src/cmd/compile/internal/gc/syntax.go - go - Git at Google

 // Copyright 2009 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.

 // “Abstract” syntax representation.

 package gc

 // A Node is a single node in the syntax tree.
 // Actually the syntax tree is a syntax DAG, because there is only one
 // node with Op=ONAME for a given instance of a variable x.
 // The same is true for Op=OTYPE and Op=OLITERAL.
 type Node struct {
 	// Tree structure.
 	// Generic recursive walks should follow these fields.
 	Left  *Node
 	Right *Node
 	Ntest *Node
 	Nincr *Node
 	Ninit *NodeList
 	Nbody *NodeList
 	Nelse *NodeList
 	List  *NodeList
 	Rlist *NodeList

 	// most nodes
 	Type  *Type
 	Orig  *Node // original form, for printing, and tracking copies of ONAMEs
 	Nname *Node

 	// func
 	Func *Func

 	// ONAME
 	Name     *Name
 	Defn     *Node // ONAME: initializing assignment; OLABEL: labeled statement
 	Pack     *Node // real package for import . names
 	Curfn    *Node // function for local variables
 	Paramfld *Type // TFIELD for this PPARAM; also for ODOT, curfn
 	Alloc    *Node // allocation call
 	*Param

 	// OPACK
 	Pkg *Pkg

 	// OARRAYLIT, OMAPLIT, OSTRUCTLIT.
 	Initplan *InitPlan

 	// Escape analysis.
 	Escflowsrc *NodeList // flow(this, src)
 	Escretval  *NodeList // on OCALLxxx, list of dummy return values

 	Sym *Sym // various

 	Opt interface{} // for optimization passes

 	// OLITERAL
 	Val Val

 	Xoffset  int64
 	Stkdelta int64 // offset added by stack frame compaction phase.

 	// Escape analysis.
 	Escloopdepth int32 // -1: global, 0: return variables, 1:function top level, increased inside function for every loop or label to mark scopes

 	Vargen  int32 // unique name for OTYPE/ONAME within a function.  Function outputs are numbered starting at one.
 	Lineno  int32
 	Iota    int32
 	Walkgen uint32

 	Funcdepth int32

 	// OREGISTER, OINDREG
 	Reg int16

 	// most nodes - smaller fields
 	Esclevel Level
 	Esc      uint16 // EscXXX

 	Op          uint8
 	Nointerface bool
 	Ullman      uint8 // sethi/ullman number
 	Addable     bool  // addressable
 	Etype       uint8 // op for OASOP, etype for OTYPE, exclam for export, 6g saved reg
 	Bounded     bool  // bounds check unnecessary
 	Class       uint8 // PPARAM, PAUTO, PEXTERN, etc
 	Embedded    uint8 // ODCLFIELD embedded type
 	Colas       bool  // OAS resulting from :=
 	Diag        uint8 // already printed error about this
 	Noescape    bool  // func arguments do not escape; TODO(rsc): move Noescape to Func struct (see CL 7360)
 	Walkdef     uint8
 	Typecheck   uint8
 	Local       bool
 	Dodata      uint8
 	Initorder   uint8
 	Used        bool
 	Isddd       bool // is the argument variadic
 	Implicit    bool
 	Addrtaken   bool // address taken, even if not moved to heap
 	Assigned    bool // is the variable ever assigned to
 	Likely      int8 // likeliness of if statement
 	Hasbreak    bool // has break statement
 }

 // Name holds Node fields used only by ONAME nodes.
 type Name struct {
 	Heapaddr  *Node // temp holding heap address of param
 	Inlvar    *Node // ONAME substitute while inlining
 	Decldepth int32 // declaration loop depth, increased for every loop or label
 	Method    bool  // OCALLMETH name
 	Readonly  bool
 	Captured  bool // is the variable captured by a closure
 	Byval     bool // is the variable captured by value or by reference
 	Needzero  bool // if it contains pointers, needs to be zeroed on function entry
 }

 type Param struct {
 	Ntype *Node

 	// ONAME func param with PHEAP
 	Outerexpr  *Node // expression copied into closure for variable
 	Stackparam *Node // OPARAM node referring to stack copy of param

