src/cmd/compile/internal/ssa/flagalloc.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

 // flagalloc allocates the flag register among all the flag-generating
 // instructions. Flag values are recomputed if they need to be
 // spilled/restored.
 func flagalloc(f *Func) {
 	// Compute the in-register flag value we want at the end of
 	// each block. This is basically a best-effort live variable
 	// analysis, so it can be much simpler than a full analysis.
 	end := make([]*Value, f.NumBlocks())
 	po := f.postorder()
 	for n := 0; n < 2; n++ {
 		for _, b := range po {
 			// Walk values backwards to figure out what flag
 			// value we want in the flag register at the start
 			// of the block.
 			var flag *Value
 			for _, c := range b.ControlValues() {
 				if c.Type.IsFlags() {
 					if flag != nil {
 						panic("cannot have multiple controls using flags")
 					}
 					flag = c
 				}
 			}
 			if flag == nil {
 				flag = end[b.ID]
 			}
 			for j := len(b.Values) - 1; j >= 0; j-- {
 				v := b.Values[j]
 				if v == flag {
 					flag = nil
 				}
 				if v.clobbersFlags() {
 					flag = nil
 				}
 				for _, a := range v.Args {
 					if a.Type.IsFlags() {
 						flag = a
 					}
 				}
 			}
 			if flag != nil {
 				for _, e := range b.Preds {
 					p := e.b
 					end[p.ID] = flag
 				}
 			}
 		}
 	}

 	// For blocks which have a flags control value, that's the only value
 	// we can leave in the flags register at the end of the block. (There
 	// is no place to put a flag regeneration instruction.)
 	for _, b := range f.Blocks {
 		if b.Kind == BlockDefer {
 			// Defer blocks internally use/clobber the flags value.
 			end[b.ID] = nil
 			continue
 		}
 		for _, v := range b.ControlValues() {
 			if v.Type.IsFlags() && end[b.ID] != v {
 				end[b.ID] = nil
 			}
 		}
 	}

 	// Compute which flags values will need to be spilled.
 	spill := map[ID]bool{}
 	for _, b := range f.Blocks {
 		var flag *Value
 		if len(b.Preds) > 0 {
 			flag = end[b.Preds[0].b.ID]
 		}
 		for _, v := range b.Values {
 			for _, a := range v.Args {
 				if !a.Type.IsFlags() {
 					continue
 				}
 				if a == flag {
 					continue
 				}
 				// a will need to be restored here.
 				spill[a.ID] = true
 				flag = a
 			}
 			if v.clobbersFlags() {
 				flag = nil
 			}
 			if v.Type.IsFlags() {
 				flag = v
 			}
 		}
 		for _, v := range b.ControlValues() {
 			if v != flag && v.Type.IsFlags() {
 				spill[v.ID] = true
 			}
 		}
 		if v := end[b.ID]; v != nil && v != flag {
 			spill[v.ID] = true
 		}
 	}

 	// Add flag spill and recomputation where they are needed.
 	var remove []*Value // values that should be checked for possible removal
 	var oldSched []*Value
 	for _, b := range f.Blocks {
 		oldSched = append(oldSched[:0], b.Values...)
 		b.Values = b.Values[:0]
 		// The current live flag value (the pre-flagalloc copy).
 		var flag *Value
 		if len(b.Preds) > 0 {
 			flag = end[b.Preds[0].b.ID]
 			// Note: the following condition depends on the lack of critical edges.
 			for _, e := range b.Preds[1:] {
 				p := e.b
 				if end[p.ID] != flag {
 					f.Fatalf("live flag in %s's predecessors not consistent", b)
 				}
 			}
 		}
 		for _, v := range oldSched {
 			if v.Op == OpPhi && v.Type.IsFlags() {
 				f.Fatalf("phi of flags not supported: %s", v.LongString())
 			}

