Go 1.23

Timer rewrite with GC-able synchronous timer channels. Range-over-func ships. unique and internal weak pointers. Linkname lockdown. v1 tracer deleted. Profile stack depth 128. First spinbit attempt (reverted). Stack-slot merging. GOARM64/GORISCV64.

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 Runtime: memory allocator and garbage collector (Go 1.18–1.28)

• Soft memory limit and GC CPU limiter — Done: GC CPU pause accounting became exact. The pause update was factored out (08a7ab97ba) and uses maxprocs instead of stwprocs (ae0a08dee9). Pause time now starts when the world is actually stopped, removing overlap with idle time (d4395ebc0c), and the time Ps spend in _Pgcstop while stopping is added back individually (e995aa95cb). The limiter consumes these values. Left open: nothing noted.
• Heap bitmap rewrite and allocation headers — Done: user-visible the experiment and old code were removed (9f3f4c64db). Headers made large pointerful zeroing preemptible (de3a3c9ebc, publication thread). Left open: nothing noted.
• Finalizers, runtime.AddCleanup and the cleanup queue — Done: user-visible doc note on method receivers in SetFinalizer (f7c330eac7). Left open: nothing noted.
• Weak pointers and the unique package — Done: user-visible internal/weak (dfc86e922c) and the unique package with Make/Handle (a088e230d4); string clone via stringslite (4dccb78582); HashTrieMap iteration (3a3837f422). Left open: the weak API was internal only.
• Object pinning (runtime.Pinner) — Done: runtime.debugPinnerV1, a self-pinning Pinner for debuggers using debugCall (7f9edb4225). Left open: nothing noted.
• Mark worker scheduling and GC work queues — Done: gcBgMarkStartWorkers uses channels instead of notes, so runnext can hand off directly (5dceddfe7c); data/bss root jobs computed in one loop (0b1b6c1589). Left open: nothing noted.
• Allocation publication, zeroing and preemptibility — Done: preemptible zeroing of large pointerful objects (de3a3c9ebc). Left open: nothing noted.
• Sweeper termination and pacing — Done: large-object stats updated before freeSpan, fixing /memory/classes/heap/unused underflow (36d32f68f4). Left open: nothing noted.

Other changes

• allocfreetrace replaced by traceallocfree user-visible — GODEBUG=allocfreetrace removed (11047345f5). The traceallocfree experiment records alloc/free events in execution traces, with a batch carrying heap base and alignment for ID compression (724bab1505, efe7a1f5d3, ca1d2ead5d).
• time.Timer no longer embeds runtime timer state internal — touches the scavenger’s timer use (bc20704c26).

Scheduler and concurrency

From Runtime: scheduler, timers, channels, locks and synchronization (Go 1.18–1.28)

• Timer rewrite: per-P timer heap, timer.lock, and GC-able synchronous timer channels — Done: Cox’s stacked refactor, deliberately in tiny steps: delete addtimer/clearDeletedTimers (4a3c3ec996, 77f0bd01fb), merge statuses (b2bd7ebafc, 09bfea95cf, 3be36e9b20, ed0ad591d3, db8c6c8c7a), introduce timer.lock and use it in deltimer/modtimer/cleantimers/runtimer/adjusttimers/moveTimers (51df232b12, 2fb5ef889b, 58911599e8, d1e8dc25ff, 1df6db8e4f, c6888d9264) with a new lock rank (b5a99eaa94), status→state and method-ification (a155a2f8a1, 370c8e983b), per-P timers struct (adc575e64c; the message doubts the pair of atomic int64 deadlines hide missed wakeups), then the state cleanup above (6135651867). user-visible GC of unstopped timers (508bb17edd) and synchronous Stop/Reset (966609ad9e). Follow-ups: use of per-M/P timers needs acquirem before choosing ts (lost-sleep flake 2965dc9895), zombie count fixes (40cc3eb278, e38be310a4), timers.wakeTime race (85bbb12141), Ticker.Reset spurious race (24fa7544b0), adjust fast path (1c864333ca), avoiding netpoll wakeups at timer creation (84b190ad3c), cleanHead optimisation (97f1b76b4b), restoring the older adjust call order because it measured faster (77d9cb8937), timer race (fae6eb5f2f), heap sift simplification (d0051be847), debug tracing (38723f2b38). Left open: Stop/Reset racing a running timer returned wrong results.
• Runtime mutex: tri-state futex/sema locks → “spinbit” mutex — Done: first attempt at futex lock redesign for profiling blame: share the sema lock2 structure with futex locks and double-link the waiting-M list (be0b569caa, d881ed6384, dfb7073bb8), plus unlock-time contention profiling (ba1c5b2c45, 8ab131fb12). All five commits reverted on 2024-05-29 (1be701a344, 5dead59add, afbbc2894b, 3f4be127bc, 9114c51521) because the patch series reduced the maximum throughput of contended runtime.mutex and regressed applications where that is the bottleneck. Left open: both a scalable lock and a correct blame-the-unlocker profile.
• Contended runtime-lock profiling (mutex profile for runtime-internal locks) — Done: acquireLockRank always acquires the M so m.locks is consistent when deciding whether profiling is safe (4bb67bc21e). Unlock-blame attempt reverted (see spinbit thread). Left open: blame semantics.
• Scheduler core: spinning Ms, work conservation, stealing, STW interaction — Done: idle-P race in injectglist (24070cf747); runnext disabled on wasm (no sysmon to prevent runnext ping-pong starvation) — added and reverted (0c64be46ac, a6a5c30d2b, 4afb155cdf); trace acquire/release across all P steals (c9d88ea2aa). Left open: nothing noted.
• Coroutines (iter.Pull runtime support) — Done: thread-lock state is bound to the coro at first newcoro; full lock state (counts and locked thread) must remain identical thereafter; simple nested/serial iter.Pull stays fast while exotic cases fail (dfe781e1eb); goexit propagation from iterator in Pull (9d2aeae72d). Left open: restrictions “may consider lifting”.
• sync package API evolution — Done: user-visible Map.Clear (5b6cd3d0cb, 3b0d39c5da). Left open: nothing noted.
• sync/atomic and runtime atomics — Done: runtime atomics moved to internal/runtime/atomic (4c2b1e0feb); user-visible atomic.And/Or (192d65e46b). Left open: nothing noted.
• Runtime lock ranking — Done: acquireLockRankAndM (4bb67bc21e); lockWithRankMayAcquire nosplit (3959d54c0b); sched/allp < timers rule removed (854dea060e). Left open: nothing noted.
• Netpoller — Done: Windows websocket handling moved out of the runtime (d0dc93c8e1); netpoll event sources identifiable on Windows (1de46564a7). Left open: nothing noted.

