go/src/cmd/internal/gc/popt.go

// Derived from Inferno utils/6c/reg.c
// http://code.google.com/p/inferno-os/source/browse/utils/6c/reg.c
//
//	Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
//	Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
//	Portions Copyright © 1997-1999 Vita Nuova Limited
//	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
//	Portions Copyright © 2004,2006 Bruce Ellis
//	Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
//	Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
//	Portions Copyright © 2009 The Go Authors.  All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

package gc

import (
	"cmd/internal/obj"
	"fmt"
	"sort"
	"strings"
)

// "Portable" optimizations.
// Compiled separately for 5g, 6g, and 8g, so allowed to use gg.h, opt.h.
// Must code to the intersection of the three back ends.

// Derived from Inferno utils/6c/gc.h
// http://code.google.com/p/inferno-os/source/browse/utils/6c/gc.h
//
//	Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
//	Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
//	Portions Copyright © 1997-1999 Vita Nuova Limited
//	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
//	Portions Copyright © 2004,2006 Bruce Ellis
//	Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
//	Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
//	Portions Copyright © 2009 The Go Authors.  All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

const (
	CLOAD = 5
	CREF  = 5
	CINF  = 1000
	LOOP  = 3
)

type Reg struct {
	set       Bits
	use1      Bits
	use2      Bits
	refbehind Bits
	refahead  Bits
	calbehind Bits
	calahead  Bits
	regdiff   Bits
	act       Bits
	regu      uint64
}

type Rgn struct {
	enter *Flow
	cost  int16
	varno int16
	regno int16
}

var Z *Node

// A Reg is a wrapper around a single Prog (one instruction) that holds
// register optimization information while the optimizer runs.
// r->prog is the instruction.

var R *Reg

const (
	NRGN = 600
)

// A Rgn represents a single regopt variable over a region of code
// where a register could potentially be dedicated to that variable.
// The code encompassed by a Rgn is defined by the flow graph,
// starting at enter, flood-filling forward while varno is refahead
// and backward while varno is refbehind, and following branches.  A
// single variable may be represented by multiple disjoint Rgns and
// each Rgn may choose a different register for that variable.
// Registers are allocated to regions greedily in order of descending
// cost.

var zreg Reg

var region [NRGN]Rgn

var rgp *Rgn

var nregion int

var nvar int

var regbits uint64

var externs Bits

var params Bits

var consts Bits

var addrs Bits

var ivar Bits

var ovar Bits

var change int

var maxnr int32

type OptStats struct {
	Ncvtreg int32
	Nspill  int32
	Nreload int32
	Ndelmov int32
	Nvar    int32
	Naddr   int32
}

var Ostats OptStats

/*
 * reg.c
 */

/*
 * peep.c
void	peep(Prog*);
void	excise(Flow*);
int	copyu(Prog*, Adr*, Adr*);
*/

/*
 * prog.c

void proginfo(ProgInfo*, Prog*);
*/
// p is a call instruction. Does the call fail to return?

var noreturn_symlist [10]*Sym

func Noreturn(p *obj.Prog) bool {
	if noreturn_symlist[0] == nil {
		noreturn_symlist[0] = Pkglookup("panicindex", Runtimepkg)
		noreturn_symlist[1] = Pkglookup("panicslice", Runtimepkg)
		noreturn_symlist[2] = Pkglookup("throwinit", Runtimepkg)
		noreturn_symlist[3] = Pkglookup("gopanic", Runtimepkg)
		noreturn_symlist[4] = Pkglookup("panicwrap", Runtimepkg)
		noreturn_symlist[5] = Pkglookup("throwreturn", Runtimepkg)
		noreturn_symlist[6] = Pkglookup("selectgo", Runtimepkg)
		noreturn_symlist[7] = Pkglookup("block", Runtimepkg)
	}

	if p.To.Node == nil {
		return false
	}
	s := ((p.To.Node).(*Node)).Sym
	if s == nil {
		return false
	}
	for i := 0; noreturn_symlist[i] != nil; i++ {
		if s == noreturn_symlist[i] {
			return true
		}
	}
	return false
}

// JMP chasing and removal.
//
// The code generator depends on being able to write out jump
// instructions that it can jump to now but fill in later.
// the linker will resolve them nicely, but they make the code
// longer and more difficult to follow during debugging.
// Remove them.