 			// If v will be spilled, and v uses memory, then we must split it
 			// into a load + a flag generator.
 			if spill[v.ID] && v.MemoryArg() != nil {
 				remove = append(remove, v)
 				if !f.Config.splitLoad(v) {
 					f.Fatalf("can't split flag generator: %s", v.LongString())
 				}
 			}

 			// Make sure any flag arg of v is in the flags register.
 			// If not, recompute it.
 			for i, a := range v.Args {
 				if !a.Type.IsFlags() {
 					continue
 				}
 				if a == flag {
 					continue
 				}
 				// Recalculate a
 				c := copyFlags(a, b)
 				// Update v.
 				v.SetArg(i, c)
 				// Remember the most-recently computed flag value.
 				flag = a
 			}
 			// Issue v.
 			b.Values = append(b.Values, v)
 			if v.clobbersFlags() {
 				flag = nil
 			}
 			if v.Type.IsFlags() {
 				flag = v
 			}
 		}
 		for i, v := range b.ControlValues() {
 			if v != flag && v.Type.IsFlags() {
 				// Recalculate control value.
 				remove = append(remove, v)
 				c := copyFlags(v, b)
 				b.ReplaceControl(i, c)
 				flag = v
 			}
 		}
 		if v := end[b.ID]; v != nil && v != flag {
 			// Need to reissue flag generator for use by
 			// subsequent blocks.
 			remove = append(remove, v)
 			copyFlags(v, b)
 			// Note: this flag generator is not properly linked up
 			// with the flag users. This breaks the SSA representation.
 			// We could fix up the users with another pass, but for now
 			// we'll just leave it. (Regalloc has the same issue for
 			// standard regs, and it runs next.)
 			// For this reason, take care not to add this flag
 			// generator to the remove list.
 		}
 	}

 	// Save live flag state for later.
 	for _, b := range f.Blocks {
 		b.FlagsLiveAtEnd = end[b.ID] != nil
 	}

 	// Remove any now-dead values.
 	// The number of values to remove is likely small,
 	// and removing them requires processing all values in a block,
 	// so minimize the number of blocks that we touch.

 	// Shrink remove to contain only dead values, and clobber those dead values.
 	for i := 0; i < len(remove); i++ {
 		v := remove[i]
 		if v.Uses == 0 {
 			v.reset(OpInvalid)
 			continue
 		}
 		// Remove v.
 		last := len(remove) - 1
 		remove[i] = remove[last]
 		remove[last] = nil
 		remove = remove[:last]
 		i-- // reprocess value at i
 	}

 	if len(remove) == 0 {
 		return
 	}

 	removeBlocks := f.newSparseSet(f.NumBlocks())
 	defer f.retSparseSet(removeBlocks)
 	for _, v := range remove {
 		removeBlocks.add(v.Block.ID)
 	}

 	// Process affected blocks, preserving value order.
 	for _, b := range f.Blocks {
 		if !removeBlocks.contains(b.ID) {
 			continue
 		}
 		i := 0
 		for j := 0; j < len(b.Values); j++ {
 			v := b.Values[j]
 			if v.Op == OpInvalid {
 				continue
 			}
 			b.Values[i] = v
 			i++
 		}
 		b.truncateValues(i)
 	}
 }

 func (v *Value) clobbersFlags() bool {
 	if opcodeTable[v.Op].clobberFlags {
 		return true
 	}
 	if v.Type.IsTuple() && (v.Type.FieldType(0).IsFlags() || v.Type.FieldType(1).IsFlags()) {
 		// This case handles the possibility where a flag value is generated but never used.
 		// In that case, there's no corresponding Select to overwrite the flags value,
 		// so we must consider flags clobbered by the tuple-generating instruction.
 		return true
 	}
 	return false
 }

