src/cmd/compile/internal/ssa/fuse.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"
 	"fmt"
 )

 // fuseEarly runs fuse(f, fuseTypePlain|fuseTypeIntInRange).
 func fuseEarly(f *Func) { fuse(f, fuseTypePlain|fuseTypeIntInRange) }

 // fuseLate runs fuse(f, fuseTypePlain|fuseTypeIf|fuseTypeBranchRedirect).
 func fuseLate(f *Func) { fuse(f, fuseTypePlain|fuseTypeIf|fuseTypeBranchRedirect) }

 type fuseType uint8

 const (
 	fuseTypePlain fuseType = 1 << iota
 	fuseTypeIf
 	fuseTypeIntInRange
 	fuseTypeBranchRedirect
 	fuseTypeShortCircuit
 )

 // fuse simplifies control flow by joining basic blocks.
 func fuse(f *Func, typ fuseType) {
 	for changed := true; changed; {
 		changed = false
 		// Be sure to avoid quadratic behavior in fuseBlockPlain. See issue 13554.
 		// Previously this was dealt with using backwards iteration, now fuseBlockPlain
 		// handles large runs of blocks.
 		for i := len(f.Blocks) - 1; i >= 0; i-- {
 			b := f.Blocks[i]
 			if typ&fuseTypeIf != 0 {
 				changed = fuseBlockIf(b) || changed
 			}
 			if typ&fuseTypeIntInRange != 0 {
 				changed = fuseIntegerComparisons(b) || changed
 			}
 			if typ&fuseTypePlain != 0 {
 				changed = fuseBlockPlain(b) || changed
 			}
 			if typ&fuseTypeShortCircuit != 0 {
 				changed = shortcircuitBlock(b) || changed
 			}
 		}

 		if typ&fuseTypeBranchRedirect != 0 {
 			changed = fuseBranchRedirect(f) || changed
 		}
 		if changed {
 			f.invalidateCFG()
 		}
 	}
 }

 // fuseBlockIf handles the following cases where s0 and s1 are empty blocks.
 //
 //	   b        b           b       b
 //	\ / \ /    | \  /    \ / |     | |
 //	 s0  s1    |  s1      s0 |     | |
 //	  \ /      | /         \ |     | |
 //	   ss      ss           ss      ss
 //
 // If all Phi ops in ss have identical variables for slots corresponding to
 // s0, s1 and b then the branch can be dropped.
 // This optimization often comes up in switch statements with multiple
 // expressions in a case clause:
 //
 //	switch n {
 //	  case 1,2,3: return 4
 //	}
 //
 // TODO: If ss doesn't contain any OpPhis, are s0 and s1 dead code anyway.
 func fuseBlockIf(b *Block) bool {
 	if b.Kind != BlockIf {
 		return false
 	}
 	// It doesn't matter how much Preds does s0 or s1 have.
 	var ss0, ss1 *Block
 	s0 := b.Succs[0].b
 	i0 := b.Succs[0].i
 	if s0.Kind != BlockPlain || !isEmpty(s0) {
 		s0, ss0 = b, s0
 	} else {
 		ss0 = s0.Succs[0].b
 		i0 = s0.Succs[0].i
 	}
 	s1 := b.Succs[1].b
 	i1 := b.Succs[1].i
 	if s1.Kind != BlockPlain || !isEmpty(s1) {
 		s1, ss1 = b, s1
 	} else {
 		ss1 = s1.Succs[0].b
 		i1 = s1.Succs[0].i
 	}
 	if ss0 != ss1 {
 		if s0.Kind == BlockPlain && isEmpty(s0) && s1.Kind == BlockPlain && isEmpty(s1) {
 			// Two special cases where both s0, s1 and ss are empty blocks.
 			if s0 == ss1 {
 				s0, ss0 = b, ss1
 			} else if ss0 == s1 {
 				s1, ss1 = b, ss0
 			} else {
 				return false
 			}
 		} else {
 			return false
 		}
 	}
 	ss := ss0

 	// s0 and s1 are equal with b if the corresponding block is missing
 	// (2nd, 3rd and 4th case in the figure).

 	for _, v := range ss.Values {
 		if v.Op == OpPhi && v.Uses > 0 && v.Args[i0] != v.Args[i1] {
 			return false
 		}
 	}