 	// ONAME closure param with PPARAMREF
 	Outer   *Node // outer PPARAMREF in nested closure
 	Closure *Node // ONAME/PHEAP <-> ONAME/PPARAMREF
 	Top     int   // top context (Ecall, Eproc, etc)
 }

 // Func holds Node fields used only with function-like nodes.
 type Func struct {
 	Shortname *Node
 	Enter     *NodeList
 	Exit      *NodeList
 	Cvars     *NodeList // closure params
 	Dcl       *NodeList // autodcl for this func/closure
 	Inldcl    *NodeList // copy of dcl for use in inlining
 	Closgen   int
 	Outerfunc *Node

 	Inl     *NodeList // copy of the body for use in inlining
 	InlCost int32

 	Endlineno int32

 	Nosplit        bool // func should not execute on separate stack
 	Nowritebarrier bool // emit compiler error instead of write barrier
 	Dupok          bool // duplicate definitions ok
 	Wrapper        bool // is method wrapper
 	Needctxt       bool // function uses context register (has closure variables)
 }

 // Node ops.
 const (
 	OXXX = iota

 	// names
 	ONAME    // var, const or func name
 	ONONAME  // unnamed arg or return value: f(int, string) (int, error) { etc }
 	OTYPE    // type name
 	OPACK    // import
 	OLITERAL // literal