/* what instruction does a JMP to p eventually land on? */
func chasejmp(p *obj.Prog, jmploop *int) *obj.Prog {
	n := 0
	for p != nil && p.As == obj.AJMP && p.To.Type == obj.TYPE_BRANCH {
		n++
		if n > 10 {
			*jmploop = 1
			break
		}

		p = p.To.U.Branch
	}

	return p
}

/*
 * reuse reg pointer for mark/sweep state.
 * leave reg==nil at end because alive==nil.
 */
var alive interface{} = nil
var dead interface{} = 1

/* mark all code reachable from firstp as alive */
func mark(firstp *obj.Prog) {
	for p := firstp; p != nil; p = p.Link {
		if p.Opt != dead {
			break
		}
		p.Opt = alive
		if p.As != obj.ACALL && p.To.Type == obj.TYPE_BRANCH && p.To.U.Branch != nil {
			mark(p.To.U.Branch)
		}
		if p.As == obj.AJMP || p.As == obj.ARET || p.As == obj.AUNDEF {
			break
		}
	}
}

func fixjmp(firstp *obj.Prog) {
	if Debug['R'] != 0 && Debug['v'] != 0 {
		fmt.Printf("\nfixjmp\n")
	}

	// pass 1: resolve jump to jump, mark all code as dead.
	jmploop := 0

	for p := firstp; p != nil; p = p.Link {
		if Debug['R'] != 0 && Debug['v'] != 0 {
			fmt.Printf("%v\n", p)
		}
		if p.As != obj.ACALL && p.To.Type == obj.TYPE_BRANCH && p.To.U.Branch != nil && p.To.U.Branch.As == obj.AJMP {
			p.To.U.Branch = chasejmp(p.To.U.Branch, &jmploop)
			if Debug['R'] != 0 && Debug['v'] != 0 {
				fmt.Printf("->%v\n", p)
			}
		}

		p.Opt = dead
	}

	if Debug['R'] != 0 && Debug['v'] != 0 {
		fmt.Printf("\n")
	}

	// pass 2: mark all reachable code alive
	mark(firstp)

	// pass 3: delete dead code (mostly JMPs).
	last := (*obj.Prog)(nil)

	for p := firstp; p != nil; p = p.Link {
		if p.Opt == dead {
			if p.Link == nil && p.As == obj.ARET && last != nil && last.As != obj.ARET {
				// This is the final ARET, and the code so far doesn't have one.
				// Let it stay. The register allocator assumes that all live code in
				// the function can be traversed by starting at all the RET instructions
				// and following predecessor links. If we remove the final RET,
				// this assumption will not hold in the case of an infinite loop
				// at the end of a function.
				// Keep the RET but mark it dead for the liveness analysis.
				p.Mode = 1
			} else {
				if Debug['R'] != 0 && Debug['v'] != 0 {
					fmt.Printf("del %v\n", p)
				}
				continue
			}
		}

		if last != nil {
			last.Link = p
		}
		last = p
	}

	last.Link = nil

	// pass 4: elide JMP to next instruction.
	// only safe if there are no jumps to JMPs anymore.
	if jmploop == 0 {
		last := (*obj.Prog)(nil)
		for p := firstp; p != nil; p = p.Link {
			if p.As == obj.AJMP && p.To.Type == obj.TYPE_BRANCH && p.To.U.Branch == p.Link {
				if Debug['R'] != 0 && Debug['v'] != 0 {
					fmt.Printf("del %v\n", p)
				}
				continue
			}

			if last != nil {
				last.Link = p
			}
			last = p
		}

		last.Link = nil
	}

	if Debug['R'] != 0 && Debug['v'] != 0 {
		fmt.Printf("\n")
		for p := firstp; p != nil; p = p.Link {
			fmt.Printf("%v\n", p)
		}
		fmt.Printf("\n")
	}
}

// Control flow analysis. The Flow structures hold predecessor and successor
// information as well as basic loop analysis.
//
//	graph = flowstart(firstp, 0);
//	... use flow graph ...
//	flowend(graph); // free graph
//
// Typical uses of the flow graph are to iterate over all the flow-relevant instructions:
//
//	for(f = graph->start; f != nil; f = f->link)
//
// or, given an instruction f, to iterate over all the predecessors, which is
// f->p1 and this list:
//
//	for(f2 = f->p2; f2 != nil; f2 = f2->p2link)
//
// The size argument to flowstart specifies an amount of zeroed memory
// to allocate in every f->data field, for use by the client.
// If size == 0, f->data will be nil.

