src/cmd/compile/internal/ssa/deadcode.go - go - Git at Google

 // Copyright 2015 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.

 package ssa

 import (
 	"cmd/internal/src"
 )

 // findlive returns the reachable blocks and live values in f.
 func findlive(f *Func) (reachable []bool, live []bool) {
 	reachable = ReachableBlocks(f)
 	live, _ = liveValues(f, reachable)
 	return
 }

 // ReachableBlocks returns the reachable blocks in f.
 func ReachableBlocks(f *Func) []bool {
 	reachable := make([]bool, f.NumBlocks())
 	reachable[f.Entry.ID] = true
 	p := make([]*Block, 0, 64) // stack-like worklist
 	p = append(p, f.Entry)
 	for len(p) > 0 {
 		// Pop a reachable block
 		b := p[len(p)-1]
 		p = p[:len(p)-1]
 		// Mark successors as reachable
 		s := b.Succs
 		if b.Kind == BlockFirst {
 			s = s[:1]
 		}
 		for _, e := range s {
 			c := e.b
 			if int(c.ID) >= len(reachable) {
 				f.Fatalf("block %s >= f.NumBlocks()=%d?", c, len(reachable))
 			}
 			if !reachable[c.ID] {
 				reachable[c.ID] = true
 				p = append(p, c) // push
 			}
 		}
 	}
 	return reachable
 }

 // liveValues returns the live values in f and a list of values that are eligible
 // to be statements in reversed data flow order.
 // The second result is used to help conserve statement boundaries for debugging.
 // reachable is a map from block ID to whether the block is reachable.
 func liveValues(f *Func, reachable []bool) (live []bool, liveOrderStmts []*Value) {
 	live = make([]bool, f.NumValues())

 	// After regalloc, consider all values to be live.
 	// See the comment at the top of regalloc.go and in deadcode for details.
 	if f.RegAlloc != nil {
 		for i := range live {
 			live[i] = true
 		}
 		return
 	}

 	// Find all live values
 	q := make([]*Value, 0, 64) // stack-like worklist of unscanned values

 	// Starting set: all control values of reachable blocks are live.
 	// Calls are live (because callee can observe the memory state).
 	for _, b := range f.Blocks {
 		if !reachable[b.ID] {
 			continue
 		}
 		if v := b.Control; v != nil && !live[v.ID] {
 			live[v.ID] = true
 			q = append(q, v)
 			if v.Pos.IsStmt() != src.PosNotStmt {
 				liveOrderStmts = append(liveOrderStmts, v)
 			}
 		}
 		for _, v := range b.Values {
 			if (opcodeTable[v.Op].call || opcodeTable[v.Op].hasSideEffects) && !live[v.ID] {
 				live[v.ID] = true
 				q = append(q, v)
 				if v.Pos.IsStmt() != src.PosNotStmt {
 					liveOrderStmts = append(liveOrderStmts, v)
 				}
 			}
 			if v.Type.IsVoid() && !live[v.ID] {
 				// The only Void ops are nil checks and inline marks.  We must keep these.
 				live[v.ID] = true
 				q = append(q, v)
 				if v.Pos.IsStmt() != src.PosNotStmt {
 					liveOrderStmts = append(liveOrderStmts, v)
 				}
 			}
 		}
 	}

 	// Compute transitive closure of live values.
 	for len(q) > 0 {
 		// pop a reachable value
 		v := q[len(q)-1]
 		q = q[:len(q)-1]
 		for i, x := range v.Args {
 			if v.Op == OpPhi && !reachable[v.Block.Preds[i].b.ID] {
 				continue
 			}
 			if !live[x.ID] {
 				live[x.ID] = true
 				q = append(q, x) // push
 				if x.Pos.IsStmt() != src.PosNotStmt {
 					liveOrderStmts = append(liveOrderStmts, x)
 				}
 			}
 		}
 	}

 	return
 }

 // deadcode removes dead code from f.
 func deadcode(f *Func) {
 	// deadcode after regalloc is forbidden for now. Regalloc
 	// doesn't quite generate legal SSA which will lead to some
 	// required moves being eliminated. See the comment at the
 	// top of regalloc.go for details.
 	if f.RegAlloc != nil {
 		f.Fatalf("deadcode after regalloc")
 	}