 	// expressions
 	OADD             // x + y
 	OSUB             // x - y
 	OOR              // x | y
 	OXOR             // x ^ y
 	OADDSTR          // s + "foo"
 	OADDR            // &x
 	OANDAND          // b0 && b1
 	OAPPEND          // append
 	OARRAYBYTESTR    // string(bytes)
 	OARRAYBYTESTRTMP // string(bytes) ephemeral
 	OARRAYRUNESTR    // string(runes)
 	OSTRARRAYBYTE    // []byte(s)
 	OSTRARRAYBYTETMP // []byte(s) ephemeral
 	OSTRARRAYRUNE    // []rune(s)
 	OAS              // x = y or x := y
 	OAS2             // x, y, z = xx, yy, zz
 	OAS2FUNC         // x, y = f()
 	OAS2RECV         // x, ok = <-c
 	OAS2MAPR         // x, ok = m["foo"]
 	OAS2DOTTYPE      // x, ok = I.(int)
 	OASOP            // x += y
 	OASWB            // OAS but with write barrier
 	OCALL            // function call, method call or type conversion, possibly preceded by defer or go.
 	OCALLFUNC        // f()
 	OCALLMETH        // t.Method()
 	OCALLINTER       // err.Error()
 	OCALLPART        // t.Method (without ())
 	OCAP             // cap
 	OCLOSE           // close
 	OCLOSURE         // f = func() { etc }
 	OCMPIFACE        // err1 == err2
 	OCMPSTR          // s1 == s2
 	OCOMPLIT         // composite literal, typechecking may convert to a more specific OXXXLIT.
 	OMAPLIT          // M{"foo":3, "bar":4}
 	OSTRUCTLIT       // T{x:3, y:4}
 	OARRAYLIT        // [2]int{3, 4}
 	OPTRLIT          // &T{x:3, y:4}
 	OCONV            // var i int; var u uint; i = int(u)
 	OCONVIFACE       // I(t)
 	OCONVNOP         // type Int int; var i int; var j Int; i = int(j)
 	OCOPY            // copy
 	ODCL             // var x int
 	ODCLFUNC         // func f() or func (r) f()
 	ODCLFIELD        // struct field, interface field, or func/method argument/return value.
 	ODCLCONST        // const pi = 3.14
 	ODCLTYPE         // type Int int
 	ODELETE          // delete
 	ODOT             // t.x
 	ODOTPTR          // p.x that is implicitly (*p).x
 	ODOTMETH         // t.Method
 	ODOTINTER        // err.Error
 	OXDOT            // t.x, typechecking may convert to a more specific ODOTXXX.
 	ODOTTYPE         // e = err.(MyErr)
 	ODOTTYPE2        // e, ok = err.(MyErr)
 	OEQ              // x == y
 	ONE              // x != y
 	OLT              // x < y
 	OLE              // x <= y
 	OGE              // x >= y
 	OGT              // x > y
 	OIND             // *p
 	OINDEX           // a[i]
 	OINDEXMAP        // m[s]
 	OKEY             // The x:3 in t{x:3, y:4}, the 1:2 in a[1:2], the 2:20 in [3]int{2:20}, etc.
 	OPARAM           // The on-stack copy of a parameter or return value that escapes.
 	OLEN             // len
 	OMAKE            // make, typechecking may convert to a more specific OMAKEXXX.
 	OMAKECHAN        // make(chan int)
 	OMAKEMAP         // make(map[string]int)
 	OMAKESLICE       // make([]int, 0)
 	OMUL             // *
 	ODIV             // x / y
 	OMOD             // x % y
 	OLSH             // x << u
 	ORSH             // x >> u
 	OAND             // x & y
 	OANDNOT          // x &^ y
 	ONEW             // new
 	ONOT             // !b
 	OCOM             // ^x
 	OPLUS            // +x
 	OMINUS           // -y
 	OOROR            // b1 || b2
 	OPANIC           // panic
 	OPRINT           // print
 	OPRINTN          // println
 	OPAREN           // (x)
 	OSEND            // c <- x
 	OSLICE           // v[1:2], typechecking may convert to a more specific OSLICEXXX.
 	OSLICEARR        // a[1:2]
 	OSLICESTR        // s[1:2]
 	OSLICE3          // v[1:2:3], typechecking may convert to OSLICE3ARR.
 	OSLICE3ARR       // a[1:2:3]
 	ORECOVER         // recover
 	ORECV            // <-c
 	ORUNESTR         // string(i)
 	OSELRECV         // case x = <-c:
 	OSELRECV2        // case x, ok = <-c:
 	OIOTA            // iota
 	OREAL            // real
 	OIMAG            // imag
 	OCOMPLEX         // complex

 	// statements
 	OBLOCK    // block of code
 	OBREAK    // break
 	OCASE     // case, after being verified by swt.c's casebody.
 	OXCASE    // case, before verification.
 	OCONTINUE // continue
 	ODEFER    // defer
 	OEMPTY    // no-op
 	OFALL     // fallthrough, after being verified by swt.c's casebody.
 	OXFALL    // fallthrough, before verification.
 	OFOR      // for
 	OGOTO     // goto
 	OIF       // if
 	OLABEL    // label:
 	OPROC     // go
 	ORANGE    // range
 	ORETURN   // return
 	OSELECT   // select
 	OSWITCH   // switch x
 	OTYPESW   // switch err.(type)

 	// types
 	OTCHAN   // chan int
 	OTMAP    // map[string]int
 	OTSTRUCT // struct{}
 	OTINTER  // interface{}
 	OTFUNC   // func()
 	OTARRAY  // []int, [8]int, [N]int or [...]int

 	// misc
 	ODDD        // func f(args ...int) or f(l...) or var a = [...]int{0, 1, 2}.
 	ODDDARG     // func f(args ...int), introduced by escape analysis.
 	OINLCALL    // intermediary representation of an inlined call.
 	OEFACE      // itable and data words of an empty-interface value.
 	OITAB       // itable word of an interface value.
 	OSPTR       // base pointer of a slice or string.
 	OCLOSUREVAR // variable reference at beginning of closure function
 	OCFUNC      // reference to c function pointer (not go func value)
 	OCHECKNIL   // emit code to ensure pointer/interface not nil
 	OVARKILL    // variable is dead

 	// thearch-specific registers
 	OREGISTER // a register, such as AX.
 	OINDREG   // offset plus indirect of a register, such as 8(SP).