Other changes

• Trace/GC CPU pause metrics internal — d4395ebc0c, e995aa95cb; stack ownership check for traces (d6a3d093c3); frame pointer in syscalls (2b82a4f488, 6737f4cee5, and later 54fe0fd43f^1.24).
• Timer plumbing leftovers internal — 8a493a6672, 3ad5cd4741, c961ab3fe9, 3c39b2ed11, 7381123235, b2e9221089; sync cleanups (5bce5362da, 36cd880878).
• Misc internal — pahole struct sizing (552faa8927); profstackdepth GODEBUG (66cc2b7ca7); pc buffers to M (2141315251); kq parameter (1c924572d0); Windows netpollBreak identification (29746b4814); mips And/Or (9623a3586f), linknames (6ec291f495); exithook safe for concurrent os.Exit (f85c40438f); casgstatus throws on systemstack (0304d035cd); crashFD init (3d20a327a1); dead code (be29a02161); legacy linkname documentation (1d3d6ae725, 05cbbf985f, 2a7ca156b8, 9a3ef86173); atomic Or doc (201129414f).

Data structures and type metadata

From Runtime: maps, built-in data structures, hashing, randomness and type metadata (Go 1.18–1.28)

• Map hashing and hash functions (memhash, aeshash, fallback hash) — Done: the 64-bit fallback hash mix() first argument was attacker controllable (xor of input with constants) so zeroing the multiplier produced many collisions; it is now xored with the process seed (ef2f3391da), then constants xors removed/odd-forcing dropped (c260de24b6, 35c619398b). hashkey was deterministic because it was initialised before rand was seeded; now uses bootstrapRand (1a3682b4c1). memhash{32,64} lost their external linkname (2e9ed44d39^1.24, landed in the 1.24 cycle). Left open: nothing noted.
• Compiler-generated equality/hash functions and type descriptor emission (reflectdata) — Done: itabs generated via the rttype mechanism (977803e796), compiler uses abi.MapMaxKeyBytes etc. (4605ce2d34), MaxPtrmaskBytes merged into internal/abi (ff4e45fbc2). Left open: nothing noted.
• internal/abi type layout unification and type metadata (rtype → abi.Type) — Done: abi.ITab in reflect (f2db96cd6c), EmptyInterface/TypeOf/NoEscape (4742c52e10), zeroVal moved to internal/abi and then reverted back to runtime (2f5b420fb5, 4c589d93ad), ifaceIndir removed then restored “for now” (55a06f7920, 2064413b54). Left open: nothing noted.
• Interface conversion: itabs, type switch and assertion caches — Done: compiler generates itabs with rttype (977803e796), 2589a89468. Left open: nothing noted.
• reflect implementation: performance, allocation and escape behaviour — Done: user-visible reflect.SliceAt (d08a957298), Type.OverflowInt/Uint/Float/Complex (33013e8ea8), iteration methods Value.Seq/Seq2, Type.CanSeq/CanSeq2 (524a774634, b44600f83f), TypeFor faster for concrete types (2413629aa8), Comparable true for nil interface (e41fabd688), String kind in Pointer/UnsafePointer (3d61f24835), StructOf sets TFlagRegularMemory (99e12bee44) with an array fix (3330c69476), anonymous field name omitted from type string (2ff89341f6). Left open: nothing noted.
• memmove / memclr / memequal / internal/bytealg assembly optimisation — Done: strings.Compare intrinsified through bytealg.CompareString→runtime_cmpstring, ~28% faster, because slices.SortFunc needs a three-way compare (fd999fda59); arm64 memequal short-circuits identical pointers (66b776b025: O(n)→O(1), geomean 43.5ns→2.4ns; all other arches already had it); wasm memchr result extension fix (90c6558b6a). Left open: nothing noted.
• Slice growth, string building and allocation-free bytes/strings helpers — Done: Builder.Grow size classes (968b71bce4). Left open: nothing noted.
• Runtime randomness: fastrand, chacha8rand and math/rand/v2 — Done: internal/byteorder use (8623c0ba95). Left open: nothing noted.
• iter.Pull and range-over-func iterator types — Done: user-visible types Seq/Seq2 exposed without experiment (5a181c5042), double next/yield rejection (ff743ce862), panic propagation (1471978bac), Goexit propagation (9d2aeae72d), stop-before-next (1634fde4f9), doc from proposal (fe36ce669c). Left open: nothing noted.