 	// Find reachable blocks.
 	reachable := ReachableBlocks(f)

 	// Get rid of edges from dead to live code.
 	for _, b := range f.Blocks {
 		if reachable[b.ID] {
 			continue
 		}
 		for i := 0; i < len(b.Succs); {
 			e := b.Succs[i]
 			if reachable[e.b.ID] {
 				b.removeEdge(i)
 			} else {
 				i++
 			}
 		}
 	}

 	// Get rid of dead edges from live code.
 	for _, b := range f.Blocks {
 		if !reachable[b.ID] {
 			continue
 		}
 		if b.Kind != BlockFirst {
 			continue
 		}
 		b.removeEdge(1)
 		b.Kind = BlockPlain
 		b.Likely = BranchUnknown
 	}

 	// Splice out any copies introduced during dead block removal.
 	copyelim(f)

 	// Find live values.
 	live, order := liveValues(f, reachable)

 	// Remove dead & duplicate entries from namedValues map.
 	s := f.newSparseSet(f.NumValues())
 	defer f.retSparseSet(s)
 	i := 0
 	for _, name := range f.Names {
 		j := 0
 		s.clear()
 		values := f.NamedValues[name]
 		for _, v := range values {
 			if live[v.ID] && !s.contains(v.ID) {
 				values[j] = v
 				j++
 				s.add(v.ID)
 			}
 		}
 		if j == 0 {
 			delete(f.NamedValues, name)
 		} else {
 			f.Names[i] = name
 			i++
 			for k := len(values) - 1; k >= j; k-- {
 				values[k] = nil
 			}
 			f.NamedValues[name] = values[:j]
 		}
 	}
 	for k := len(f.Names) - 1; k >= i; k-- {
 		f.Names[k] = LocalSlot{}
 	}
 	f.Names = f.Names[:i]

 	pendingLines := f.cachedLineStarts // Holds statement boundaries that need to be moved to a new value/block
 	pendingLines.clear()

 	// Unlink values and conserve statement boundaries
 	for i, b := range f.Blocks {
 		if !reachable[b.ID] {
 			// TODO what if control is statement boundary? Too late here.
 			b.SetControl(nil)
 		}
 		for _, v := range b.Values {
 			if !live[v.ID] {
 				v.resetArgs()
 				if v.Pos.IsStmt() == src.PosIsStmt && reachable[b.ID] {
 					pendingLines.set(v.Pos.Line(), int32(i)) // TODO could be more than one pos for a line
 				}
 			}
 		}
 	}

 	// Find new homes for lost lines -- require earliest in data flow with same line that is also in same block
 	for i := len(order) - 1; i >= 0; i-- {
 		w := order[i]
 		if j := pendingLines.get(w.Pos.Line()); j > -1 && f.Blocks[j] == w.Block {
 			w.Pos = w.Pos.WithIsStmt()
 			pendingLines.remove(w.Pos.Line())
 		}
 	}

 	// Any boundary that failed to match a live value can move to a block end
 	for i := 0; i < pendingLines.size(); i++ {
 		l, bi := pendingLines.getEntry(i)
 		b := f.Blocks[bi]
 		if b.Pos.Line() == l {
 			b.Pos = b.Pos.WithIsStmt()
 		}
 	}

 	// Remove dead values from blocks' value list. Return dead
 	// values to the allocator.
 	for _, b := range f.Blocks {
 		i := 0
 		for _, v := range b.Values {
 			if live[v.ID] {
 				b.Values[i] = v
 				i++
 			} else {
 				f.freeValue(v)
 			}
 		}
 		// aid GC
 		tail := b.Values[i:]
 		for j := range tail {
 			tail[j] = nil
 		}
 		b.Values = b.Values[:i]
 	}

 	// Remove dead blocks from WBLoads list.
 	i = 0
 	for _, b := range f.WBLoads {
 		if reachable[b.ID] {
 			f.WBLoads[i] = b
 			i++
 		}
 	}
 	for j := i; j < len(f.WBLoads); j++ {
 		f.WBLoads[j] = nil
 	}
 	f.WBLoads = f.WBLoads[:i]