 	// arch-specific opcodes
 	OCMP    // compare: ACMP.
 	ODEC    // decrement: ADEC.
 	OINC    // increment: AINC.
 	OEXTEND // extend: ACWD/ACDQ/ACQO.
 	OHMUL   // high mul: AMUL/AIMUL for unsigned/signed (OMUL uses AIMUL for both).
 	OLROT   // left rotate: AROL.
 	ORROTC  // right rotate-carry: ARCR.
 	ORETJMP // return to other function
 	OPS     // compare parity set (for x86 NaN check)
 	OPC     // compare parity clear (for x86 NaN check)
 	OSQRT   // sqrt(float64), on systems that have hw support
 	OGETG   // runtime.getg() (read g pointer)

 	OEND
 )

 /*
  * Every node has a walkgen field.
  * If you want to do a traversal of a node graph that
  * might contain duplicates and want to avoid
  * visiting the same nodes twice, increment walkgen
  * before starting.  Then before processing a node, do
  *
  *	if(n->walkgen == walkgen)
  *		return;
  *	n->walkgen = walkgen;
  *
  * Such a walk cannot call another such walk recursively,
  * because of the use of the global walkgen.
  */
 var walkgen uint32

 // A NodeList is a linked list of nodes.
 // TODO(rsc): Some uses of NodeList should be made into slices.
 // The remaining ones probably just need a simple linked list,
 // not one with concatenation support.
 type NodeList struct {
 	N    *Node
 	Next *NodeList
 	End  *NodeList
 }

 // concat returns the concatenation of the lists a and b.
 // The storage taken by both is reused for the result.
 func concat(a *NodeList, b *NodeList) *NodeList {
 	if a == nil {
 		return b
 	}
 	if b == nil {
 		return a
 	}

 	a.End.Next = b
 	a.End = b.End
 	b.End = nil
 	return a
 }

 // list1 returns a one-element list containing n.
 func list1(n *Node) *NodeList {
 	if n == nil {
 		return nil
 	}
 	if n.Op == OBLOCK && n.Ninit == nil {
 		// Flatten list and steal storage.
 		// Poison pointer to catch errant uses.
 		l := n.List

 		n.List = nil
 		return l
 	}

 	l := new(NodeList)
 	l.N = n
 	l.End = l
 	return l
 }

 // list returns the result of appending n to l.
 func list(l *NodeList, n *Node) *NodeList {
 	return concat(l, list1(n))
 }

 // listsort sorts *l in place according to the 3-way comparison function f.
 // The algorithm is mergesort, so it is guaranteed to be O(n log n).
 func listsort(l **NodeList, f func(*Node, *Node) int) {
 	if *l == nil || (*l).Next == nil {
 		return
 	}

 	l1 := *l
 	l2 := *l
 	for {
 		l2 = l2.Next
 		if l2 == nil {
 			break
 		}
 		l2 = l2.Next
 		if l2 == nil {
 			break
 		}
 		l1 = l1.Next
 	}

 	l2 = l1.Next
 	l1.Next = nil
 	l2.End = (*l).End
 	(*l).End = l1

 	l1 = *l
 	listsort(&l1, f)
 	listsort(&l2, f)

 	if f(l1.N, l2.N) < 0 {
 		*l = l1
 	} else {
 		*l = l2
 		l2 = l1
 		l1 = *l
 	}

 	// now l1 == *l; and l1 < l2

 	var le *NodeList
 	for (l1 != nil) && (l2 != nil) {
 		for (l1.Next != nil) && f(l1.Next.N, l2.N) < 0 {
 			l1 = l1.Next
 		}