Other changes

• Legacy //go:linkname documentation internal — a set of commits listing linkname users by dependent count (5fee159bc2, bde905af5b, 5fc5555feb, ef225d1c57, 4cac885741, 9a8995b8b6).
• Panic values framed in tracebacks internal — 69e75c8581.

Diagnostics and knobs

From Runtime: tracing, profiling, metrics and diagnostics (Go 1.18–1.28)

• The new execution tracer (trace v2): generations, per-M buffers, new parser — Done: v1 tracer removed and v2 becomes the only one (2c5849dc40, −3415 lines; flag removed 55e4097cba); old-format reader (6133c1e4e2) and PC-based stacks (d3d390984b); GoStatusStack event gives stacks to goroutines blocked for a whole generation (5879bf7e38); coroutine events GoCreateBlocked, GoSwitch, GoSwitchDestroy for iter.Pull, trace version 1.23 (c9c88d73f5); rewritten scalable traceMap (5bba5b256c); user-visible experimental-event mechanism and GODEBUG=traceallocfree which adds 9 expensive events for heap/span/stack alloc and free plus a type-table batch (724bab1505, 197101a84a, efe7a1f5d3, b6fa505c4a); traceAdvance before exit (20f4b6d8f9); package moved to internal/trace (5890b023a5, 664088b898). Attempted push-down of systemstack requirement reverted for breaking longtest (5ec7395afc, 9f13665088). Left open: tracer not reentrant; alloc/free events are experimental.
• Frame-pointer unwinding in tracing and profiling; profile stack depth — Done: FP unwinding for block and mutex profilers (f4494522dc); user-visible profile stack depth raised 32 → 128 for alloc/mutex/block/threadcreate/goroutine profiles using new internal/profilerecord types with unbounded stacks (1b9dc3e178), GODEBUG=profstackdepth (up to 1024, back to 32) (66cc2b7ca7), regression fix (47187a4f4f); frame pointer tracked while in syscall (2b82a4f488); mutex profile record dedup (183a40db6d, reverted 6948b4df8c, relanded 87abb4afb6 after fixing a bounds error and inline expansion). Left open: CPU/heap profiles still use the old unwinder; MutexProfile root-frame regression (fixed 1.24).
• Mutex and block profiles, including runtime-internal lock contention — Done: reverts of in-flight work (ca7d300509 restoring the GODEBUG, e6b8b2f3e2); FP unwinding for the profiler (f4494522dc) and dedup fix (87abb4afb6). Left open: unlocker blame still missing.
• CPU profiling, goroutine profiling, pprof labels and the protobuf writer — Done: legacy profile parser removed from internal/profile (2f6a25f447); one fewer STW when reading goroutine profile config (f96d9a6432); linker no longer disables memory profiling when only WriteHeapProfile is reachable (82c14346d8); stack depth 128 (see FP thread). Left open: nothing noted.
• runtime/metrics and MemStats — Done: GC pause CPU stats updated inside the STW to remove a race, tests tightened by 10x (017478a963); uses maxprocs not stwprocs (ae0a08dee9); IncNonDefault coverage test (74726defe9, 418e6d559e, 20b79fd577). Left open: nothing noted.
• Coverage redesign: runtime/coverage, internal/coverage, covdata — Done: go:linkname uses removed (180ea45566, df4e49366c, cd41d71785); -coverpkg percentage fix (a85b8810c4, 7196db9e66). Left open: nothing noted.
• Race detector, ASan, MSan and Valgrind integration — Done: internal/asan and internal/msan packages for use from std (db99b9b17c, e1e466f80c, 8f7df2256e); wakeableSleep modeled in the race detector (0b12e3d81c); arm64 racecall saved-FP clobber fixed (234390e741); user-visible public sync/atomic And/Or with race instrumentation (192d65e46b). Left open: nothing noted.
• Crash output, runtime/debug, BuildInfo and debuglog — Done: user-visible SetCrashOutput (1bb947b2eb, example f0a862ee4d, dup semantics b27d02c07b, CrashOptions param 3f7a030e02, variadic removed a22cb5cabe). Left open: “placeholder for future options (e.g. JSON)”.