 // copyFlags copies v (flag generator) into b, returns the copy.
 // If v's arg is also flags, copy recursively.
 func copyFlags(v *Value, b *Block) *Value {
 	flagsArgs := make(map[int]*Value)
 	for i, a := range v.Args {
 		if a.Type.IsFlags() || a.Type.IsTuple() {
 			flagsArgs[i] = copyFlags(a, b)
 		}
 	}
 	c := v.copyInto(b)
 	for i, a := range flagsArgs {
 		c.SetArg(i, a)
 	}
 	return c
 }
	// 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

	// flagalloc allocates the flag register among all the flag-generating
	// instructions. Flag values are recomputed if they need to be
	// spilled/restored.
	func flagalloc(f *Func) {
	// Compute the in-register flag value we want at the end of
	// each block. This is basically a best-effort live variable
	// analysis, so it can be much simpler than a full analysis.
	end := make([]*Value, f.NumBlocks())
	po := f.postorder()
	for n := 0; n < 2; n++ {
	for _, b := range po {
	// Walk values backwards to figure out what flag
	// value we want in the flag register at the start
	// of the block.
	var flag *Value
	for _, c := range b.ControlValues() {
	if c.Type.IsFlags() {
	if flag != nil {
	panic("cannot have multiple controls using flags")
	}
	flag = c
	}
	}
	if flag == nil {
	flag = end[b.ID]
	}
	for j := len(b.Values) - 1; j >= 0; j-- {
	v := b.Values[j]
	if v == flag {
	flag = nil
	}
	if v.clobbersFlags() {
	flag = nil
	}
	for _, a := range v.Args {
	if a.Type.IsFlags() {
	flag = a
	}
	}
	}
	if flag != nil {
	for _, e := range b.Preds {
	p := e.b
	end[p.ID] = flag
	}
	}
	}
	}

	// For blocks which have a flags control value, that's the only value
	// we can leave in the flags register at the end of the block. (There
	// is no place to put a flag regeneration instruction.)
	for _, b := range f.Blocks {
	if b.Kind == BlockDefer {
	// Defer blocks internally use/clobber the flags value.
	end[b.ID] = nil
	continue
	}
	for _, v := range b.ControlValues() {
	if v.Type.IsFlags() && end[b.ID] != v {
	end[b.ID] = nil
	}
	}
	}

	// Compute which flags values will need to be spilled.
	spill := map[ID]bool{}
	for _, b := range f.Blocks {
	var flag *Value
	if len(b.Preds) > 0 {
	flag = end[b.Preds[0].b.ID]
	}
	for _, v := range b.Values {
	for _, a := range v.Args {
	if !a.Type.IsFlags() {
	continue
	}
	if a == flag {
	continue
	}
	// a will need to be restored here.
	spill[a.ID] = true
	flag = a
	}
	if v.clobbersFlags() {
	flag = nil
	}
	if v.Type.IsFlags() {
	flag = v
	}
	}
	for _, v := range b.ControlValues() {
	if v != flag && v.Type.IsFlags() {
	spill[v.ID] = true
	}
	}
	if v := end[b.ID]; v != nil && v != flag {
	spill[v.ID] = true
	}
	}

	// Add flag spill and recomputation where they are needed.
	var remove []*Value // values that should be checked for possible removal
	var oldSched []*Value
	for _, b := range f.Blocks {
	oldSched = append(oldSched[:0], b.Values...)
	b.Values = b.Values[:0]
	// The current live flag value (the pre-flagalloc copy).
	var flag *Value
	if len(b.Preds) > 0 {
	flag = end[b.Preds[0].b.ID]
	// Note: the following condition depends on the lack of critical edges.
	for _, e := range b.Preds[1:] {
	p := e.b
	if end[p.ID] != flag {
	f.Fatalf("live flag in %s's predecessors not consistent", b)
	}
	}
	}
	for _, v := range oldSched {
	if v.Op == OpPhi && v.Type.IsFlags() {
	f.Fatalf("phi of flags not supported: %s", v.LongString())
	}