 	// We do not need to redirect the Preds of s0 and s1 to ss,
 	// the following optimization will do this.
 	b.removeEdge(0)
 	if s0 != b && len(s0.Preds) == 0 {
 		s0.removeEdge(0)
 		// Move any (dead) values in s0 to b,
 		// where they will be eliminated by the next deadcode pass.
 		for _, v := range s0.Values {
 			v.Block = b
 		}
 		b.Values = append(b.Values, s0.Values...)
 		// Clear s0.
 		s0.Kind = BlockInvalid
 		s0.Values = nil
 		s0.Succs = nil
 		s0.Preds = nil
 	}

 	b.Kind = BlockPlain
 	b.Likely = BranchUnknown
 	b.ResetControls()
 	// The values in b may be dead codes, and clearing them in time may
 	// obtain new optimization opportunities.
 	// First put dead values that can be deleted into a slice walkValues.
 	// Then put their arguments in walkValues before resetting the dead values
 	// in walkValues, because the arguments may also become dead values.
 	walkValues := []*Value{}
 	for _, v := range b.Values {
 		if v.Uses == 0 && v.removeable() {
 			walkValues = append(walkValues, v)
 		}
 	}
 	for len(walkValues) != 0 {
 		v := walkValues[len(walkValues)-1]
 		walkValues = walkValues[:len(walkValues)-1]
 		if v.Uses == 0 && v.removeable() {
 			walkValues = append(walkValues, v.Args...)
 			v.reset(OpInvalid)
 		}
 	}
 	return true
 }

 // isEmpty reports whether b contains any live values.
 // There may be false positives.
 func isEmpty(b *Block) bool {
 	for _, v := range b.Values {
 		if v.Uses > 0 || v.Op.IsCall() || v.Op.HasSideEffects() || v.Type.IsVoid() || opcodeTable[v.Op].nilCheck {
 			return false
 		}
 	}
 	return true
 }

 // fuseBlockPlain handles a run of blocks with length >= 2,
 // whose interior has single predecessors and successors,
 // b must be BlockPlain, allowing it to be any node except the
 // last (multiple successors means not BlockPlain).
 // Cycles are handled and merged into b's successor.
 func fuseBlockPlain(b *Block) bool {
 	if b.Kind != BlockPlain {
 		return false
 	}

 	c := b.Succs[0].b
 	if len(c.Preds) != 1 || c == b { // At least 2 distinct blocks.
 		return false
 	}

 	// find earliest block in run.  Avoid simple cycles.
 	for len(b.Preds) == 1 && b.Preds[0].b != c && b.Preds[0].b.Kind == BlockPlain {
 		b = b.Preds[0].b
 	}

 	// find latest block in run.  Still beware of simple cycles.
 	for {
 		if c.Kind != BlockPlain {
 			break
 		} // Has exactly 1 successor
 		cNext := c.Succs[0].b
 		if cNext == b {
 			break
 		} // not a cycle
 		if len(cNext.Preds) != 1 {
 			break
 		} // no other incoming edge
 		c = cNext
 	}

 	// Try to preserve any statement marks on the ends of blocks; move values to C
 	var b_next *Block
 	for bx := b; bx != c; bx = b_next {
 		// For each bx with an end-of-block statement marker,
 		// try to move it to a value in the next block,
 		// or to the next block's end, if possible.
 		b_next = bx.Succs[0].b
 		if bx.Pos.IsStmt() == src.PosIsStmt {
 			l := bx.Pos.Line() // looking for another place to mark for line l
 			outOfOrder := false
 			for _, v := range b_next.Values {
 				if v.Pos.IsStmt() == src.PosNotStmt {
 					continue
 				}
 				if l == v.Pos.Line() { // Found a Value with same line, therefore done.
 					v.Pos = v.Pos.WithIsStmt()
 					l = 0
 					break
 				}
 				if l < v.Pos.Line() {
 					// The order of values in a block is not specified so OOO in a block is not interesting,
 					// but they do all come before the end of the block, so this disqualifies attaching to end of b_next.
 					outOfOrder = true
 				}
 			}
 			if l != 0 && !outOfOrder && (b_next.Pos.Line() == l || b_next.Pos.IsStmt() != src.PosIsStmt) {
 				b_next.Pos = bx.Pos.WithIsStmt()
 			}
 		}
 		// move all of bx's values to c (note containing loop excludes c)
 		for _, v := range bx.Values {
 			v.Block = c
 		}
 	}

 	// Compute the total number of values and find the largest value slice in the run, to maximize chance of storage reuse.
 	total := 0
 	totalBeforeMax := 0 // number of elements preceding the maximum block (i.e. its position in the result).
 	max_b := b          // block with maximum capacity

 	for bx := b; ; bx = bx.Succs[0].b {
 		if cap(bx.Values) > cap(max_b.Values) {
 			totalBeforeMax = total
 			max_b = bx
 		}
 		total += len(bx.Values)
 		if bx == c {
 			break
 		}
 	}

 	// Use c's storage if fused blocks will fit, else use the max if that will fit, else allocate new storage.