func Flowstart(firstp *obj.Prog, newData func() interface{}) *Graph {
	var info ProgInfo

	// Count and mark instructions to annotate.
	nf := 0

	for p := firstp; p != nil; p = p.Link {
		p.Opt = nil // should be already, but just in case
		Thearch.Proginfo(&info, p)
		if info.Flags&Skip != 0 {
			continue
		}
		p.Opt = interface{}(1)
		nf++
	}

	if nf == 0 {
		return nil
	}

	if nf >= 20000 {
		// fatal("%S is too big (%d instructions)", curfn->nname->sym, nf);
		return nil
	}

	// Allocate annotations and assign to instructions.
	graph := new(Graph)
	ff := make([]Flow, nf)
	start := &ff[0]
	id := 0
	var last *Flow
	for p := firstp; p != nil; p = p.Link {
		if p.Opt == nil {
			continue
		}
		f := &ff[0]
		ff = ff[1:]
		p.Opt = f
		f.Prog = p
		if last != nil {
			last.Link = f
		}
		last = f
		if newData != nil {
			f.Data = newData()
		}
		f.Id = int32(id)
		id++
	}

	// Fill in pred/succ information.
	var f1 *Flow
	var p *obj.Prog
	for f := start; f != nil; f = f.Link {
		p = f.Prog
		Thearch.Proginfo(&info, p)
		if info.Flags&Break == 0 {
			f1 = f.Link
			f.S1 = f1
			f1.P1 = f
		}

		if p.To.Type == obj.TYPE_BRANCH {
			if p.To.U.Branch == nil {
				Fatal("pnil %v", p)
			}
			f1 = p.To.U.Branch.Opt.(*Flow)
			if f1 == nil {
				Fatal("fnil %v / %v", p, p.To.U.Branch)
			}
			if f1 == f {
				//fatal("self loop %P", p);
				continue
			}

			f.S2 = f1
			f.P2link = f1.P2
			f1.P2 = f
		}
	}

	graph.Start = start
	graph.Num = nf
	return graph
}

func Flowend(graph *Graph) {
	for f := graph.Start; f != nil; f = f.Link {
		f.Prog.Opt = nil
	}
}

/*
 * find looping structure
 *
 * 1) find reverse postordering
 * 2) find approximate dominators,
 *	the actual dominators if the flow graph is reducible
 *	otherwise, dominators plus some other non-dominators.
 *	See Matthew S. Hecht and Jeffrey D. Ullman,
 *	"Analysis of a Simple Algorithm for Global Data Flow Problems",
 *	Conf.  Record of ACM Symp. on Principles of Prog. Langs, Boston, Massachusetts,
 *	Oct. 1-3, 1973, pp.  207-217.
 * 3) find all nodes with a predecessor dominated by the current node.
 *	such a node is a loop head.
 *	recursively, all preds with a greater rpo number are in the loop
 */
func postorder(r *Flow, rpo2r []*Flow, n int32) int32 {
	r.Rpo = 1
	r1 := r.S1
	if r1 != nil && r1.Rpo == 0 {
		n = postorder(r1, rpo2r, n)
	}
	r1 = r.S2
	if r1 != nil && r1.Rpo == 0 {
		n = postorder(r1, rpo2r, n)
	}
	rpo2r[n] = r
	n++
	return n
}

func rpolca(idom []int32, rpo1 int32, rpo2 int32) int32 {
	if rpo1 == -1 {
		return rpo2
	}
	var t int32
	for rpo1 != rpo2 {
		if rpo1 > rpo2 {
			t = rpo2
			rpo2 = rpo1
			rpo1 = t
		}

		for rpo1 < rpo2 {
			t = idom[rpo2]
			if t >= rpo2 {
				Fatal("bad idom")
			}
			rpo2 = t
		}
	}

	return rpo1
}

func doms(idom []int32, r int32, s int32) bool {
	for s > r {
		s = idom[s]
	}
	return s == r
}

