9, 8c741c97f7), m/g for ARM (5d081792b6), MOVBS/MOVHS (3670337097).

Other changes

• Non-escaping &x internal — &x panics if x does, inserting checks (999a36f9af) with redundancy removal (aa0439ba65), checknil fix (b75a08d03c).
• Temporary lifetimes internal — greedy temporary merging, with the back ends tracking only 128 variables (fa72679f07).
• Plan 9 and portability internal — consistent BGETC/BPUTC (79dca0327e), UB removals (7d734d9252, 8d5ec52e6c, fd7ddad160), simplified bitmap loop (af2a3193af).
• Deferred call line numbers user-visible — traceback line number of the deferred call frame (7e97d39879).
• cgo internal — retain Go pointer passed to C for duration of the call (5639d2754b).
• Remaining small code-generation, optimizer and tooling fixes (3 commits) internal — per-operator and per-back-end bug fixes and cleanups that do not carry design of their own: proglist prototypes, R13/F3 clash on 5g, 8g instruction descriptions (4fc7ff497d, bf58d20999, b27e09331c)
• Build, portability and warning fixes (1 commits) internal — Plan 9/gcc/clang warnings, yacc/bison, format verbs, buffer sizes, output-file handling and Windows paths in the compilers/assemblers/liblink (acd887ba57)
• Compiler op constants and misc internal — Reordered gc O* constants, renamed ORRC to ORROTC, and used ArgsSizeUnknown for functions with unknown argument size. (6fc49c1854)

Linker and object files

From Linker and object files (Go 1.0–1.9)

New in-memory symbol table and contiguous pclntab with FUNCDATA GC metadata and argument bitmaps; ARM external linking and TLS merge.

• Go symbol table, pc/line table and function metadata (gosymtab, pclntab, findfunc) — Done: new in-memory symtab (5d363c6357), contiguous pcln table (c3de91bb15), funcdata for GC info (48769bf546), argument pointer bitmaps (4e0a51c210), file table shrunk (d011f0aa89), -s no longer discards pclntab (0d85d5423b), -v reports pclntab/funcdata sizes (d43ee1b714). Left open: linker still emitted per-function pcln from instruction lists.
• Stack splitting and the linker’s nosplit overflow check — Done: large-frame stack split for the preempt check fixed (031c107cad) and the stack-split code refactored (1a6576db34).
• DWARF generation in the linker — Done: .debug_frame relocations in external link mode (28bbc6c27a); fieldtrack line fix (a14e143c21).
• External (host) linking and cgo — Done: ARM external linking and -shared in terms of external linking (45233734e2); .tbss for dynamic internal linking (a848475324); duplicate static symbols in COFF/Mach-O (9add729a1f); PE label symbols (c742179c49).
• Build modes: -shared, c-archive, c-shared, shared, PIE, plugin — Done: -shared through external linking on ARM (45233734e2).
• Runtime type data, GC metadata and sections laid out by the linker — Done: gcdata/gcbss fix (251baea1af), textflag constants in a shared header (5a54696d78), GC metadata via funcdata (48769bf546).
• Container writers: Windows PE, Mach-O and Plan 9 — Done: COFF duplicates and labels (9add729a1f, c742179c49), _image_base__ fix (60513bf2f4).
• Architecture and OS ports in the linker (arm, power64, arm64, mips, s390x, nacl, BSDs, Solaris) and trampolines — Done: DragonFly (71dc91db0f, fd0af9b56c), UNDEF fix for 5l (ed6dce6f9d), MOVBS/MOVHS (3670337097, 5636b60b70), PCLMULQDQ (6bea504b94).
• Build IDs, ABI hashes, flags and determinism — Done: -s documented and passed to gcc (71051953e0, e50e4f7ec1).

Other changes

• Misc linker work internal — use one-byte XCHG forms (aad4720b51), NOP of $x+10(SP) (26d43a0f22), PCDATA accepted (567818224e), error buffer size (6c99b5c0d3), iself check cleanup (3b4da67ee3), -installsuffix (0a033a18ad), libbio BGETC macros (79dca0327e), dragonfly interp path, wrapper RET.EQ fix (3acddba2ec), fix of 4984e6e9fd frame-size printing, “undefined behavior” cleanups (6034406eae).
• Remaining 1.2 ld work internal — read-only data placed in a non-executable segment (d6d83c918c), -v verbosity levels (b99fa81555), Plan 9 and build fixes (0b88587d22, 328ec95878, b015af4117, 32a3ce6799, 4984e6e9fd, 308a3e6c52, e18ed3c111, 80a153dd51, e38eddbfe4).