 	// Take care to avoid c.Values pointing to b.valstorage.
 	// See golang.org/issue/18602.

 	// It's important to keep the elements in the same order; maintenance of
 	// debugging information depends on the order of *Values in Blocks.
 	// This can also cause changes in the order (which may affect other
 	// optimizations and possibly compiler output) for 32-vs-64 bit compilation
 	// platforms (word size affects allocation bucket size affects slice capacity).

 	// figure out what slice will hold the values,
 	// preposition the destination elements if not allocating new storage
 	var t []*Value
 	if total <= len(c.valstorage) {
 		t = c.valstorage[:total]
 		max_b = c
 		totalBeforeMax = total - len(c.Values)
 		copy(t[totalBeforeMax:], c.Values)
 	} else if total <= cap(max_b.Values) { // in place, somewhere
 		t = max_b.Values[0:total]
 		copy(t[totalBeforeMax:], max_b.Values)
 	} else {
 		t = make([]*Value, total)
 		max_b = nil
 	}

 	// copy the values
 	copyTo := 0
 	for bx := b; ; bx = bx.Succs[0].b {
 		if bx != max_b {
 			copy(t[copyTo:], bx.Values)
 		} else if copyTo != totalBeforeMax { // trust but verify.
 			panic(fmt.Errorf("totalBeforeMax (%d) != copyTo (%d), max_b=%v, b=%v, c=%v", totalBeforeMax, copyTo, max_b, b, c))
 		}
 		if bx == c {
 			break
 		}
 		copyTo += len(bx.Values)
 	}
 	c.Values = t

 	// replace b->c edge with preds(b) -> c
 	c.predstorage[0] = Edge{}
 	if len(b.Preds) > len(b.predstorage) {
 		c.Preds = b.Preds
 	} else {
 		c.Preds = append(c.predstorage[:0], b.Preds...)
 	}
 	for i, e := range c.Preds {
 		p := e.b
 		p.Succs[e.i] = Edge{c, i}
 	}
 	f := b.Func
 	if f.Entry == b {
 		f.Entry = c
 	}

 	// trash b's fields, just in case
 	for bx := b; bx != c; bx = b_next {
 		b_next = bx.Succs[0].b

 		bx.Kind = BlockInvalid
 		bx.Values = nil
 		bx.Preds = nil
 		bx.Succs = nil
 	}
 	return true
 }
	// 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"
	"fmt"
	)

	// fuseEarly runs fuse(f, fuseTypePlain\|fuseTypeIntInRange).
	func fuseEarly(f *Func) { fuse(f, fuseTypePlain\|fuseTypeIntInRange) }

	// fuseLate runs fuse(f, fuseTypePlain\|fuseTypeIf\|fuseTypeBranchRedirect).
	func fuseLate(f *Func) { fuse(f, fuseTypePlain\|fuseTypeIf\|fuseTypeBranchRedirect) }

	type fuseType uint8

	const (
	fuseTypePlain fuseType = 1 << iota
	fuseTypeIf
	fuseTypeIntInRange
	fuseTypeBranchRedirect
	fuseTypeShortCircuit
	)

	// fuse simplifies control flow by joining basic blocks.
	func fuse(f *Func, typ fuseType) {
	for changed := true; changed; {
	changed = false
	// Be sure to avoid quadratic behavior in fuseBlockPlain. See issue 13554.
	// Previously this was dealt with using backwards iteration, now fuseBlockPlain
	// handles large runs of blocks.
	for i := len(f.Blocks) - 1; i >= 0; i-- {
	b := f.Blocks[i]
	if typ&fuseTypeIf != 0 {
	changed = fuseBlockIf(b) \|\| changed
	}
	if typ&fuseTypeIntInRange != 0 {
	changed = fuseIntegerComparisons(b) \|\| changed
	}
	if typ&fuseTypePlain != 0 {
	changed = fuseBlockPlain(b) \|\| changed
	}
	if typ&fuseTypeShortCircuit != 0 {
	changed = shortcircuitBlock(b) \|\| changed
	}
	}

	if typ&fuseTypeBranchRedirect != 0 {
	changed = fuseBranchRedirect(f) \|\| changed
	}
	if changed {
	f.invalidateCFG()
	}
	}
	}

