github.com/shogo82148/std@v1.22.1-0.20240327122250-4e474527810c/runtime/mgcstack.go

// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Garbage collector: stack objects and stack tracing
// See the design doc at https://docs.google.com/document/d/1un-Jn47yByHL7I0aVIP_uVCMxjdM5mpelJhiKlIqxkE/edit?usp=sharing
// Also see issue 22350.

// Stack tracing solves the problem of determining which parts of the
// stack are live and should be scanned. It runs as part of scanning
// a single goroutine stack.
//
// Normally determining which parts of the stack are live is easy to
// do statically, as user code has explicit references (reads and
// writes) to stack variables. The compiler can do a simple dataflow
// analysis to determine liveness of stack variables at every point in
// the code. See cmd/compile/internal/gc/plive.go for that analysis.
//
// However, when we take the address of a stack variable, determining
// whether that variable is still live is less clear. We can still
// look for static accesses, but accesses through a pointer to the
// variable are difficult in general to track statically. That pointer
// can be passed among functions on the stack, conditionally retained,
// etc.
//
// Instead, we will track pointers to stack variables dynamically.
// All pointers to stack-allocated variables will themselves be on the
// stack somewhere (or in associated locations, like defer records), so
// we can find them all efficiently.
//
// Stack tracing is organized as a mini garbage collection tracing
// pass. The objects in this garbage collection are all the variables
// on the stack whose address is taken, and which themselves contain a
// pointer. We call these variables "stack objects".
//
// We begin by determining all the stack objects on the stack and all
// the statically live pointers that may point into the stack. We then
// process each pointer to see if it points to a stack object. If it
// does, we scan that stack object. It may contain pointers into the
// heap, in which case those pointers are passed to the main garbage
// collection. It may also contain pointers into the stack, in which
// case we add them to our set of stack pointers.
//
// Once we're done processing all the pointers (including the ones we
// added during processing), we've found all the stack objects that
// are live. Any dead stack objects are not scanned and their contents
// will not keep heap objects live. Unlike the main garbage
// collection, we can't sweep the dead stack objects; they live on in
// a moribund state until the stack frame that contains them is
// popped.
//
// A stack can look like this:
//
// +----------+
// | foo()    |
// | +------+ |
// | |  A   | | <---\
// | +------+ |     |
// |          |     |
// | +------+ |     |
// | |  B   | |     |
// | +------+ |     |
// |          |     |
// +----------+     |
// | bar()    |     |
// | +------+ |     |
// | |  C   | | <-\ |
// | +----|-+ |   | |
// |      |   |   | |
// | +----v-+ |   | |
// | |  D  ---------/
// | +------+ |   |
// |          |   |
// +----------+   |
// | baz()    |   |
// | +------+ |   |
// | |  E  -------/
// | +------+ |
// |      ^   |
// | F: --/   |
// |          |
// +----------+
//
// foo() calls bar() calls baz(). Each has a frame on the stack.
// foo() has stack objects A and B.
// bar() has stack objects C and D, with C pointing to D and D pointing to A.
// baz() has a stack object E pointing to C, and a local variable F pointing to E.
//
// Starting from the pointer in local variable F, we will eventually
// scan all of E, C, D, and A (in that order). B is never scanned
// because there is no live pointer to it. If B is also statically
// dead (meaning that foo() never accesses B again after it calls
// bar()), then B's pointers into the heap are not considered live.

package runtime