// Copyright 2018 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Garbage collector: stack objects and stack tracing
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// See the design doc at https://docs.google.com/document/d/1un-Jn47yByHL7I0aVIP_uVCMxjdM5mpelJhiKlIqxkE/edit?usp=sharing
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// Also see issue 22350.
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// Stack tracing solves the problem of determining which parts of the
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// stack are live and should be scanned. It runs as part of scanning
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// a single goroutine stack.
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//
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// Normally determining which parts of the stack are live is easy to
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// do statically, as user code has explicit references (reads and
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// writes) to stack variables. The compiler can do a simple dataflow
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// analysis to determine liveness of stack variables at every point in
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// the code. See cmd/compile/internal/gc/plive.go for that analysis.
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//
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// However, when we take the address of a stack variable, determining
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// whether that variable is still live is less clear. We can still
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// look for static accesses, but accesses through a pointer to the
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// variable are difficult in general to track statically. That pointer
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// can be passed among functions on the stack, conditionally retained,
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// etc.
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//
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// Instead, we will track pointers to stack variables dynamically.
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// All pointers to stack-allocated variables will themselves be on the
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// stack somewhere (or in associated locations, like defer records), so
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// we can find them all efficiently.
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//
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// Stack tracing is organized as a mini garbage collection tracing
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// pass. The objects in this garbage collection are all the variables
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// on the stack whose address is taken, and which themselves contain a
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// pointer. We call these variables "stack objects".
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//
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// We begin by determining all the stack objects on the stack and all
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// the statically live pointers that may point into the stack. We then
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// process each pointer to see if it points to a stack object. If it
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// does, we scan that stack object. It may contain pointers into the
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// heap, in which case those pointers are passed to the main garbage
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// collection. It may also contain pointers into the stack, in which
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// case we add them to our set of stack pointers.
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//
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// Once we're done processing all the pointers (including the ones we
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// added during processing), we've found all the stack objects that
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// are live. Any dead stack objects are not scanned and their contents
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// will not keep heap objects live. Unlike the main garbage
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// collection, we can't sweep the dead stack objects; they live on in
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// a moribund state until the stack frame that contains them is
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// popped.
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//
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// A stack can look like this:
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//
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// +----------+
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// | foo() |
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// | +------+ |
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// | | A | | <---\
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// | +------+ | |
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// | | |
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// | +------+ | |
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// | | B | | |
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// | +------+ | |
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// | | |
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// +----------+ |
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// | bar() | |
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// | +------+ | |
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// | | C | | <-\ |
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// | +----|-+ | | |
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// | | | | |
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// | +----v-+ | | |
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// | | D ---------/
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// | +------+ | |
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// | | |
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// +----------+ |
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// | baz() | |
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// | +------+ | |
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// | | E -------/
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// | +------+ |
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// | ^ |
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// | F: --/ |
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// | |
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// +----------+
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//
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// foo() calls bar() calls baz(). Each has a frame on the stack.
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// foo() has stack objects A and B.
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// bar() has stack objects C and D, with C pointing to D and D pointing to A.
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// baz() has a stack object E pointing to C, and a local variable F pointing to E.
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//
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// Starting from the pointer in local variable F, we will eventually
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// scan all of E, C, D, and A (in that order). B is never scanned
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// because there is no live pointer to it. If B is also statically
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// dead (meaning that foo() never accesses B again after it calls
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// bar()), then B's pointers into the heap are not considered live.
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package runtime
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import (
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"runtime/internal/sys"
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"unsafe"
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)
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const stackTraceDebug = false
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// Buffer for pointers found during stack tracing.
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// Must be smaller than or equal to workbuf.
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//
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//go:notinheap
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type stackWorkBuf struct {
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stackWorkBufHdr
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obj [(_WorkbufSize - unsafe.Sizeof(stackWorkBufHdr{})) / sys.PtrSize]uintptr
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}
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// Header declaration must come after the buf declaration above, because of issue #14620.
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//
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//go:notinheap
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type stackWorkBufHdr struct {
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workbufhdr
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next *stackWorkBuf // linked list of workbufs
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// Note: we could theoretically repurpose lfnode.next as this next pointer.
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// It would save 1 word, but that probably isn't worth busting open
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// the lfnode API.
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}
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// Buffer for stack objects found on a goroutine stack.
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// Must be smaller than or equal to workbuf.
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//
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//go:notinheap
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type stackObjectBuf struct {
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stackObjectBufHdr
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obj [(_WorkbufSize - unsafe.Sizeof(stackObjectBufHdr{})) / unsafe.Sizeof(stackObject{})]stackObject
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}
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//go:notinheap
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type stackObjectBufHdr struct {
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workbufhdr
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next *stackObjectBuf
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}
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func init() {
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if unsafe.Sizeof(stackWorkBuf{}) > unsafe.Sizeof(workbuf{}) {
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panic("stackWorkBuf too big")
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}
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if unsafe.Sizeof(stackObjectBuf{}) > unsafe.Sizeof(workbuf{}) {
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panic("stackObjectBuf too big")
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}
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}
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// A stackObject represents a variable on the stack that has had
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// its address taken.
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//
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//go:notinheap
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type stackObject struct {
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off uint32 // offset above stack.lo
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size uint32 // size of object
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typ *_type // type info (for ptr/nonptr bits). nil if object has been scanned.
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left *stackObject // objects with lower addresses
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right *stackObject // objects with higher addresses
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}
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// obj.typ = typ, but with no write barrier.