 		// l1 is last one from l1 that is < l2
 		le = l1.Next // le is the rest of l1, first one that is >= l2
 		if le != nil {
 			le.End = (*l).End
 		}

 		(*l).End = l1       // cut *l at l1
 		*l = concat(*l, l2) // glue l2 to *l's tail

 		l1 = l2 // l1 is the first element of *l that is < the new l2
 		l2 = le // ... because l2 now is the old tail of l1
 	}

 	*l = concat(*l, l2) // any remainder
 }

 // count returns the length of the list l.
 func count(l *NodeList) int {
 	n := int64(0)
 	for ; l != nil; l = l.Next {
 		n++
 	}
 	if int64(int(n)) != n { // Overflow.
 		Yyerror("too many elements in list")
 	}
 	return int(n)
 }
	// Copyright 2009 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.

	// “Abstract” syntax representation.

	package gc

	// A Node is a single node in the syntax tree.
	// Actually the syntax tree is a syntax DAG, because there is only one
	// node with Op=ONAME for a given instance of a variable x.
	// The same is true for Op=OTYPE and Op=OLITERAL.
	type Node struct {
	// Tree structure.
	// Generic recursive walks should follow these fields.
	Left *Node
	Right *Node
	Ntest *Node
	Nincr *Node
	Ninit *NodeList
	Nbody *NodeList
	Nelse *NodeList
	List *NodeList
	Rlist *NodeList

	// most nodes
	Type *Type
	Orig *Node // original form, for printing, and tracking copies of ONAMEs
	Nname *Node

	// func
	Func *Func

	// ONAME
	Name *Name
	Defn *Node // ONAME: initializing assignment; OLABEL: labeled statement
	Pack *Node // real package for import . names
	Curfn *Node // function for local variables
	Paramfld *Type // TFIELD for this PPARAM; also for ODOT, curfn
	Alloc *Node // allocation call
	*Param

	// OPACK
	Pkg *Pkg

	// OARRAYLIT, OMAPLIT, OSTRUCTLIT.
	Initplan *InitPlan

	// Escape analysis.
	Escflowsrc *NodeList // flow(this, src)
	Escretval *NodeList // on OCALLxxx, list of dummy return values

	Sym *Sym // various

	Opt interface{} // for optimization passes

	// OLITERAL
	Val Val

	Xoffset int64
	Stkdelta int64 // offset added by stack frame compaction phase.

	// Escape analysis.
	Escloopdepth int32 // -1: global, 0: return variables, 1:function top level, increased inside function for every loop or label to mark scopes

	Vargen int32 // unique name for OTYPE/ONAME within a function. Function outputs are numbered starting at one.
	Lineno int32
	Iota int32
	Walkgen uint32

	Funcdepth int32

	// OREGISTER, OINDREG
	Reg int16

	// most nodes - smaller fields
	Esclevel Level
	Esc uint16 // EscXXX

	Op uint8
	Nointerface bool
	Ullman uint8 // sethi/ullman number
	Addable bool // addressable
	Etype uint8 // op for OASOP, etype for OTYPE, exclam for export, 6g saved reg
	Bounded bool // bounds check unnecessary
	Class uint8 // PPARAM, PAUTO, PEXTERN, etc
	Embedded uint8 // ODCLFIELD embedded type
	Colas bool // OAS resulting from :=
	Diag uint8 // already printed error about this
	Noescape bool // func arguments do not escape; TODO(rsc): move Noescape to Func struct (see CL 7360)
	Walkdef uint8
	Typecheck uint8
	Local bool
	Dodata uint8
	Initorder uint8
	Used bool
	Isddd bool // is the argument variadic
	Implicit bool
	Addrtaken bool // address taken, even if not moved to heap
	Assigned bool // is the variable ever assigned to
	Likely int8 // likeliness of if statement
	Hasbreak bool // has break statement
	}