func loophead(idom []int32, r *Flow) bool {
	src := r.Rpo
	if r.P1 != nil && doms(idom, src, r.P1.Rpo) {
		return true
	}
	for r = r.P2; r != nil; r = r.P2link {
		if doms(idom, src, r.Rpo) {
			return true
		}
	}
	return false
}

func loopmark(rpo2r **Flow, head int32, r *Flow) {
	if r.Rpo < head || r.Active == head {
		return
	}
	r.Active = head
	r.Loop += LOOP
	if r.P1 != nil {
		loopmark(rpo2r, head, r.P1)
	}
	for r = r.P2; r != nil; r = r.P2link {
		loopmark(rpo2r, head, r)
	}
}

func flowrpo(g *Graph) {
	g.Rpo = make([]*Flow, g.Num)
	idom := make([]int32, g.Num)

	for r1 := g.Start; r1 != nil; r1 = r1.Link {
		r1.Active = 0
	}

	rpo2r := g.Rpo
	d := postorder(g.Start, rpo2r, 0)
	nr := int32(g.Num)
	if d > nr {
		Fatal("too many reg nodes %d %d", d, nr)
	}
	nr = d
	var r1 *Flow
	for i := int32(0); i < nr/2; i++ {
		r1 = rpo2r[i]
		rpo2r[i] = rpo2r[nr-1-i]
		rpo2r[nr-1-i] = r1
	}

	for i := int32(0); i < nr; i++ {
		rpo2r[i].Rpo = i
	}

	idom[0] = 0
	var me int32
	for i := int32(0); i < nr; i++ {
		r1 = rpo2r[i]
		me = r1.Rpo
		d = -1

		// rpo2r[r->rpo] == r protects against considering dead code,
		// which has r->rpo == 0.
		if r1.P1 != nil && rpo2r[r1.P1.Rpo] == r1.P1 && r1.P1.Rpo < me {
			d = r1.P1.Rpo
		}
		for r1 = r1.P2; r1 != nil; r1 = r1.P2link {
			if rpo2r[r1.Rpo] == r1 && r1.Rpo < me {
				d = rpolca(idom, d, r1.Rpo)
			}
		}
		idom[i] = d
	}

	for i := int32(0); i < nr; i++ {
		r1 = rpo2r[i]
		r1.Loop++
		if r1.P2 != nil && loophead(idom, r1) {
			loopmark(&rpo2r[0], i, r1)
		}
	}

	for r1 := g.Start; r1 != nil; r1 = r1.Link {
		r1.Active = 0
	}
}

func Uniqp(r *Flow) *Flow {
	r1 := r.P1
	if r1 == nil {
		r1 = r.P2
		if r1 == nil || r1.P2link != nil {
			return nil
		}
	} else if r.P2 != nil {
		return nil
	}
	return r1
}

func Uniqs(r *Flow) *Flow {
	r1 := r.S1
	if r1 == nil {
		r1 = r.S2
		if r1 == nil {
			return nil
		}
	} else if r.S2 != nil {
		return nil
	}
	return r1
}

// The compilers assume they can generate temporary variables
// as needed to preserve the right semantics or simplify code
// generation and the back end will still generate good code.
// This results in a large number of ephemeral temporary variables.
// Merge temps with non-overlapping lifetimes and equal types using the
// greedy algorithm in Poletto and Sarkar, "Linear Scan Register Allocation",
// ACM TOPLAS 1999.

type TempVar struct {
	node     *Node
	def      *Flow
	use      *Flow
	freelink *TempVar
	merge    *TempVar
	start    int64
	end      int64
	addr     uint8
	removed  uint8
}

type startcmp []*TempVar

func (x startcmp) Len() int {
	return len(x)
}

func (x startcmp) Swap(i, j int) {
	x[i], x[j] = x[j], x[i]
}

func (x startcmp) Less(i, j int) bool {
	a := x[i]
	b := x[j]

	if a.start < b.start {
		return true
	}
	if a.start > b.start {
		return false
	}

	// Order what's left by id or symbol name,
	// just so that sort is forced into a specific ordering,
	// so that the result of the sort does not depend on
	// the sort implementation.
	if a.def != b.def {
		return int(a.def.Id-b.def.Id) < 0
	}
	if a.node != b.node {
		return stringsCompare(a.node.Sym.Name, b.node.Sym.Name) < 0
	}
	return false
}