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//go:nowritebarrier
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func (obj *stackObject) setType(typ *_type) {
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// Types of stack objects are always in read-only memory, not the heap.
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// So not using a write barrier is ok.
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*(*uintptr)(unsafe.Pointer(&obj.typ)) = uintptr(unsafe.Pointer(typ))
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}
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// A stackScanState keeps track of the state used during the GC walk
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// of a goroutine.
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//
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//go:notinheap
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type stackScanState struct {
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cache pcvalueCache
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// stack limits
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stack stack
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// buf contains the set of possible pointers to stack objects.
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// Organized as a LIFO linked list of buffers.
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// All buffers except possibly the head buffer are full.
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buf *stackWorkBuf
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freeBuf *stackWorkBuf // keep around one free buffer for allocation hysteresis
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// list of stack objects
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// Objects are in increasing address order.
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head *stackObjectBuf
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tail *stackObjectBuf
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nobjs int
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// root of binary tree for fast object lookup by address
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// Initialized by buildIndex.
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root *stackObject
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}
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// Add p as a potential pointer to a stack object.
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// p must be a stack address.
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func (s *stackScanState) putPtr(p uintptr) {
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if p < s.stack.lo || p >= s.stack.hi {
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throw("address not a stack address")
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}
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buf := s.buf
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if buf == nil {
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// Initial setup.
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buf = (*stackWorkBuf)(unsafe.Pointer(getempty()))
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buf.nobj = 0
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buf.next = nil
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s.buf = buf
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} else if buf.nobj == len(buf.obj) {
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if s.freeBuf != nil {
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buf = s.freeBuf
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s.freeBuf = nil
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} else {
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buf = (*stackWorkBuf)(unsafe.Pointer(getempty()))
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}
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buf.nobj = 0
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buf.next = s.buf
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s.buf = buf
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}
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buf.obj[buf.nobj] = p
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buf.nobj++
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}
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// Remove and return a potential pointer to a stack object.
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// Returns 0 if there are no more pointers available.
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func (s *stackScanState) getPtr() uintptr {
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buf := s.buf
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if buf == nil {
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// Never had any data.
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return 0
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}
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if buf.nobj == 0 {
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if s.freeBuf != nil {
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// Free old freeBuf.
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putempty((*workbuf)(unsafe.Pointer(s.freeBuf)))
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}
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// Move buf to the freeBuf.
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s.freeBuf = buf
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buf = buf.next
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s.buf = buf
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if buf == nil {
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// No more data.
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putempty((*workbuf)(unsafe.Pointer(s.freeBuf)))
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s.freeBuf = nil
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return 0
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}
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}
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buf.nobj--
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return buf.obj[buf.nobj]
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}
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// addObject adds a stack object at addr of type typ to the set of stack objects.
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func (s *stackScanState) addObject(addr uintptr, typ *_type) {
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x := s.tail
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if x == nil {
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// initial setup
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x = (*stackObjectBuf)(unsafe.Pointer(getempty()))
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x.next = nil
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s.head = x
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s.tail = x
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}
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if x.nobj > 0 && uint32(addr-s.stack.lo) < x.obj[x.nobj-1].off+x.obj[x.nobj-1].size {
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throw("objects added out of order or overlapping")
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}
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if x.nobj == len(x.obj) {
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// full buffer - allocate a new buffer, add to end of linked list
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y := (*stackObjectBuf)(unsafe.Pointer(getempty()))
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y.next = nil
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x.next = y
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s.tail = y
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x = y
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}
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obj := &x.obj[x.nobj]
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x.nobj++
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obj.off = uint32(addr - s.stack.lo)
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obj.size = uint32(typ.size)
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obj.setType(typ)
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// obj.left and obj.right will be initalized by buildIndex before use.
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s.nobjs++
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}
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// buildIndex initializes s.root to a binary search tree.
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// It should be called after all addObject calls but before
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// any call of findObject.
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func (s *stackScanState) buildIndex() {
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s.root, _, _ = binarySearchTree(s.head, 0, s.nobjs)
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}
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// Build a binary search tree with the n objects in the list
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// x.obj[idx], x.obj[idx+1], ..., x.next.obj[0], ...
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// Returns the root of that tree, and the buf+idx of the nth object after x.obj[idx].
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// (The first object that was not included in the binary search tree.)
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// If n == 0, returns nil, x.
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func binarySearchTree(x *stackObjectBuf, idx int, n int) (root *stackObject, restBuf *stackObjectBuf, restIdx int) {
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if n == 0 {
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return nil, x, idx
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}
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var left, right *stackObject
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left, x, idx = binarySearchTree(x, idx, n/2)
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root = &x.obj[idx]
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idx++
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if idx == len(x.obj) {
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x = x.next
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idx = 0
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}
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right, x, idx = binarySearchTree(x, idx, n-n/2-1)
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root.left = left
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root.right = right
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return root, x, idx
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}
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// findObject returns the stack object containing address a, if any.
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// Must have called buildIndex previously.
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func (s *stackScanState) findObject(a uintptr) *stackObject {
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off := uint32(a - s.stack.lo)
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obj := s.root
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for {
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if obj == nil {
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return nil
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}
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if off < obj.off {
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obj = obj.left
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continue
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}
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if off >= obj.off+obj.size {
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obj = obj.right
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continue
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}
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return obj
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}
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}
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