 	// Remove unreachable blocks. Return dead blocks to allocator.
 	i = 0
 	for _, b := range f.Blocks {
 		if reachable[b.ID] {
 			f.Blocks[i] = b
 			i++
 		} else {
 			if len(b.Values) > 0 {
 				b.Fatalf("live values in unreachable block %v: %v", b, b.Values)
 			}
 			f.freeBlock(b)
 		}
 	}
 	// zero remainder to help GC
 	tail := f.Blocks[i:]
 	for j := range tail {
 		tail[j] = nil
 	}
 	f.Blocks = f.Blocks[:i]
 }

 // removeEdge removes the i'th outgoing edge from b (and
 // the corresponding incoming edge from b.Succs[i].b).
 func (b *Block) removeEdge(i int) {
 	e := b.Succs[i]
 	c := e.b
 	j := e.i

 	// Adjust b.Succs
 	b.removeSucc(i)

 	// Adjust c.Preds
 	c.removePred(j)

 	// Remove phi args from c's phis.
 	n := len(c.Preds)
 	for _, v := range c.Values {
 		if v.Op != OpPhi {
 			continue
 		}
 		v.Args[j].Uses--
 		v.Args[j] = v.Args[n]
 		v.Args[n] = nil
 		v.Args = v.Args[:n]
 		phielimValue(v)
 		// Note: this is trickier than it looks. Replacing
 		// a Phi with a Copy can in general cause problems because
 		// Phi and Copy don't have exactly the same semantics.
 		// Phi arguments always come from a predecessor block,
 		// whereas copies don't. This matters in loops like:
 		// 1: x = (Phi y)
 		//    y = (Add x 1)
 		//    goto 1
 		// If we replace Phi->Copy, we get
 		// 1: x = (Copy y)
 		//    y = (Add x 1)
 		//    goto 1
 		// (Phi y) refers to the *previous* value of y, whereas
 		// (Copy y) refers to the *current* value of y.
 		// The modified code has a cycle and the scheduler
 		// will barf on it.
 		//
 		// Fortunately, this situation can only happen for dead
 		// code loops. We know the code we're working with is
 		// not dead, so we're ok.
 		// Proof: If we have a potential bad cycle, we have a
 		// situation like this:
 		//   x = (Phi z)
 		//   y = (op1 x ...)
 		//   z = (op2 y ...)
 		// Where opX are not Phi ops. But such a situation
 		// implies a cycle in the dominator graph. In the
 		// example, x.Block dominates y.Block, y.Block dominates
 		// z.Block, and z.Block dominates x.Block (treating
 		// "dominates" as reflexive).  Cycles in the dominator
 		// graph can only happen in an unreachable cycle.
 	}
 }
	// Copyright 2015 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.

	package ssa

	import (
	"cmd/internal/src"
	)

	// findlive returns the reachable blocks and live values in f.
	func findlive(f *Func) (reachable []bool, live []bool) {
	reachable = ReachableBlocks(f)
	live, _ = liveValues(f, reachable)
	return
	}

	// ReachableBlocks returns the reachable blocks in f.
	func ReachableBlocks(f *Func) []bool {
	reachable := make([]bool, f.NumBlocks())
	reachable[f.Entry.ID] = true
	p := make([]*Block, 0, 64) // stack-like worklist
	p = append(p, f.Entry)
	for len(p) > 0 {
	// Pop a reachable block
	b := p[len(p)-1]
	p = p[:len(p)-1]
	// Mark successors as reachable
	s := b.Succs
	if b.Kind == BlockFirst {
	s = s[:1]
	}
	for _, e := range s {
	c := e.b
	if int(c.ID) >= len(reachable) {
	f.Fatalf("block %s >= f.NumBlocks()=%d?", c, len(reachable))
	}
	if !reachable[c.ID] {
	reachable[c.ID] = true
	p = append(p, c) // push
	}
	}
	}
	return reachable
	}

	// liveValues returns the live values in f and a list of values that are eligible
	// to be statements in reversed data flow order.
	// The second result is used to help conserve statement boundaries for debugging.
	// reachable is a map from block ID to whether the block is reachable.
	func liveValues(f Func, reachable []bool) (live []bool, liveOrderStmts []Value) {
	live = make([]bool, f.NumValues())

	// After regalloc, consider all values to be live.
	// See the comment at the top of regalloc.go and in deadcode for details.
	if f.RegAlloc != nil {
	for i := range live {
	live[i] = true
	}
	return
	}