// Is n available for merging?
func canmerge(n *Node) bool {
	return n.Class == PAUTO && strings.HasPrefix(n.Sym.Name, "autotmp")
}

func mergetemp(firstp *obj.Prog) {
	const (
		debugmerge = 1
	)

	g := Flowstart(firstp, nil)
	if g == nil {
		return
	}

	// Build list of all mergeable variables.
	nvar := 0
	for l := Curfn.Dcl; l != nil; l = l.Next {
		if canmerge(l.N) {
			nvar++
		}
	}

	var_ := make([]TempVar, nvar)
	nvar = 0
	var n *Node
	var v *TempVar
	for l := Curfn.Dcl; l != nil; l = l.Next {
		n = l.N
		if canmerge(n) {
			v = &var_[nvar]
			nvar++
			n.Opt = v
			v.node = n
		}
	}

	// Build list of uses.
	// We assume that the earliest reference to a temporary is its definition.
	// This is not true of variables in general but our temporaries are all
	// single-use (that's why we have so many!).
	var p *obj.Prog
	var info ProgInfo
	for f := g.Start; f != nil; f = f.Link {
		p = f.Prog
		Thearch.Proginfo(&info, p)

		if p.From.Node != nil && ((p.From.Node).(*Node)).Opt != nil && p.To.Node != nil && ((p.To.Node).(*Node)).Opt != nil {
			Fatal("double node %v", p)
		}
		v = nil
		n, _ = p.From.Node.(*Node)
		if n != nil {
			v, _ = n.Opt.(*TempVar)
		}
		if v == nil {
			n, _ = p.To.Node.(*Node)
			if n != nil {
				v, _ = n.Opt.(*TempVar)
			}
		}
		if v != nil {
			if v.def == nil {
				v.def = f
			}
			f.Data = v.use
			v.use = f
			if n == p.From.Node && (info.Flags&LeftAddr != 0) {
				v.addr = 1
			}
		}
	}

	if debugmerge > 1 && Debug['v'] != 0 {
		Dumpit("before", g.Start, 0)
	}

	nkill := 0

	// Special case.
	var p1 *obj.Prog
	var info1 ProgInfo
	var f *Flow
	for i := 0; i < len(var_); i++ {
		v = &var_[i]
		if v.addr != 0 {
			continue
		}

		// Used in only one instruction, which had better be a write.
		f = v.use
		if f != nil && f.Data.(*Flow) == nil {
			p = f.Prog
			Thearch.Proginfo(&info, p)
			if p.To.Node == v.node && (info.Flags&RightWrite != 0) && info.Flags&RightRead == 0 {
				p.As = obj.ANOP
				p.To = obj.Addr{}
				v.removed = 1
				if debugmerge > 0 && Debug['v'] != 0 {
					fmt.Printf("drop write-only %v\n", Sconv(v.node.Sym, 0))
				}
			} else {
				Fatal("temp used and not set: %v", p)
			}
			nkill++
			continue
		}

		// Written in one instruction, read in the next, otherwise unused,
		// no jumps to the next instruction. Happens mainly in 386 compiler.
		f = v.use
		if f != nil && f.Link == f.Data.(*Flow) && (f.Data.(*Flow)).Data.(*Flow) == nil && Uniqp(f.Link) == f {
			p = f.Prog
			Thearch.Proginfo(&info, p)
			p1 = f.Link.Prog
			Thearch.Proginfo(&info1, p1)
			const (
				SizeAny = SizeB | SizeW | SizeL | SizeQ | SizeF | SizeD
			)
			if p.From.Node == v.node && p1.To.Node == v.node && (info.Flags&Move != 0) && (info.Flags|info1.Flags)&(LeftAddr|RightAddr) == 0 && info.Flags&SizeAny == info1.Flags&SizeAny {
				p1.From = p.From
				Thearch.Excise(f)
				v.removed = 1
				if debugmerge > 0 && Debug['v'] != 0 {
					fmt.Printf("drop immediate-use %v\n", Sconv(v.node.Sym, 0))
				}
			}

			nkill++
			continue
		}
	}

	// Traverse live range of each variable to set start, end.
	// Each flood uses a new value of gen so that we don't have
	// to clear all the r->active words after each variable.
	gen := int32(0)

	for i := 0; i < len(var_); i++ {
		v = &var_[i]
		gen++
		for f = v.use; f != nil; f = f.Data.(*Flow) {
			mergewalk(v, f, uint32(gen))
		}
		if v.addr != 0 {
			gen++
			for f = v.use; f != nil; f = f.Data.(*Flow) {
				varkillwalk(v, f, uint32(gen))
			}
		}
	}