Other changes

• Tracer v2 residue (1.23 log) internal — EvGoStop Gosched reason (287f791845), syscalling M IDs (c46966653f), Windows EvFrequency (707eba9b94), traceMap cleared without write barriers (829f2ce3ba), non-atomic enabled check (52b5f164ae), traceReadCPU off the system stack (9fa153b729), parser spilled-batch recovery (4513f1a1c1), experimental data/arg fixes (81d424ee9a, 1ed8166192), dead code (be29a02161), ordering.Advance split (c2b1463153), gotraceeventstats tool (c3bd543cc3). The runtimecontentionstacks removal (87e930f728) was reverted (ca7d300509) and finally redone in 1.25.
• os.FindProcess via pidfd reverted (cdf3249d74, c4e4afc90e) user-visible.
• Exit hooks moved to internal/runtime/exithook, safe for concurrent os.Exit internal (ff2070d939, f85c40438f).
• Memory profile/lock tests internal — f9ba2cff22 reverted by e6b8b2f3e2.

OS layer, stacks and panics

From Runtime: OS and platform layer, stacks, panics and tracebacks (Go 1.18–1.28)

• Defer, panic and recover re-implementation — Done: freedefer simplified (5a61d8d36b); user-visible panic values printed so every newline is followed by a tab, so a crafted panic value cannot fake a goroutine stack in tracebacks (69e75c8581); rangefunc panic messages reworded (6dd79d02f8). “Preempt more aggressively when panicking” (13766fe7d8) was reverted (18840865d2). Left open: nothing noted.
• Unwinder: gentraceback → unwinder iterator, inline unwinder, pcvalue cache — Done: exported runtime methods printed in tracebacks (a0d477cb6d), mcall unwinding on Windows fixed (a8ba163695); an attempt to print pc=0x.. for inline frames was reverted (643d816c8b, 386dcf4c93). Left open: nothing noted.
• Symbol tables, runtime.Caller and CallersFrames — Done: user-visible CallersFrames input may omit virtual PCs for inline frames (22f5e33031) so a []uintptr captured with skip works without changing the textual traceback format. Left open: nothing noted.
• Stack growth, stack bounds and the crash stack — Done: remaining arches: s390x, loong64, arm, 386, mips (cad6629144, f719d5cffd, 727e8d3863, c764d56ff1, ff0bc4669e). Left open: nothing noted.
• C-to-Go callbacks: extra-M reuse, system-stack bounds, init gating — Done: stack bounds always updated at cgocallback (1ffc296717), saved/restored (76a8409eb8^1.24), x_cgo_getstackbound pthread attr initialized (35ef4a9f33), runtime/cgo stack-bounds note (e45c262fe5), fatalf marked noreturn (0286a0822b). Left open: nothing noted.
• Security hardening: AT_SECURE, std fds, AT_RANDOM, gated debug features — Done: user-visible debug.SetCrashOutput (fd for fatal panics, 1bb947b2eb, plus 638b9023e3, 3d20a327a1); traceback framing (69e75c8581); cheaper permission changes via mprotect (cb12198de5). Left open: nothing noted.
• Signals and async-preemption machinery — Done: ignored signal not re-raised (be003bd00b), debugPinnerV1 (7f9edb4225). Left open: nothing noted.
• Syscall layer: AllThreadsSyscall, internal/runtime/syscall, libc and Windows SyscallN — Done: internal/syscall → internal/runtime (05c0579621); eventfd for epoll wake (d068c2cb62); EVFILT_USER for kqueue (519f6a00e4); no lockOSThread per Windows syscall (1f354a60ff). Left open: nothing noted.
• Windows runtime port — Done: IOCP high-res timer (cf52e70997), winmm avoided (558919b454), websocket handling out of runtime (d0dc93c8e1), long-path check removed (c098400548), RtlGetVersion (cff7267e0d), no per-syscall LockOSThread (1f354a60ff). Left open: nothing noted.