	// Find all live values
	q := make([]*Value, 0, 64) // stack-like worklist of unscanned values

	// Starting set: all control values of reachable blocks are live.
	// Calls are live (because callee can observe the memory state).
	for _, b := range f.Blocks {
	if !reachable[b.ID] {
	continue
	}
	if v := b.Control; v != nil && !live[v.ID] {
	live[v.ID] = true
	q = append(q, v)
	if v.Pos.IsStmt() != src.PosNotStmt {
	liveOrderStmts = append(liveOrderStmts, v)
	}
	}
	for _, v := range b.Values {
	if (opcodeTable[v.Op].call \|\| opcodeTable[v.Op].hasSideEffects) && !live[v.ID] {
	live[v.ID] = true
	q = append(q, v)
	if v.Pos.IsStmt() != src.PosNotStmt {
	liveOrderStmts = append(liveOrderStmts, v)
	}
	}
	if v.Type.IsVoid() && !live[v.ID] {
	// The only Void ops are nil checks and inline marks. We must keep these.
	live[v.ID] = true
	q = append(q, v)
	if v.Pos.IsStmt() != src.PosNotStmt {
	liveOrderStmts = append(liveOrderStmts, v)
	}
	}
	}
	}

	// Compute transitive closure of live values.
	for len(q) > 0 {
	// pop a reachable value
	v := q[len(q)-1]
	q = q[:len(q)-1]
	for i, x := range v.Args {
	if v.Op == OpPhi && !reachable[v.Block.Preds[i].b.ID] {
	continue
	}
	if !live[x.ID] {
	live[x.ID] = true
	q = append(q, x) // push
	if x.Pos.IsStmt() != src.PosNotStmt {
	liveOrderStmts = append(liveOrderStmts, x)
	}
	}
	}
	}

	return
	}

	// deadcode removes dead code from f.
	func deadcode(f *Func) {
	// deadcode after regalloc is forbidden for now. Regalloc
	// doesn't quite generate legal SSA which will lead to some
	// required moves being eliminated. See the comment at the
	// top of regalloc.go for details.
	if f.RegAlloc != nil {
	f.Fatalf("deadcode after regalloc")
	}

	// Find reachable blocks.
	reachable := ReachableBlocks(f)

	// Get rid of edges from dead to live code.
	for _, b := range f.Blocks {
	if reachable[b.ID] {
	continue
	}
	for i := 0; i < len(b.Succs); {
	e := b.Succs[i]
	if reachable[e.b.ID] {
	b.removeEdge(i)
	} else {
	i++
	}
	}
	}

	// Get rid of dead edges from live code.
	for _, b := range f.Blocks {
	if !reachable[b.ID] {
	continue
	}
	if b.Kind != BlockFirst {
	continue
	}
	b.removeEdge(1)
	b.Kind = BlockPlain
	b.Likely = BranchUnknown
	}

	// Splice out any copies introduced during dead block removal.
	copyelim(f)

	// Find live values.
	live, order := liveValues(f, reachable)

	// Remove dead & duplicate entries from namedValues map.
	s := f.newSparseSet(f.NumValues())
	defer f.retSparseSet(s)
	i := 0
	for _, name := range f.Names {
	j := 0
	s.clear()
	values := f.NamedValues[name]
	for _, v := range values {
	if live[v.ID] && !s.contains(v.ID) {
	values[j] = v
	j++
	s.add(v.ID)
	}
	}
	if j == 0 {
	delete(f.NamedValues, name)
	} else {
	f.Names[i] = name
	i++
	for k := len(values) - 1; k >= j; k-- {
	values[k] = nil
	}
	f.NamedValues[name] = values[:j]
	}
	}
	for k := len(f.Names) - 1; k >= i; k-- {
	f.Names[k] = LocalSlot{}
	}
	f.Names = f.Names[:i]

	pendingLines := f.cachedLineStarts // Holds statement boundaries that need to be moved to a new value/block
	pendingLines.clear()