	// Name holds Node fields used only by ONAME nodes.
	type Name struct {
	Heapaddr *Node // temp holding heap address of param
	Inlvar *Node // ONAME substitute while inlining
	Decldepth int32 // declaration loop depth, increased for every loop or label
	Method bool // OCALLMETH name
	Readonly bool
	Captured bool // is the variable captured by a closure
	Byval bool // is the variable captured by value or by reference
	Needzero bool // if it contains pointers, needs to be zeroed on function entry
	}

	type Param struct {
	Ntype *Node

	// ONAME func param with PHEAP
	Outerexpr *Node // expression copied into closure for variable
	Stackparam *Node // OPARAM node referring to stack copy of param

	// ONAME closure param with PPARAMREF
	Outer *Node // outer PPARAMREF in nested closure
	Closure *Node // ONAME/PHEAP <-> ONAME/PPARAMREF
	Top int // top context (Ecall, Eproc, etc)
	}

	// Func holds Node fields used only with function-like nodes.
	type Func struct {
	Shortname *Node
	Enter *NodeList
	Exit *NodeList
	Cvars *NodeList // closure params
	Dcl *NodeList // autodcl for this func/closure
	Inldcl *NodeList // copy of dcl for use in inlining
	Closgen int
	Outerfunc *Node

	Inl *NodeList // copy of the body for use in inlining
	InlCost int32

	Endlineno int32

	Nosplit bool // func should not execute on separate stack
	Nowritebarrier bool // emit compiler error instead of write barrier
	Dupok bool // duplicate definitions ok
	Wrapper bool // is method wrapper
	Needctxt bool // function uses context register (has closure variables)
	}

	// Node ops.
	const (
	OXXX = iota

	// names
	ONAME // var, const or func name
	ONONAME // unnamed arg or return value: f(int, string) (int, error) { etc }
	OTYPE // type name
	OPACK // import
	OLITERAL // literal