	// Sort variables by start.
	bystart := make([]*TempVar, len(var_))

	for i := 0; i < len(var_); i++ {
		bystart[i] = &var_[i]
	}
	sort.Sort(startcmp(bystart[:len(var_)]))

	// List of in-use variables, sorted by end, so that the ones that
	// will last the longest are the earliest ones in the array.
	// The tail inuse[nfree:] holds no-longer-used variables.
	// In theory we should use a sorted tree so that insertions are
	// guaranteed O(log n) and then the loop is guaranteed O(n log n).
	// In practice, it doesn't really matter.
	inuse := make([]*TempVar, len(var_))

	ninuse := 0
	nfree := len(var_)
	var t *Type
	var v1 *TempVar
	var j int
	for i := 0; i < len(var_); i++ {
		v = bystart[i]
		if debugmerge > 0 && Debug['v'] != 0 {
			fmt.Printf("consider %v: removed=%d\n", Nconv(v.node, obj.FmtSharp), v.removed)
		}

		if v.removed != 0 {
			continue
		}

		// Expire no longer in use.
		for ninuse > 0 && inuse[ninuse-1].end < v.start {
			ninuse--
			v1 = inuse[ninuse]
			nfree--
			inuse[nfree] = v1
		}

		if debugmerge > 0 && Debug['v'] != 0 {
			fmt.Printf("consider %v: removed=%d nfree=%d nvar=%d\n", Nconv(v.node, obj.FmtSharp), v.removed, nfree, len(var_))
		}

		// Find old temp to reuse if possible.
		t = v.node.Type

		for j = nfree; j < len(var_); j++ {
			v1 = inuse[j]
			if debugmerge > 0 && Debug['v'] != 0 {
				fmt.Printf("consider %v: maybe %v: type=%v,%v addrtaken=%d,%d\n", Nconv(v.node, obj.FmtSharp), Nconv(v1.node, obj.FmtSharp), Tconv(t, 0), Tconv(v1.node.Type, 0), v.node.Addrtaken, v1.node.Addrtaken)
			}

			// Require the types to match but also require the addrtaken bits to match.
			// If a variable's address is taken, that disables registerization for the individual
			// words of the variable (for example, the base,len,cap of a slice).
			// We don't want to merge a non-addressed var with an addressed one and
			// inhibit registerization of the former.
			if Eqtype(t, v1.node.Type) && v.node.Addrtaken == v1.node.Addrtaken {
				inuse[j] = inuse[nfree]
				nfree++
				if v1.merge != nil {
					v.merge = v1.merge
				} else {
					v.merge = v1
				}
				nkill++
				break
			}
		}

		// Sort v into inuse.
		j = ninuse
		ninuse++

		for j > 0 && inuse[j-1].end < v.end {
			inuse[j] = inuse[j-1]
			j--
		}

		inuse[j] = v
	}

	if debugmerge > 0 && Debug['v'] != 0 {
		fmt.Printf("%v [%d - %d]\n", Sconv(Curfn.Nname.Sym, 0), len(var_), nkill)
		var v *TempVar
		for i := 0; i < len(var_); i++ {
			v = &var_[i]
			fmt.Printf("var %v %v %d-%d", Nconv(v.node, obj.FmtSharp), Tconv(v.node.Type, 0), v.start, v.end)
			if v.addr != 0 {
				fmt.Printf(" addr=1")
			}
			if v.removed != 0 {
				fmt.Printf(" dead=1")
			}
			if v.merge != nil {
				fmt.Printf(" merge %v", Nconv(v.merge.node, obj.FmtSharp))
			}
			if v.start == v.end && v.def != nil {
				fmt.Printf(" %v", v.def.Prog)
			}
			fmt.Printf("\n")
		}

		if debugmerge > 1 && Debug['v'] != 0 {
			Dumpit("after", g.Start, 0)
		}
	}

	// Update node references to use merged temporaries.
	for f := g.Start; f != nil; f = f.Link {
		p = f.Prog
		n, _ = p.From.Node.(*Node)
		if n != nil {
			v, _ = n.Opt.(*TempVar)
			if v != nil && v.merge != nil {
				p.From.Node = v.merge.node
			}
		}
		n, _ = p.To.Node.(*Node)
		if n != nil {
			v, _ = n.Opt.(*TempVar)
			if v != nil && v.merge != nil {
				p.To.Node = v.merge.node
			}
		}
	}