Other changes

• Misc internal — throw workaround pending issue 67274 and write-barrier-free convTstring (9eebdca8a9, bf0b605dfb), PPC64 asmcgocall stack usage (0c39dc1ff7), traceAdvance before exit (20f4b6d8f9), Windows SyscallX parameter counts and nosplit cleanup (35fa852d6d, dda4b17ee4), cgo warning flags (4a7f3ac8eb, 0784fd1b2f, 8705675614), loong64/s390x And/Or atomics (76ff0caa59, 1a8ee033f8), testing uses QPC on Windows user-visible (1c4704991a).
• kqueue/epoll wakeups internal — see syscall thread.
• runtime/internal → internal/runtime internal — (05c0579621, later fc5073bc15^1.24 in 1.24 for sys).
• Linkname policy internal — document legacy //go:linkname for modules with ≥2,000 dependents (519b0116a1; later guidance 8d0bef7ffe^1.26 in 1.26).

Compiler frontend · types2

From Compiler frontend: the types2 type checker (Go 1.18–1.28)

• Instantiation, Named types, Context deduplication and lazy expansion — Done: 7020beeffd optimizes Named method lookup; 210f051d6b documents the deterministic method order.
• Type inference and unification — Done: e85968670e better error when inference fails in a call; alias handling in unification (ee29dbe43e, 0d478d8e07).
• Generic type aliases and Alias type nodes — Done: 15cec430d7 flips the default to gotypesalias=1 in types2 (go/types had its own flip earlier in the cycle), though “the compiler still uses the non-default setting due to a panic” (9101bf1916 controls creation via types2.Config). 1a6498e1cb initial parameterized aliases, f294ddeb29 instantiation, 4ed358b57e represents any using Alias, d68d485405 gates alias type parameters on GOEXPERIMENT=aliastypeparams, bf279b71e2 adds Alias.{TypeParams, SetTypeParams, TypeArgs, Origin}, 9b9de261bd adds Alias.Rhs, 13c49096fd rejects uninstantiated signature aliases, f1039180bd enables aliases for TestStdlib. Left open: 1a6498e1cb lists “instantiation of alias types” and API additions as missing at that point; the dev experiment remained.
• Type declaration cycles, valid-type detection and the object path — Done: a66a3bf494 tracing, 9f9008ce66 disable an incorrect optimization in validType, 13e5fd95f5 better position for infinite types, 83b2b47b5d, d186dde818/a555ac6332 alias cycles.
• Per-file language versions and version gating — Done: 8121604559 do not fail fast on version errors, 133c8fc9bb no version checks for embedded types of imported interfaces, 5e00352b9b singular wording, 502347f121 duplicate-method error in go1.13.
• Language features as seen by the type checker (by release) — Done: user-visible range over func: e15cc7ab82 iteration variables may be omitted, 26ed0528c7 error for floating-point iteration variable, 4e77872d16, fd99157f9d refactor of rangeStmt for clarity; generic aliases (alias thread).
• Error reporting: codes, messages, and sub-errors — Done: 96e9838f39, 5e3c4016a4, dc6a5cfca1 quote user-supplied names, f81e498673 better errors for non-existing fields or methods.
• Sharing code between go/types and types2 — Done: eight generated files.

Other changes

• Position/syntax helpers internal — bdb0a1abfc name position in struct processing, 67361bf868, 7731fd9cd3, a56834922f, 8e02e7b26a.
• Typed-API additions user-visible — 01064622a2 Func.Signature, 9b9de261bd Alias.Rhs, (f176f5e1a3^1.28 is 1.28).
• Switch/Type fixes internal — 69e7b2bcd6 typecheck cases even if switch expression invalid, ad87d44047, 814e72f2ef, 2c6f24c825.
• Small fixes, unifier/assignment refactors, message and tracing tweaks internal — 11 commits: 8435659a43, 10a65649a3, 720306359a, 61a3ee5441, e8f5c04c1b, cfbe6cd9bb, c51f6c6257, e718aee5f5, 799968dfc3, 06478e4b46, 2fe2e4d593

Compiler frontend · syntax, IR, generics

From Compiler frontend: syntax, IR, unified IR and the generics implementation (Go 1.18–1.28)