	// fuseBlockIf handles the following cases where s0 and s1 are empty blocks.
	//
	// b b b b
	// \ / \ / \| \ / \ / \| \| \|
	// s0 s1 \| s1 s0 \| \| \|
	// \ / \| / \ \| \| \|
	// ss ss ss ss
	//
	// If all Phi ops in ss have identical variables for slots corresponding to
	// s0, s1 and b then the branch can be dropped.
	// This optimization often comes up in switch statements with multiple
	// expressions in a case clause:
	//
	// switch n {
	// case 1,2,3: return 4
	// }
	//
	// TODO: If ss doesn't contain any OpPhis, are s0 and s1 dead code anyway.
	func fuseBlockIf(b *Block) bool {
	if b.Kind != BlockIf {
	return false
	}
	// It doesn't matter how much Preds does s0 or s1 have.
	var ss0, ss1 *Block
	s0 := b.Succs[0].b
	i0 := b.Succs[0].i
	if s0.Kind != BlockPlain \|\| !isEmpty(s0) {
	s0, ss0 = b, s0
	} else {
	ss0 = s0.Succs[0].b
	i0 = s0.Succs[0].i
	}
	s1 := b.Succs[1].b
	i1 := b.Succs[1].i
	if s1.Kind != BlockPlain \|\| !isEmpty(s1) {
	s1, ss1 = b, s1
	} else {
	ss1 = s1.Succs[0].b
	i1 = s1.Succs[0].i
	}
	if ss0 != ss1 {
	if s0.Kind == BlockPlain && isEmpty(s0) && s1.Kind == BlockPlain && isEmpty(s1) {
	// Two special cases where both s0, s1 and ss are empty blocks.
	if s0 == ss1 {
	s0, ss0 = b, ss1
	} else if ss0 == s1 {
	s1, ss1 = b, ss0
	} else {
	return false
	}
	} else {
	return false
	}
	}
	ss := ss0

	// s0 and s1 are equal with b if the corresponding block is missing
	// (2nd, 3rd and 4th case in the figure).

	for _, v := range ss.Values {
	if v.Op == OpPhi && v.Uses > 0 && v.Args[i0] != v.Args[i1] {
	return false
	}
	}

	// We do not need to redirect the Preds of s0 and s1 to ss,
	// the following optimization will do this.
	b.removeEdge(0)
	if s0 != b && len(s0.Preds) == 0 {
	s0.removeEdge(0)
	// Move any (dead) values in s0 to b,
	// where they will be eliminated by the next deadcode pass.
	for _, v := range s0.Values {
	v.Block = b
	}
	b.Values = append(b.Values, s0.Values...)
	// Clear s0.
	s0.Kind = BlockInvalid
	s0.Values = nil
	s0.Succs = nil
	s0.Preds = nil
	}

	b.Kind = BlockPlain
	b.Likely = BranchUnknown
	b.ResetControls()
	// The values in b may be dead codes, and clearing them in time may
	// obtain new optimization opportunities.
	// First put dead values that can be deleted into a slice walkValues.
	// Then put their arguments in walkValues before resetting the dead values
	// in walkValues, because the arguments may also become dead values.
	walkValues := []*Value{}
	for _, v := range b.Values {
	if v.Uses == 0 && v.removeable() {
	walkValues = append(walkValues, v)
	}
	}
	for len(walkValues) != 0 {
	v := walkValues[len(walkValues)-1]
	walkValues = walkValues[:len(walkValues)-1]
	if v.Uses == 0 && v.removeable() {
	walkValues = append(walkValues, v.Args...)
	v.reset(OpInvalid)
	}
	}
	return true
	}

	// isEmpty reports whether b contains any live values.
	// There may be false positives.
	func isEmpty(b *Block) bool {
	for _, v := range b.Values {
	if v.Uses > 0 \|\| v.Op.IsCall() \|\| v.Op.HasSideEffects() \|\| v.Type.IsVoid() \|\| opcodeTable[v.Op].nilCheck {
	return false
	}
	}
	return true
	}

	// fuseBlockPlain handles a run of blocks with length >= 2,
	// whose interior has single predecessors and successors,
	// b must be BlockPlain, allowing it to be any node except the
	// last (multiple successors means not BlockPlain).
	// Cycles are handled and merged into b's successor.
	func fuseBlockPlain(b *Block) bool {
	if b.Kind != BlockPlain {
	return false
	}

	c := b.Succs[0].b
	if len(c.Preds) != 1 \|\| c == b { // At least 2 distinct blocks.
	return false
	}

	// find earliest block in run. Avoid simple cycles.
	for len(b.Preds) == 1 && b.Preds[0].b != c && b.Preds[0].b.Kind == BlockPlain {
	b = b.Preds[0].b
	}