	// expressions
	OADD // x + y
	OSUB // x - y
	OOR // x \| y
	OXOR // x ^ y
	OADDSTR // s + "foo"
	OADDR // &x
	OANDAND // b0 && b1
	OAPPEND // append
	OARRAYBYTESTR // string(bytes)
	OARRAYBYTESTRTMP // string(bytes) ephemeral
	OARRAYRUNESTR // string(runes)
	OSTRARRAYBYTE // []byte(s)
	OSTRARRAYBYTETMP // []byte(s) ephemeral
	OSTRARRAYRUNE // []rune(s)
	OAS // x = y or x := y
	OAS2 // x, y, z = xx, yy, zz
	OAS2FUNC // x, y = f()
	OAS2RECV // x, ok = <-c
	OAS2MAPR // x, ok = m["foo"]
	OAS2DOTTYPE // x, ok = I.(int)
	OASOP // x += y
	OASWB // OAS but with write barrier
	OCALL // function call, method call or type conversion, possibly preceded by defer or go.
	OCALLFUNC // f()
	OCALLMETH // t.Method()
	OCALLINTER // err.Error()
	OCALLPART // t.Method (without ())
	OCAP // cap
	OCLOSE // close
	OCLOSURE // f = func() { etc }
	OCMPIFACE // err1 == err2
	OCMPSTR // s1 == s2
	OCOMPLIT // composite literal, typechecking may convert to a more specific OXXXLIT.
	OMAPLIT // M{"foo":3, "bar":4}
	OSTRUCTLIT // T{x:3, y:4}
	OARRAYLIT // [2]int{3, 4}
	OPTRLIT // &T{x:3, y:4}
	OCONV // var i int; var u uint; i = int(u)
	OCONVIFACE // I(t)
	OCONVNOP // type Int int; var i int; var j Int; i = int(j)
	OCOPY // copy
	ODCL // var x int
	ODCLFUNC // func f() or func (r) f()
	ODCLFIELD // struct field, interface field, or func/method argument/return value.
	ODCLCONST // const pi = 3.14
	ODCLTYPE // type Int int
	ODELETE // delete
	ODOT // t.x
	ODOTPTR // p.x that is implicitly (*p).x
	ODOTMETH // t.Method
	ODOTINTER // err.Error
	OXDOT // t.x, typechecking may convert to a more specific ODOTXXX.
	ODOTTYPE // e = err.(MyErr)
	ODOTTYPE2 // e, ok = err.(MyErr)
	OEQ // x == y
	ONE // x != y
	OLT // x < y
	OLE // x <= y
	OGE // x >= y
	OGT // x > y
	OIND // *p
	OINDEX // a[i]
	OINDEXMAP // m[s]
	OKEY // The x:3 in t{x:3, y:4}, the 1:2 in a[1:2], the 2:20 in [3]int{2:20}, etc.
	OPARAM // The on-stack copy of a parameter or return value that escapes.
	OLEN // len
	OMAKE // make, typechecking may convert to a more specific OMAKEXXX.
	OMAKECHAN // make(chan int)
	OMAKEMAP // make(map[string]int)
	OMAKESLICE // make([]int, 0)
	OMUL // *
	ODIV // x / y
	OMOD // x % y
	OLSH // x << u
	ORSH // x >> u
	OAND // x & y
	OANDNOT // x &^ y
	ONEW // new
	ONOT // !b
	OCOM // ^x
	OPLUS // +x
	OMINUS // -y
	OOROR // b1 \|\| b2
	OPANIC // panic
	OPRINT // print
	OPRINTN // println
	OPAREN // (x)
	OSEND // c <- x
	OSLICE // v[1:2], typechecking may convert to a more specific OSLICEXXX.
	OSLICEARR // a[1:2]
	OSLICESTR // s[1:2]
	OSLICE3 // v[1:2:3], typechecking may convert to OSLICE3ARR.
	OSLICE3ARR // a[1:2:3]
	ORECOVER // recover
	ORECV // <-c
	ORUNESTR // string(i)
	OSELRECV // case x = <-c:
	OSELRECV2 // case x, ok = <-c:
	OIOTA // iota
	OREAL // real
	OIMAG // imag
	OCOMPLEX // complex

	// statements
	OBLOCK // block of code
	OBREAK // break
	OCASE // case, after being verified by swt.c's casebody.
	OXCASE // case, before verification.
	OCONTINUE // continue
	ODEFER // defer
	OEMPTY // no-op
	OFALL // fallthrough, after being verified by swt.c's casebody.
	OXFALL // fallthrough, before verification.
	OFOR // for
	OGOTO // goto
	OIF // if
	OLABEL // label:
	OPROC // go
	ORANGE // range
	ORETURN // return
	OSELECT // select
	OSWITCH // switch x
	OTYPESW // switch err.(type)

	// types
	OTCHAN // chan int
	OTMAP // map[string]int
	OTSTRUCT // struct{}
	OTINTER // interface{}
	OTFUNC // func()
	OTARRAY // []int, [8]int, [N]int or [...]int

	// misc
	ODDD // func f(args ...int) or f(l...) or var a = [...]int{0, 1, 2}.
	ODDDARG // func f(args ...int), introduced by escape analysis.
	OINLCALL // intermediary representation of an inlined call.
	OEFACE // itable and data words of an empty-interface value.
	OITAB // itable word of an interface value.
	OSPTR // base pointer of a slice or string.
	OCLOSUREVAR // variable reference at beginning of closure function
	OCFUNC // reference to c function pointer (not go func value)
	OCHECKNIL // emit code to ensure pointer/interface not nil
	OVARKILL // variable is dead

	// thearch-specific registers
	OREGISTER // a register, such as AX.
	OINDREG // offset plus indirect of a register, such as 8(SP).