• Unified IR: one serialization for noding, stenciling, inlining and export — Done: alias uses preserved in export data (f9c0423ec8, via pkgbits.TypeNamed), materialized aliases exported/imported (b24ec88bb9), 1e4de0584e describes the export data.
• Old typechecker and IR node-model cleanup — Done: OPAREN skipping more selective (6af27c49bc), wrapName removed (334835e6b3), remaining constant bounds check removed from typecheck (5428cc4f14).
• Static initialization and package init — Done: user-visible constants before variables in init order (8f618c1f53, a types2 change affecting program behaviour); static-init safety fixes (973befe714 ODEREF, 5038ce82b6 OASOP in mayModifyPkgVar); init fixes be152920b9.
• Compile-time performance, concurrency and the self-PGO’d compiler — Done: PGO profile refresh (7c52c064df, Template -3.5%, Unicode -5.1%, GoTypes -3.9%); cmd/preprofile tool to pre-parse PGO profiles (b7ae16e04b, e069587c8d); eq/hash algorithm and ptrBytes computed once in CalcSize (fcfd824e31, b40dc30d24) rather than on demand; telemetry counters (483a913a55, acda0107ba).
• Frontend support for new language features and spec changes — Done: user-visible range-over-func enabled for Go 1.23 files (d278d5bbdd), generic alias types (477ad7dd51, 31c8150082 enables aliases in the type checker), len(ch)/cap(ch) via chanlen/chancap (b6753baaed, 61d6817c83), -lang=go1 accepted (08370dcd9a), linkname referring to instantiations disallowed (849770dec9), rangefunc robustness (5a9dabc2ba).
• The syntax package (parser, scanner, tree) — Done: KeyValueExpr start position (4256ec1661), Pos.FileBase (8b990f2592).
• Types package internals: size, algorithm kind, TFlag, SIMD flags — Done: algorithm kinds simplified (d255792933), computed in CalcSize (fcfd824e31), ptrBytes computed during CalcSize (b40dc30d24); “mark pointer to noalg type as noalg” (b8c76effd9); itabs generated using the rttype mechanism (977803e796).
• Method wrappers and promoted-method sharing — Done: method wrapper comments; wrapper handling for aliases.

Other changes

• Stack slot merging internal — MergeLocals + AllocFrame overlap disjoint-lifetime autos (>3*PtrSize, same size / GC shape): merged 89f7805c2e, reverted 0bf6071066, re-landed off by default 875332b8a2, default-on in ecfddf4841.
• Telemetry internal — compiler counters (483a913a55, d79c350916).
• Rangefunc internal — see language features; 5f50b1e3bf fixes miscompilation switching over channels.
• Unified IR: noder/writer/reader refinements and pkgbits (1.23) internal — RType/implicit-conversion wiring, *At helpers, optional expression handling, UIR documentation and grammar, pkgbits.Index/SectionKind renames, synthetic closure positions, unsafe.Sizeof handling and related reader/writer fixes (532c6f1c8d)
• Generics: dictionary/shape fixes in the stenciler (1.23) internal — fixes in dictionary passes for type assertions, comparisons, switches, nested instantiation packages, and recursive generic interface instantiation (cda1e40b44, 8b48290895)
• Old typechecker and IR node-model cleanup (1.23) internal — removal of unused typecheck/ir/types code (Ntype, CurFunc uses, vargen, Resolve dependencies), more typed IR node construction, simplified DeclFunc/NewMethodExpr and constant handling (dcf046f021, 508e761978, bf91eb3a8b)
• Syntax package: error messages and parser tolerance (1.23) internal — better syntax and go/parser error messages, accepted type-parameter lists, parser simplifications (cc95d85fe4, 71f9dbb1e4)
• Housekeeping (1.23) internal — dead-code removal, ioutil/seek/slices/atomic-type replacements, README edits, debug-flag removal, assorted small bug fixes and doc comments (109c7bbf8f, 856355a913, b847d4cd2c, 28e0052ee7, 4d923e3f07, de487d5616, a245aced26, 9a028e14a5, a49952445f, 9702cd980f, 3c72dd513c, b8a410c434, 1cb13ac7f6, 0b72631a82, b3b4556c24, ad77cefeb2)

Middle end and SSA optimizer

From Compiler: middle end and SSA optimization (Go 1.18–1.28)

