Initial commit: Go 1.23 release state

src/runtime/HACKING.md

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+This is a living document and at times it will be out of date. It is
+intended to articulate how programming in the Go runtime differs from
+writing normal Go. It focuses on pervasive concepts rather than
+details of particular interfaces.
+
+Scheduler structures
+====================
+
+The scheduler manages three types of resources that pervade the
+runtime: Gs, Ms, and Ps. It's important to understand these even if
+you're not working on the scheduler.
+
+Gs, Ms, Ps
+----------
+
+A "G" is simply a goroutine. It's represented by type `g`. When a
+goroutine exits, its `g` object is returned to a pool of free `g`s and
+can later be reused for some other goroutine.
+
+An "M" is an OS thread that can be executing user Go code, runtime
+code, a system call, or be idle. It's represented by type `m`. There
+can be any number of Ms at a time since any number of threads may be
+blocked in system calls.
+
+Finally, a "P" represents the resources required to execute user Go
+code, such as scheduler and memory allocator state. It's represented
+by type `p`. There are exactly `GOMAXPROCS` Ps. A P can be thought of
+like a CPU in the OS scheduler and the contents of the `p` type like
+per-CPU state. This is a good place to put state that needs to be
+sharded for efficiency, but doesn't need to be per-thread or
+per-goroutine.
+
+The scheduler's job is to match up a G (the code to execute), an M
+(where to execute it), and a P (the rights and resources to execute
+it). When an M stops executing user Go code, for example by entering a
+system call, it returns its P to the idle P pool. In order to resume
+executing user Go code, for example on return from a system call, it
+must acquire a P from the idle pool.
+
+All `g`, `m`, and `p` objects are heap allocated, but are never freed,
+so their memory remains type stable. As a result, the runtime can
+avoid write barriers in the depths of the scheduler.
+
+`getg()` and `getg().m.curg`
+----------------------------
+
+To get the current user `g`, use `getg().m.curg`.
+
+`getg()` alone returns the current `g`, but when executing on the
+system or signal stacks, this will return the current M's "g0" or
+"gsignal", respectively. This is usually not what you want.
+
+To determine if you're running on the user stack or the system stack,
+use `getg() == getg().m.curg`.
+
+Stacks
+======
+
+Every non-dead G has a *user stack* associated with it, which is what
+user Go code executes on. User stacks start small (e.g., 2K) and grow
+or shrink dynamically.
+
+Every M has a *system stack* associated with it (also known as the M's
+"g0" stack because it's implemented as a stub G) and, on Unix
+platforms, a *signal stack* (also known as the M's "gsignal" stack).
+System and signal stacks cannot grow, but are large enough to execute
+runtime and cgo code (8K in a pure Go binary; system-allocated in a
+cgo binary).
+
+Runtime code often temporarily switches to the system stack using
+`systemstack`, `mcall`, or `asmcgocall` to perform tasks that must not
+be preempted, that must not grow the user stack, or that switch user
+goroutines. Code running on the system stack is implicitly
+non-preemptible and the garbage collector does not scan system stacks.
+While running on the system stack, the current user stack is not used
+for execution.
+
+nosplit functions
+-----------------
+
+Most functions start with a prologue that inspects the stack pointer
+and the current G's stack bound and calls `morestack` if the stack
+needs to grow.
+
+Functions can be marked `//go:nosplit` (or `NOSPLIT` in assembly) to
+indicate that they should not get this prologue. This has several
+uses:
+
+- Functions that must run on the user stack, but must not call into
+  stack growth, for example because this would cause a deadlock, or
+  because they have untyped words on the stack.
+
+- Functions that must not be preempted on entry.
+
+- Functions that may run without a valid G. For example, functions
+  that run in early runtime start-up, or that may be entered from C
+  code such as cgo callbacks or the signal handler.
+
+Splittable functions ensure there's some amount of space on the stack
+for nosplit functions to run in and the linker checks that any static
+chain of nosplit function calls cannot exceed this bound.
+
+Any function with a `//go:nosplit` annotation should explain why it is
+nosplit in its documentation comment.
+
+Error handling and reporting
+============================
+
+Errors that can reasonably be recovered from in user code should use
+`panic` like usual. However, there are some situations where `panic`
+will cause an immediate fatal error, such as when called on the system
+stack or when called during `mallocgc`.