	// find latest block in run. Still beware of simple cycles.
	for {
	if c.Kind != BlockPlain {
	break
	} // Has exactly 1 successor
	cNext := c.Succs[0].b
	if cNext == b {
	break
	} // not a cycle
	if len(cNext.Preds) != 1 {
	break
	} // no other incoming edge
	c = cNext
	}

	// Try to preserve any statement marks on the ends of blocks; move values to C
	var b_next *Block
	for bx := b; bx != c; bx = b_next {
	// For each bx with an end-of-block statement marker,
	// try to move it to a value in the next block,
	// or to the next block's end, if possible.
	b_next = bx.Succs[0].b
	if bx.Pos.IsStmt() == src.PosIsStmt {
	l := bx.Pos.Line() // looking for another place to mark for line l
	outOfOrder := false
	for _, v := range b_next.Values {
	if v.Pos.IsStmt() == src.PosNotStmt {
	continue
	}
	if l == v.Pos.Line() { // Found a Value with same line, therefore done.
	v.Pos = v.Pos.WithIsStmt()
	l = 0
	break
	}
	if l < v.Pos.Line() {
	// The order of values in a block is not specified so OOO in a block is not interesting,
	// but they do all come before the end of the block, so this disqualifies attaching to end of b_next.
	outOfOrder = true
	}
	}
	if l != 0 && !outOfOrder && (b_next.Pos.Line() == l \|\| b_next.Pos.IsStmt() != src.PosIsStmt) {
	b_next.Pos = bx.Pos.WithIsStmt()
	}
	}
	// move all of bx's values to c (note containing loop excludes c)
	for _, v := range bx.Values {
	v.Block = c
	}
	}

	// Compute the total number of values and find the largest value slice in the run, to maximize chance of storage reuse.
	total := 0
	totalBeforeMax := 0 // number of elements preceding the maximum block (i.e. its position in the result).
	max_b := b // block with maximum capacity

	for bx := b; ; bx = bx.Succs[0].b {
	if cap(bx.Values) > cap(max_b.Values) {
	totalBeforeMax = total
	max_b = bx
	}
	total += len(bx.Values)
	if bx == c {
	break
	}
	}

	// Use c's storage if fused blocks will fit, else use the max if that will fit, else allocate new storage.

	// Take care to avoid c.Values pointing to b.valstorage.
	// See golang.org/issue/18602.

	// It's important to keep the elements in the same order; maintenance of
	// debugging information depends on the order of *Values in Blocks.
	// This can also cause changes in the order (which may affect other
	// optimizations and possibly compiler output) for 32-vs-64 bit compilation
	// platforms (word size affects allocation bucket size affects slice capacity).

	// figure out what slice will hold the values,
	// preposition the destination elements if not allocating new storage
	var t []*Value
	if total <= len(c.valstorage) {
	t = c.valstorage[:total]
	max_b = c
	totalBeforeMax = total - len(c.Values)
	copy(t[totalBeforeMax:], c.Values)
	} else if total <= cap(max_b.Values) { // in place, somewhere
	t = max_b.Values[0:total]
	copy(t[totalBeforeMax:], max_b.Values)
	} else {
	t = make([]*Value, total)
	max_b = nil
	}

	// copy the values
	copyTo := 0
	for bx := b; ; bx = bx.Succs[0].b {
	if bx != max_b {
	copy(t[copyTo:], bx.Values)
	} else if copyTo != totalBeforeMax { // trust but verify.
	panic(fmt.Errorf("totalBeforeMax (%d) != copyTo (%d), max_b=%v, b=%v, c=%v", totalBeforeMax, copyTo, max_b, b, c))
	}
	if bx == c {
	break
	}
	copyTo += len(bx.Values)
	}
	c.Values = t

	// replace b->c edge with preds(b) -> c
	c.predstorage[0] = Edge{}
	if len(b.Preds) > len(b.predstorage) {
	c.Preds = b.Preds
	} else {
	c.Preds = append(c.predstorage[:0], b.Preds...)
	}
	for i, e := range c.Preds {
	p := e.b
	p.Succs[e.i] = Edge{c, i}
	}
	f := b.Func
	if f.Entry == b {
	f.Entry = c
	}

	// trash b's fields, just in case
	for bx := b; bx != c; bx = b_next {
	b_next = bx.Succs[0].b

	bx.Kind = BlockInvalid
	bx.Values = nil
	bx.Preds = nil
	bx.Succs = nil
	}
	return true
	}