• Profile-guided optimization (PGO): hot-call inlining, devirtualization, profile preprocessing — Done:
  • cmd/preprofile pre-parses profiles into a compact text format. The first version b7ae16e04b was reverted in a807295d43 and reimplemented in e069587c8d.
  • The compiler checks the header without reading the whole file (9819d8265f).
  • The shared parsing moved into cmd/internal/pgo, with entries sorted by weight so deserialization needs no sort (63deaf00ea). The message reports PGO parsing falling from 13.7% to 1.3% of compile CPU.
  • pgo was renamed pgoir (0c5612092d).
  • Call-site offsets are now assigned correctly (185f31bf30).
  • Export-data lookup gained a debug log (d037bba19b), method lookup bails on interface types (508e761978), and devirtualization gained a post-lookup cleanup hook (ca5ba146da).
  Left open: nothing noted.
• Inliner: budget, closures, and the interleaved inline/devirtualize driver — Done:
  • Cost analysis was separated from applying inlining (b158ca9ae3).
  • Runtime functions are not inlined under -d=checkptr (f0de94ff12).
  • A fixpoint refactor (a245aced26, 28e0052ee7) was reverted in 9159d71a37 (#66261).
  Left open: the fixpoint restructure came back in 1.24 as the interleave refactor.
• Prove pass and bounds-check elimination: from poset relations to a dense limit table, topo-sorted walk, and division-aware proofs — Done: integer comparisons with numeric bounds improved (bbd0bc22ea). Bug workarounds were removed from loop inversion (ff35c382eb). Left open: nothing noted.
• Memory-op optimization: memcombine, dead stores, store-to-load forwarding, alias analysis, mem2reg — Done: bool constants are included in memcombine (802473cfda). DSE treats equivalent LocalAddrs as equal (57026007c8). Left open: 802473cfda points to “trickier cases … see TODO at memcombine.go:503”.
• Lowering of builtins to memory ops: memmove/memclr/memequal inlining and generated equality — Done:
  • Redundant cmpstring calls are CSE’d (1a0b86375f, rule fixed in 9f9dd2bfd8).
  • Zeroing for append(s, make([]T, n)...) avoids a past-the-end pointer (93e3696b5d).
  Left open: 1a0b86375f notes a redundant </<= test remains “because the compiler doesn’t know that cmpstring only ever returns -1,0,1”.
• Stack-slot merging (mergelocals) — Done:
  • user-visible Initial merging (89f7805c2e) was reverted (0bf6071066) and relanded default-off (875332b8a2).
  • Address-taken candidates were added (82e929e4f8), and region formation was enhanced (e01b1eb289).
  • The flag was flipped on by default (ecfddf4841).
  Left open: 89f7805c2e notes that the merge-locals liveness pass would ideally fold into the stack-map liveness pass, but “this may be difficult”.
• Switch lowering: jump tables, type-switch caches, and switch lookup tables — Done: the jump table symbol is static (2908352980). Left open: nothing noted.
• Range-loop and range-over-func lowering in walk — Done: the closure pointer stays live for range-body closures (44079f39eb). The body closure is named #yield%d (f27a40ce5f). Rangefunc constants are deduplicated into internal/abi (997b6969fd). Left open: f27a40ce5f says the name “may not actually be necessary”, pending confirmation with Delve.
• SSA values for zero-sized and multi-field direct-interface types — Done: 0-sized value simplification in copyelim (6f5d77454e).

Other changes

• Small internal — merge-locals trace tweak and stack var sort refactor (26a1f4ae00, 593daf785f), ppc64 rule fix (c6d142c4a7).
• internal/abi constants internal — ITab offsets, map consts, trace consts, byteorder package (b138b19d2f, 4605ce2d34, 7611d2e610, f3a743981d).
• Combined passes and compile speed internal — phielim+copyelim merged (69aa1974f3), rewrite-cycle detection delayed for huge functions (c686783cab), cap(ch) as chancap call (61d6817c83), ARM64 LSE atomics without dynamic checks (0ae8468b20).
• Fixes internal — zero-extension over signed values (7f90b960a9), args not assumed zero-extended (6bf8b76b95), CAS atomics SSA bug (643865856c).

Backends and assemblers · architectures, ABI

From Compiler backend: architectures, register allocation, ABI and assemblers (Go 1.18–1.28)

• amd64 code generation and GOAMD64 microarchitecture levels — Done: user-visible PCALIGNMAX align,amount directive for fine-grained loop alignment (useful only on amd64 per measurements, none on Apple Silicon) 18d0e6a14f.
• arm64 code generation and assembler — Done: atomic And/Or intrinsics on arm64 (using register temp v.RegTmp, results = old value) 63c1e141bc; asm tests run for cross builds 0def9d5c02; FMOVQ large offsets (1.26) 0c4444e13d^1.26.
• riscv64 compiler and assembler (RVA profiles, vector, compressed) — Done: user-visible rounding-mode suffixes on FP conversions (FCVTLD.RNE, default RTZ) 3580c212c2; hardware float min/max with qNaN pre-check 09ed9a6585; Zba/Zbb/Zbs in the assembler db423dde85, native rotations for rva22u64 (significant SHA-512 gain) 3659b8756a, rotation pseudo-instructions and canRotate daa58db486,997636760e; rva22u64 MOV improvements a6ecdf29e3.
• loong64 backend and assembler — Done: atomic memory access instructions 504212bbd7; recheck jump offsets after loop alignment f7f56ded01.
• PPC64 / Power10 backend (GOPPC64, prefixed instructions, PC-relative code) — Done: float min/max dfb17c126c; ADDZE c7065bb9db; CMP against $0 408739fc96,cc673d2ec5; ADDconst/ANDconst reintroduced dca577d882; RLWINM combos (7994da4cc1,0222a028f1); 32-bit constant adds 6e5398bad1.
• Stack slot merging, liveness and stack maps — Done: intervals 7979c8f588, merging enhancements 82e929e4f8,26a1f4ae00,e01b1eb289.
• Prologue, frame pointer, stack split and function alignment (cmd/internal/obj) — Done: PCALIGNMAX 18d0e6a14f.
• Atomics and memory-ordering code generation — Done: arm64 And/Or intrinsics (sync/atomic aliases) 63c1e141bc.