+
+Most errors in the runtime are not recoverable. For these, use
+`throw`, which dumps the traceback and immediately terminates the
+process. In general, `throw` should be passed a string constant to
+avoid allocating in perilous situations. By convention, additional
+details are printed before `throw` using `print` or `println` and the
+messages are prefixed with "runtime:".
+
+For unrecoverable errors where user code is expected to be at fault for the
+failure (such as racing map writes), use `fatal`.
+
+For runtime error debugging, it may be useful to run with `GOTRACEBACK=system`
+or `GOTRACEBACK=crash`. The output of `panic` and `fatal` is as described by
+`GOTRACEBACK`. The output of `throw` always includes runtime frames, metadata
+and all goroutines regardless of `GOTRACEBACK` (i.e., equivalent to
+`GOTRACEBACK=system`). Whether `throw` crashes or not is still controlled by
+`GOTRACEBACK`.
+
+Synchronization
+===============
+
+The runtime has multiple synchronization mechanisms. They differ in
+semantics and, in particular, in whether they interact with the
+goroutine scheduler or the OS scheduler.
+
+The simplest is `mutex`, which is manipulated using `lock` and
+`unlock`. This should be used to protect shared structures for short
+periods. Blocking on a `mutex` directly blocks the M, without
+interacting with the Go scheduler. This means it is safe to use from
+the lowest levels of the runtime, but also prevents any associated G
+and P from being rescheduled. `rwmutex` is similar.
+
+For one-shot notifications, use `note`, which provides `notesleep` and
+`notewakeup`. Unlike traditional UNIX `sleep`/`wakeup`, `note`s are
+race-free, so `notesleep` returns immediately if the `notewakeup` has
+already happened. A `note` can be reset after use with `noteclear`,
+which must not race with a sleep or wakeup. Like `mutex`, blocking on
+a `note` blocks the M. However, there are different ways to sleep on a
+`note`:`notesleep` also prevents rescheduling of any associated G and
+P, while `notetsleepg` acts like a blocking system call that allows
+the P to be reused to run another G. This is still less efficient than
+blocking the G directly since it consumes an M.
+
+To interact directly with the goroutine scheduler, use `gopark` and
+`goready`. `gopark` parks the current goroutine—putting it in the
+"waiting" state and removing it from the scheduler's run queue—and
+schedules another goroutine on the current M/P. `goready` puts a
+parked goroutine back in the "runnable" state and adds it to the run
+queue.
+
+In summary,
+
+<table>
+<tr><th></th><th colspan="3">Blocks</th></tr>
+<tr><th>Interface</th><th>G</th><th>M</th><th>P</th></tr>
+<tr><td>(rw)mutex</td><td>Y</td><td>Y</td><td>Y</td></tr>
+<tr><td>note</td><td>Y</td><td>Y</td><td>Y/N</td></tr>
+<tr><td>park</td><td>Y</td><td>N</td><td>N</td></tr>
+</table>
+
+Atomics
+=======
+
+The runtime uses its own atomics package at `internal/runtime/atomic`.
+This corresponds to `sync/atomic`, but functions have different names
+for historical reasons and there are a few additional functions needed
+by the runtime.
+
+In general, we think hard about the uses of atomics in the runtime and
+try to avoid unnecessary atomic operations. If access to a variable is
+sometimes protected by another synchronization mechanism, the
+already-protected accesses generally don't need to be atomic. There
+are several reasons for this:
+
+1. Using non-atomic or atomic access where appropriate makes the code
+   more self-documenting. Atomic access to a variable implies there's
+   somewhere else that may concurrently access the variable.
+
+2. Non-atomic access allows for automatic race detection. The runtime
+   doesn't currently have a race detector, but it may in the future.
+   Atomic access defeats the race detector, while non-atomic access
+   allows the race detector to check your assumptions.
+
+3. Non-atomic access may improve performance.
+
+Of course, any non-atomic access to a shared variable should be
+documented to explain how that access is protected.
+
+Some common patterns that mix atomic and non-atomic access are:
+
+* Read-mostly variables where updates are protected by a lock. Within
+  the locked region, reads do not need to be atomic, but the write
+  does. Outside the locked region, reads need to be atomic.
+
+* Reads that only happen during STW, where no writes can happen during
+  STW, do not need to be atomic.
+
+That said, the advice from the Go memory model stands: "Don't be
+[too] clever." The performance of the runtime matters, but its
+robustness matters more.