	// Delete merged nodes from declaration list.
	var l *NodeList
	for lp := &Curfn.Dcl; ; {
		l = *lp
		if l == nil {
			break
		}

		Curfn.Dcl.End = l
		n = l.N
		v, _ = n.Opt.(*TempVar)
		if v != nil && (v.merge != nil || v.removed != 0) {
			*lp = l.Next
			continue
		}

		lp = &l.Next
	}

	// Clear aux structures.
	for i := 0; i < len(var_); i++ {
		var_[i].node.Opt = nil
	}

	Flowend(g)
}

func mergewalk(v *TempVar, f0 *Flow, gen uint32) {
	var p *obj.Prog
	var f1 *Flow

	for f1 = f0; f1 != nil; f1 = f1.P1 {
		if uint32(f1.Active) == gen {
			break
		}
		f1.Active = int32(gen)
		p = f1.Prog
		if v.end < p.Pc {
			v.end = p.Pc
		}
		if f1 == v.def {
			v.start = p.Pc
			break
		}
	}

	var f2 *Flow
	for f := f0; f != f1; f = f.P1 {
		for f2 = f.P2; f2 != nil; f2 = f2.P2link {
			mergewalk(v, f2, gen)
		}
	}
}

func varkillwalk(v *TempVar, f0 *Flow, gen uint32) {
	var p *obj.Prog
	var f1 *Flow

	for f1 = f0; f1 != nil; f1 = f1.S1 {
		if uint32(f1.Active) == gen {
			break
		}
		f1.Active = int32(gen)
		p = f1.Prog
		if v.end < p.Pc {
			v.end = p.Pc
		}
		if v.start > p.Pc {
			v.start = p.Pc
		}
		if p.As == obj.ARET || (p.As == obj.AVARKILL && p.To.Node == v.node) {
			break
		}
	}

	for f := f0; f != f1; f = f.S1 {
		varkillwalk(v, f.S2, gen)
	}
}

// Eliminate redundant nil pointer checks.
//
// The code generation pass emits a CHECKNIL for every possibly nil pointer.
// This pass removes a CHECKNIL if every predecessor path has already
// checked this value for nil.
//
// Simple backwards flood from check to definition.
// Run prog loop backward from end of program to beginning to avoid quadratic
// behavior removing a run of checks.
//
// Assume that stack variables with address not taken can be loaded multiple times
// from memory without being rechecked. Other variables need to be checked on
// each load.
type NilVar struct {
}

var killed int // f->data is either nil or &killed

func nilopt(firstp *obj.Prog) {
	g := Flowstart(firstp, nil)
	if g == nil {
		return
	}

	if Debug_checknil > 1 { /* || strcmp(curfn->nname->sym->name, "f1") == 0 */
		Dumpit("nilopt", g.Start, 0)
	}

	ncheck := 0
	nkill := 0
	var p *obj.Prog
	for f := g.Start; f != nil; f = f.Link {
		p = f.Prog
		if p.As != obj.ACHECKNIL || !Thearch.Regtyp(&p.From) {
			continue
		}
		ncheck++
		if Thearch.Stackaddr(&p.From) {
			if Debug_checknil != 0 && p.Lineno > 1 {
				Warnl(int(p.Lineno), "removed nil check of SP address")
			}
			f.Data = &killed
			continue
		}

		nilwalkfwd(f)
		if f.Data != nil {
			if Debug_checknil != 0 && p.Lineno > 1 {
				Warnl(int(p.Lineno), "removed nil check before indirect")
			}
			continue
		}

		nilwalkback(f)
		if f.Data != nil {
			if Debug_checknil != 0 && p.Lineno > 1 {
				Warnl(int(p.Lineno), "removed repeated nil check")
			}
			continue
		}
	}

	for f := g.Start; f != nil; f = f.Link {
		if f.Data != nil {
			nkill++
			Thearch.Excise(f)
		}
	}