	// If v will be spilled, and v uses memory, then we must split it
	// into a load + a flag generator.
	if spill[v.ID] && v.MemoryArg() != nil {
	remove = append(remove, v)
	if !f.Config.splitLoad(v) {
	f.Fatalf("can't split flag generator: %s", v.LongString())
	}
	}

	// Make sure any flag arg of v is in the flags register.
	// If not, recompute it.
	for i, a := range v.Args {
	if !a.Type.IsFlags() {
	continue
	}
	if a == flag {
	continue
	}
	// Recalculate a
	c := copyFlags(a, b)
	// Update v.
	v.SetArg(i, c)
	// Remember the most-recently computed flag value.
	flag = a
	}
	// Issue v.
	b.Values = append(b.Values, v)
	if v.clobbersFlags() {
	flag = nil
	}
	if v.Type.IsFlags() {
	flag = v
	}
	}
	for i, v := range b.ControlValues() {
	if v != flag && v.Type.IsFlags() {
	// Recalculate control value.
	remove = append(remove, v)
	c := copyFlags(v, b)
	b.ReplaceControl(i, c)
	flag = v
	}
	}
	if v := end[b.ID]; v != nil && v != flag {
	// Need to reissue flag generator for use by
	// subsequent blocks.
	remove = append(remove, v)
	copyFlags(v, b)
	// Note: this flag generator is not properly linked up
	// with the flag users. This breaks the SSA representation.
	// We could fix up the users with another pass, but for now
	// we'll just leave it. (Regalloc has the same issue for
	// standard regs, and it runs next.)
	// For this reason, take care not to add this flag
	// generator to the remove list.
	}
	}

	// Save live flag state for later.
	for _, b := range f.Blocks {
	b.FlagsLiveAtEnd = end[b.ID] != nil
	}

	// Remove any now-dead values.
	// The number of values to remove is likely small,
	// and removing them requires processing all values in a block,
	// so minimize the number of blocks that we touch.

	// Shrink remove to contain only dead values, and clobber those dead values.
	for i := 0; i < len(remove); i++ {
	v := remove[i]
	if v.Uses == 0 {
	v.reset(OpInvalid)
	continue
	}
	// Remove v.
	last := len(remove) - 1
	remove[i] = remove[last]
	remove[last] = nil
	remove = remove[:last]
	i-- // reprocess value at i
	}

	if len(remove) == 0 {
	return
	}

	removeBlocks := f.newSparseSet(f.NumBlocks())
	defer f.retSparseSet(removeBlocks)
	for _, v := range remove {
	removeBlocks.add(v.Block.ID)
	}

	// Process affected blocks, preserving value order.
	for _, b := range f.Blocks {
	if !removeBlocks.contains(b.ID) {
	continue
	}
	i := 0
	for j := 0; j < len(b.Values); j++ {
	v := b.Values[j]
	if v.Op == OpInvalid {
	continue
	}
	b.Values[i] = v
	i++
	}
	b.truncateValues(i)
	}
	}

	func (v *Value) clobbersFlags() bool {
	if opcodeTable[v.Op].clobberFlags {
	return true
	}
	if v.Type.IsTuple() && (v.Type.FieldType(0).IsFlags() \|\| v.Type.FieldType(1).IsFlags()) {
	// This case handles the possibility where a flag value is generated but never used.
	// In that case, there's no corresponding Select to overwrite the flags value,
	// so we must consider flags clobbered by the tuple-generating instruction.
	return true
	}
	return false
	}

	// copyFlags copies v (flag generator) into b, returns the copy.
	// If v's arg is also flags, copy recursively.
	func copyFlags(v Value, b Block) *Value {
	flagsArgs := make(map[int]*Value)
	for i, a := range v.Args {
	if a.Type.IsFlags() \|\| a.Type.IsTuple() {
	flagsArgs[i] = copyFlags(a, b)
	}
	}
	c := v.copyInto(b)
	for i, a := range flagsArgs {
	c.SetArg(i, a)
	}
	return c
	}