	// Unlink values and conserve statement boundaries
	for i, b := range f.Blocks {
	if !reachable[b.ID] {
	// TODO what if control is statement boundary? Too late here.
	b.SetControl(nil)
	}
	for _, v := range b.Values {
	if !live[v.ID] {
	v.resetArgs()
	if v.Pos.IsStmt() == src.PosIsStmt && reachable[b.ID] {
	pendingLines.set(v.Pos.Line(), int32(i)) // TODO could be more than one pos for a line
	}
	}
	}
	}

	// Find new homes for lost lines -- require earliest in data flow with same line that is also in same block
	for i := len(order) - 1; i >= 0; i-- {
	w := order[i]
	if j := pendingLines.get(w.Pos.Line()); j > -1 && f.Blocks[j] == w.Block {
	w.Pos = w.Pos.WithIsStmt()
	pendingLines.remove(w.Pos.Line())
	}
	}

	// Any boundary that failed to match a live value can move to a block end
	for i := 0; i < pendingLines.size(); i++ {
	l, bi := pendingLines.getEntry(i)
	b := f.Blocks[bi]
	if b.Pos.Line() == l {
	b.Pos = b.Pos.WithIsStmt()
	}
	}

	// Remove dead values from blocks' value list. Return dead
	// values to the allocator.
	for _, b := range f.Blocks {
	i := 0
	for _, v := range b.Values {
	if live[v.ID] {
	b.Values[i] = v
	i++
	} else {
	f.freeValue(v)
	}
	}
	// aid GC
	tail := b.Values[i:]
	for j := range tail {
	tail[j] = nil
	}
	b.Values = b.Values[:i]
	}

	// Remove dead blocks from WBLoads list.
	i = 0
	for _, b := range f.WBLoads {
	if reachable[b.ID] {
	f.WBLoads[i] = b
	i++
	}
	}
	for j := i; j < len(f.WBLoads); j++ {
	f.WBLoads[j] = nil
	}
	f.WBLoads = f.WBLoads[:i]

	// Remove unreachable blocks. Return dead blocks to allocator.
	i = 0
	for _, b := range f.Blocks {
	if reachable[b.ID] {
	f.Blocks[i] = b
	i++
	} else {
	if len(b.Values) > 0 {
	b.Fatalf("live values in unreachable block %v: %v", b, b.Values)
	}
	f.freeBlock(b)
	}
	}
	// zero remainder to help GC
	tail := f.Blocks[i:]
	for j := range tail {
	tail[j] = nil
	}
	f.Blocks = f.Blocks[:i]
	}

	// removeEdge removes the i'th outgoing edge from b (and
	// the corresponding incoming edge from b.Succs[i].b).
	func (b *Block) removeEdge(i int) {
	e := b.Succs[i]
	c := e.b
	j := e.i

	// Adjust b.Succs
	b.removeSucc(i)

	// Adjust c.Preds
	c.removePred(j)

	// Remove phi args from c's phis.
	n := len(c.Preds)
	for _, v := range c.Values {
	if v.Op != OpPhi {
	continue
	}
	v.Args[j].Uses--
	v.Args[j] = v.Args[n]
	v.Args[n] = nil
	v.Args = v.Args[:n]
	phielimValue(v)
	// Note: this is trickier than it looks. Replacing
	// a Phi with a Copy can in general cause problems because
	// Phi and Copy don't have exactly the same semantics.
	// Phi arguments always come from a predecessor block,
	// whereas copies don't. This matters in loops like:
	// 1: x = (Phi y)
	// y = (Add x 1)
	// goto 1
	// If we replace Phi->Copy, we get
	// 1: x = (Copy y)
	// y = (Add x 1)
	// goto 1
	// (Phi y) refers to the previous value of y, whereas
	// (Copy y) refers to the current value of y.
	// The modified code has a cycle and the scheduler
	// will barf on it.
	//
	// Fortunately, this situation can only happen for dead
	// code loops. We know the code we're working with is
	// not dead, so we're ok.
	// Proof: If we have a potential bad cycle, we have a
	// situation like this:
	// x = (Phi z)
	// y = (op1 x ...)
	// z = (op2 y ...)
	// Where opX are not Phi ops. But such a situation
	// implies a cycle in the dominator graph. In the
	// example, x.Block dominates y.Block, y.Block dominates
	// z.Block, and z.Block dominates x.Block (treating
	// "dominates" as reflexive). Cycles in the dominator
	// graph can only happen in an unreachable cycle.
	}
	}