Other changes

• structs.HostLayout user-visible 22344e11f2.
• Telemetry for cmd tools user-visible eef288da1e,dbe03e4831.
• linkname restrictions for new internal functions user-visible 4721f95058.
• args_stackmap for ABI0 asm internal 0b72631a82.
• 1 small obj/loong64 and loong64 asm commits internal — e.g. obj/loong64: improve the definition of plan9 assembly format. (9ea4770e14)
• 8 small obj/ppc64 and PPC64 rules commits internal — e.g. obj/ppc64: combine C_SBRA and C_LBRA optab classes; obj/ppc64: add new constant optab classifications; obj/ppc64: generate smaller machine code for OR/XOR of uint3. (adbf71eb98,69f1290fcb,288615ddb5,d2eb72fcff,da732dd1c0,3aad2d0765,105ac94486,d5e5b14305)
• 6 small ssa/regalloc/liveness/obj generic commits internal — e.g. cc: soften type matching when allocating stack slots; cc: fix sign/zero-extension removal; ssa: fix the description of auxCCop as auxInt. (e930413331,a46ecdca36,734fd7a949,18dd682672,8056540772,a130fb6309)
• 1 small obj/riscv and riscv64 rules commits internal — e.g. obj/riscv: check immediate for rotation instructions. (20f052c83c)
• 1 small obj/x86 and amd64/386 rules commits internal — e.g. obj/x86: make SEH symbols content-addressable. (27603028eb)
• 1 small obj/arm64 and arm64 rules commits internal — e.g. obj/arm64: fix return with register. (b1fd047508)

Linker and object files

From Linker, object files and debug info (Go 1.18–1.28)

• Relocation-free pclntab and funcdata (offsets instead of relocations) — Done: pcbucketsize and minfunc constants merged into internal/abi 284e553035, 40fab294de. Left open: nothing noted.
• //go:linkname restrictions and the blocklist — Done: user-visible blocklist for newly added internal functions 4721f95058; pull-only linknames disallowed (-checklinkname, off) c4772d30bf; default on 003683f41d; push linknames added for legacy pulls 41aab30bd2; vdso linkname fix 0bc684a994; flag documented 148755a27b. Left open: only standard-library definitions enforced.
• DWARF quality for debuggers (types, variables, trampolines) — Done: DW_AT_go_runtime_type for unsafe.Pointer b89ad46451; captured-variable DIEs carry a closure offset attribute (+0.06% size on a Delve sample) f048829d70; putvar/putAbstractVar refactored to a single abbrev table 08029be9fc; range-over-func internal vars dropped 82c371a307. Left open: nothing noted.
• Dead code elimination and reflection-aware pruning — Done: deadcoded function symbols excluded in shared mode 98529a8e7c. Left open: nothing noted.
• Linker performance and memory — Done: -randlayout=seed randomizes function order to test benchmark sensitivity 5258d4ed60; profile files closed 7b3c38045b, 8bf9f4bb66; telemetry 7fcf38d715. Left open: nothing noted.
• Reproducible build IDs, buildinfo and symbol tables — Done: LC_UUID rewritten for external Darwin links 3bcefa5276. Left open: nothing noted.
• ELF/relro/hardening and external-link policy — Done: -bindnow 88480fadcc; .dynamic and .got into relro 2dddc7ef88 (after revert of earlier attempt be58fd0070); relro overlap fix a548dee75c; no-BTI marking on openbsd 1e433915ce. Left open: nothing noted.
• Per-platform object formats and internal linking (Mach-O, PE, XCOFF, wasm, ppc64, riscv64, loong64) — Done: loong64 reloc naming unified 0584574797; openbsd/arm64 PIE 68d3a9e417; riscv64 local text symbols b5a64ba62e; PE .reloc alignment f71c00b616. Left open: nothing noted.

Other changes

• Darwin and misc linker changes internal — unified OS/SDK versions and Mac Catalyst (d90a57ffe8, cecbf4f28e), runtime.text.N symbols in dynlink (9a9dd72d5c), runtime.end placement (b589478af7), obj timestamp cleaning (3454ac0d63), host-obj capture/trace (a428387e97, ddc6e165fd), loadelf static calls (3b51581261), objIdx removal (c2c4a32f9e), abi map constants (b1a12dd2ed), loong64 musl path (128a99decd, 7a072fc250), FromAssembly propagation (5c7d774538), ppc64 elf_* symbols (d2eb72fcff), dsymutil handling (5f319b7507), MinGW CRLF (e8ee1dc4f9), -c-archive ar path (7bfc82429c).
• GOROOT_FINAL removed user-visible (507d1b22f4).
• Runtime itab moved to internal/abi internal (e0ee5b1afa).