	// arch-specific opcodes
	OCMP // compare: ACMP.
	ODEC // decrement: ADEC.
	OINC // increment: AINC.
	OEXTEND // extend: ACWD/ACDQ/ACQO.
	OHMUL // high mul: AMUL/AIMUL for unsigned/signed (OMUL uses AIMUL for both).
	OLROT // left rotate: AROL.
	ORROTC // right rotate-carry: ARCR.
	ORETJMP // return to other function
	OPS // compare parity set (for x86 NaN check)
	OPC // compare parity clear (for x86 NaN check)
	OSQRT // sqrt(float64), on systems that have hw support
	OGETG // runtime.getg() (read g pointer)

	OEND
	)

	/*
	* Every node has a walkgen field.
	* If you want to do a traversal of a node graph that
	* might contain duplicates and want to avoid
	* visiting the same nodes twice, increment walkgen
	* before starting. Then before processing a node, do
	*
	* if(n->walkgen == walkgen)
	* return;
	* n->walkgen = walkgen;
	*
	* Such a walk cannot call another such walk recursively,
	* because of the use of the global walkgen.
	*/
	var walkgen uint32

	// A NodeList is a linked list of nodes.
	// TODO(rsc): Some uses of NodeList should be made into slices.
	// The remaining ones probably just need a simple linked list,
	// not one with concatenation support.
	type NodeList struct {
	N *Node
	Next *NodeList
	End *NodeList
	}

	// concat returns the concatenation of the lists a and b.
	// The storage taken by both is reused for the result.
	func concat(a NodeList, b NodeList) *NodeList {
	if a == nil {
	return b
	}
	if b == nil {
	return a
	}

	a.End.Next = b
	a.End = b.End
	b.End = nil
	return a
	}

	// list1 returns a one-element list containing n.
	func list1(n Node) NodeList {
	if n == nil {
	return nil
	}
	if n.Op == OBLOCK && n.Ninit == nil {
	// Flatten list and steal storage.
	// Poison pointer to catch errant uses.
	l := n.List

	n.List = nil
	return l
	}

	l := new(NodeList)
	l.N = n
	l.End = l
	return l
	}

	// list returns the result of appending n to l.
	func list(l NodeList, n Node) *NodeList {
	return concat(l, list1(n))
	}

	// listsort sorts *l in place according to the 3-way comparison function f.
	// The algorithm is mergesort, so it is guaranteed to be O(n log n).
	func listsort(l *NodeList, f func(Node, *Node) int) {
	if l == nil \|\| (l).Next == nil {
	return
	}

	l1 := *l
	l2 := *l
	for {
	l2 = l2.Next
	if l2 == nil {
	break
	}
	l2 = l2.Next
	if l2 == nil {
	break
	}
	l1 = l1.Next
	}

	l2 = l1.Next
	l1.Next = nil
	l2.End = (*l).End
	(*l).End = l1

	l1 = *l
	listsort(&l1, f)
	listsort(&l2, f)

	if f(l1.N, l2.N) < 0 {
	*l = l1
	} else {
	*l = l2
	l2 = l1
	l1 = *l
	}

	// now l1 == *l; and l1 < l2

	var le *NodeList
	for (l1 != nil) && (l2 != nil) {
	for (l1.Next != nil) && f(l1.Next.N, l2.N) < 0 {
	l1 = l1.Next
	}

	// l1 is last one from l1 that is < l2
	le = l1.Next // le is the rest of l1, first one that is >= l2
	if le != nil {
	le.End = (*l).End
	}

	(l).End = l1 // cut l at l1
	l = concat(l, l2) // glue l2 to *l's tail

	l1 = l2 // l1 is the first element of *l that is < the new l2
	l2 = le // ... because l2 now is the old tail of l1
	}

	l = concat(l, l2) // any remainder
	}

	// count returns the length of the list l.
	func count(l *NodeList) int {
	n := int64(0)
	for ; l != nil; l = l.Next {
	n++
	}
	if int64(int(n)) != n { // Overflow.
	Yyerror("too many elements in list")
	}
	return int(n)
	}