+
+Unmanaged memory
+================
+
+In general, the runtime tries to use regular heap allocation. However,
+in some cases the runtime must allocate objects outside of the garbage
+collected heap, in *unmanaged memory*. This is necessary if the
+objects are part of the memory manager itself or if they must be
+allocated in situations where the caller may not have a P.
+
+There are three mechanisms for allocating unmanaged memory:
+
+* sysAlloc obtains memory directly from the OS. This comes in whole
+  multiples of the system page size, but it can be freed with sysFree.
+
+* persistentalloc combines multiple smaller allocations into a single
+  sysAlloc to avoid fragmentation. However, there is no way to free
+  persistentalloced objects (hence the name).
+
+* fixalloc is a SLAB-style allocator that allocates objects of a fixed
+  size. fixalloced objects can be freed, but this memory can only be
+  reused by the same fixalloc pool, so it can only be reused for
+  objects of the same type.
+
+In general, types that are allocated using any of these should be
+marked as not in heap by embedding `runtime/internal/sys.NotInHeap`.
+
+Objects that are allocated in unmanaged memory **must not** contain
+heap pointers unless the following rules are also obeyed:
+
+1. Any pointers from unmanaged memory to the heap must be garbage
+   collection roots. More specifically, any pointer must either be
+   accessible through a global variable or be added as an explicit
+   garbage collection root in `runtime.markroot`.
+
+2. If the memory is reused, the heap pointers must be zero-initialized
+   before they become visible as GC roots. Otherwise, the GC may
+   observe stale heap pointers. See "Zero-initialization versus
+   zeroing".
+
+Zero-initialization versus zeroing
+==================================
+
+There are two types of zeroing in the runtime, depending on whether
+the memory is already initialized to a type-safe state.
+
+If memory is not in a type-safe state, meaning it potentially contains
+"garbage" because it was just allocated and it is being initialized
+for first use, then it must be *zero-initialized* using
+`memclrNoHeapPointers` or non-pointer writes. This does not perform
+write barriers.
+
+If memory is already in a type-safe state and is simply being set to
+the zero value, this must be done using regular writes, `typedmemclr`,
+or `memclrHasPointers`. This performs write barriers.
+
+Runtime-only compiler directives
+================================
+
+In addition to the "//go:" directives documented in "go doc compile",
+the compiler supports additional directives only in the runtime.
+
+go:systemstack
+--------------
+
+`go:systemstack` indicates that a function must run on the system
+stack. This is checked dynamically by a special function prologue.
+
+go:nowritebarrier
+-----------------
+
+`go:nowritebarrier` directs the compiler to emit an error if the
+following function contains any write barriers. (It *does not*
+suppress the generation of write barriers; it is simply an assertion.)
+
+Usually you want `go:nowritebarrierrec`. `go:nowritebarrier` is
+primarily useful in situations where it's "nice" not to have write
+barriers, but not required for correctness.
+
+go:nowritebarrierrec and go:yeswritebarrierrec
+----------------------------------------------
+
+`go:nowritebarrierrec` directs the compiler to emit an error if the
+following function or any function it calls recursively, up to a
+`go:yeswritebarrierrec`, contains a write barrier.
+
+Logically, the compiler floods the call graph starting from each
+`go:nowritebarrierrec` function and produces an error if it encounters
+a function containing a write barrier. This flood stops at
+`go:yeswritebarrierrec` functions.
+
+`go:nowritebarrierrec` is used in the implementation of the write
+barrier to prevent infinite loops.
+
+Both directives are used in the scheduler. The write barrier requires
+an active P (`getg().m.p != nil`) and scheduler code often runs
+without an active P. In this case, `go:nowritebarrierrec` is used on
+functions that release the P or may run without a P and
+`go:yeswritebarrierrec` is used when code re-acquires an active P.
+Since these are function-level annotations, code that releases or
+acquires a P may need to be split across two functions.
+
+go:uintptrkeepalive
+-------------------
+
+The //go:uintptrkeepalive directive must be followed by a function declaration.
+
+It specifies that the function's uintptr arguments may be pointer values that
+have been converted to uintptr and must be kept alive for the duration of the
+call, even though from the types alone it would appear that the object is no
+longer needed during the call.
+
+This directive is similar to //go:uintptrescapes, but it does not force
+arguments to escape. Since stack growth does not understand these arguments,
+this directive must be used with //go:nosplit (in the marked function and all
+transitive calls) to prevent stack growth.
+
+The conversion from pointer to uintptr must appear in the argument list of any
+call to this function. This directive is used for some low-level system call
+implementations.