	Flowend(g)

	if Debug_checknil > 1 {
		fmt.Printf("%v: removed %d of %d nil checks\n", Sconv(Curfn.Nname.Sym, 0), nkill, ncheck)
	}
}

func nilwalkback(fcheck *Flow) {
	var p *obj.Prog
	var info ProgInfo

	for f := fcheck; f != nil; f = Uniqp(f) {
		p = f.Prog
		Thearch.Proginfo(&info, p)
		if (info.Flags&RightWrite != 0) && Thearch.Sameaddr(&p.To, &fcheck.Prog.From) {
			// Found initialization of value we're checking for nil.
			// without first finding the check, so this one is unchecked.
			return
		}

		if f != fcheck && p.As == obj.ACHECKNIL && Thearch.Sameaddr(&p.From, &fcheck.Prog.From) {
			fcheck.Data = &killed
			return
		}
	}
}

// Here is a more complex version that scans backward across branches.
// It assumes fcheck->kill = 1 has been set on entry, and its job is to find a reason
// to keep the check (setting fcheck->kill = 0).
// It doesn't handle copying of aggregates as well as I would like,
// nor variables with their address taken,
// and it's too subtle to turn on this late in Go 1.2. Perhaps for Go 1.3.
/*
for(f1 = f0; f1 != nil; f1 = f1->p1) {
	if(f1->active == gen)
		break;
	f1->active = gen;
	p = f1->prog;

	// If same check, stop this loop but still check
	// alternate predecessors up to this point.
	if(f1 != fcheck && p->as == ACHECKNIL && thearch.sameaddr(&p->from, &fcheck->prog->from))
		break;

	thearch.proginfo(&info, p);
	if((info.flags & RightWrite) && thearch.sameaddr(&p->to, &fcheck->prog->from)) {
		// Found initialization of value we're checking for nil.
		// without first finding the check, so this one is unchecked.
		fcheck->kill = 0;
		return;
	}

	if(f1->p1 == nil && f1->p2 == nil) {
		print("lost pred for %P\n", fcheck->prog);
		for(f1=f0; f1!=nil; f1=f1->p1) {
			thearch.proginfo(&info, f1->prog);
			print("\t%P %d %d %D %D\n", r1->prog, info.flags&RightWrite, thearch.sameaddr(&f1->prog->to, &fcheck->prog->from), &f1->prog->to, &fcheck->prog->from);
		}
		fatal("lost pred trail");
	}
}

for(f = f0; f != f1; f = f->p1)
	for(f2 = f->p2; f2 != nil; f2 = f2->p2link)
		nilwalkback(fcheck, f2, gen);
*/
func nilwalkfwd(fcheck *Flow) {
	var p *obj.Prog
	var info ProgInfo

	// If the path down from rcheck dereferences the address
	// (possibly with a small offset) before writing to memory
	// and before any subsequent checks, it's okay to wait for
	// that implicit check. Only consider this basic block to
	// avoid problems like:
	//	_ = *x // should panic
	//	for {} // no writes but infinite loop may be considered visible
	last := (*Flow)(nil)

	for f := Uniqs(fcheck); f != nil; f = Uniqs(f) {
		p = f.Prog
		Thearch.Proginfo(&info, p)

		if (info.Flags&LeftRead != 0) && Thearch.Smallindir(&p.From, &fcheck.Prog.From) {
			fcheck.Data = &killed
			return
		}

		if (info.Flags&(RightRead|RightWrite) != 0) && Thearch.Smallindir(&p.To, &fcheck.Prog.From) {
			fcheck.Data = &killed
			return
		}

		// Stop if another nil check happens.
		if p.As == obj.ACHECKNIL {
			return
		}

		// Stop if value is lost.
		if (info.Flags&RightWrite != 0) && Thearch.Sameaddr(&p.To, &fcheck.Prog.From) {
			return
		}

		// Stop if memory write.
		if (info.Flags&RightWrite != 0) && !Thearch.Regtyp(&p.To) {
			return
		}

		// Stop if we jump backward.
		if last != nil && f.Id <= last.Id {
			return
		}
		last = f
	}
}