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

 // Copyright 2016 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 (
 	"fmt"
 )

 type loop struct {
 	header *Block // The header node of this (reducible) loop
 	outer  *loop  // loop containing this loop
 	// Next two fields not currently used, but cheap to maintain,
 	// and aid in computation of inner-ness and list of blocks.
 	nBlocks int32 // Number of blocks in this loop but not within inner loops
 	isInner bool  // True if never discovered to contain a loop
 }

 // outerinner records that outer contains inner
 func (sdom sparseTree) outerinner(outer, inner *loop) {
 	oldouter := inner.outer
 	if oldouter == nil || sdom.isAncestorEq(oldouter.header, outer.header) {
 		inner.outer = outer
 		outer.isInner = false
 	}
 }

 type loopnest struct {
 	f     *Func
 	b2l   []*loop
 	po    []*Block
 	sdom  sparseTree
 	loops []*loop
 }

 func min8(a, b int8) int8 {
 	if a < b {
 		return a
 	}
 	return b
 }

 func max8(a, b int8) int8 {
 	if a > b {
 		return a
 	}
 	return b
 }

 const (
 	blDEFAULT = 0
 	blMin     = blDEFAULT
 	blCALL    = 1
 	blRET     = 2
 	blEXIT    = 3
 )

 var bllikelies [4]string = [4]string{"default", "call", "ret", "exit"}

 func describePredictionAgrees(b *Block, prediction BranchPrediction) string {
 	s := ""
 	if prediction == b.Likely {
 		s = " (agrees with previous)"
 	} else if b.Likely != BranchUnknown {
 		s = " (disagrees with previous, ignored)"
 	}
 	return s
 }

 func describeBranchPrediction(f *Func, b *Block, likely, not int8, prediction BranchPrediction) {
 	f.Config.Warnl(int(b.Line), "Branch prediction rule %s < %s%s",
 		bllikelies[likely-blMin], bllikelies[not-blMin], describePredictionAgrees(b, prediction))
 }

 func likelyadjust(f *Func) {
 	// The values assigned to certain and local only matter
 	// in their rank order.  0 is default, more positive
 	// is less likely.  It's possible to assign a negative
 	// unlikeliness (though not currently the case).
 	certain := make([]int8, f.NumBlocks()) // In the long run, all outcomes are at least this bad. Mainly for Exit
 	local := make([]int8, f.NumBlocks())   // for our immediate predecessors.

 	nest := loopnestfor(f)
 	po := nest.po
 	b2l := nest.b2l

 	for _, b := range po {
 		switch b.Kind {
 		case BlockExit:
 			// Very unlikely.
 			local[b.ID] = blEXIT
 			certain[b.ID] = blEXIT

 			// Ret, it depends.
 		case BlockRet, BlockRetJmp:
 			local[b.ID] = blRET
 			certain[b.ID] = blRET

 			// Calls. TODO not all calls are equal, names give useful clues.
 			// Any name-based heuristics are only relative to other calls,
 			// and less influential than inferences from loop structure.
 		case BlockCall:
 			local[b.ID] = blCALL
 			certain[b.ID] = max8(blCALL, certain[b.Succs[0].ID])

 		default:
 			if len(b.Succs) == 1 {
 				certain[b.ID] = certain[b.Succs[0].ID]
 			} else if len(b.Succs) == 2 {
 				// If successor is an unvisited backedge, it's in loop and we don't care.
 				// Its default unlikely is also zero which is consistent with favoring loop edges.
 				// Notice that this can act like a "reset" on unlikeliness at loops; the
 				// default "everything returns" unlikeliness is erased by min with the
 				// backedge likeliness; however a loop with calls on every path will be
 				// tagged with call cost.  Net effect is that loop entry is favored.
 				b0 := b.Succs[0].ID
 				b1 := b.Succs[1].ID
 				certain[b.ID] = min8(certain[b0], certain[b1])

 				l := b2l[b.ID]
 				l0 := b2l[b0]
 				l1 := b2l[b1]

 				prediction := b.Likely
 				// Weak loop heuristic -- both source and at least one dest are in loops,
 				// and there is a difference in the destinations.
 				// TODO what is best arrangement for nested loops?
 				if l != nil && l0 != l1 {
 					noprediction := false
 					switch {
 					// prefer not to exit loops
 					case l1 == nil:
 						prediction = BranchLikely
 					case l0 == nil:
 						prediction = BranchUnlikely

 						// prefer to stay in loop, not exit to outer.
 					case l == l0:
 						prediction = BranchLikely
 					case l == l1:
 						prediction = BranchUnlikely
 					default:
 						noprediction = true
 					}
 					if f.pass.debug > 0 && !noprediction {
 						f.Config.Warnl(int(b.Line), "Branch prediction rule stay in loop%s",
 							describePredictionAgrees(b, prediction))
 					}

 				} else {
 					// Lacking loop structure, fall back on heuristics.
 					if certain[b1] > certain[b0] {
 						prediction = BranchLikely
 						if f.pass.debug > 0 {
 							describeBranchPrediction(f, b, certain[b0], certain[b1], prediction)
 						}
 					} else if certain[b0] > certain[b1] {
 						prediction = BranchUnlikely
 						if f.pass.debug > 0 {
 							describeBranchPrediction(f, b, certain[b1], certain[b0], prediction)
 						}
 					} else if local[b1] > local[b0] {
 						prediction = BranchLikely
 						if f.pass.debug > 0 {
 							describeBranchPrediction(f, b, local[b0], local[b1], prediction)
 						}
 					} else if local[b0] > local[b1] {
 						prediction = BranchUnlikely
 						if f.pass.debug > 0 {
 							describeBranchPrediction(f, b, local[b1], local[b0], prediction)
 						}
 					}
 				}
 				if b.Likely != prediction {
 					if b.Likely == BranchUnknown {
 						b.Likely = prediction
 					}
 				}
 			}
 		}
 		if f.pass.debug > 2 {
 			f.Config.Warnl(int(b.Line), "BP: Block %s, local=%s, certain=%s", b, bllikelies[local[b.ID]-blMin], bllikelies[certain[b.ID]-blMin])
 		}

 	}
 }

 func (l *loop) String() string {
 	return fmt.Sprintf("hdr:%s", l.header)
 }

 func (l *loop) LongString() string {
 	i := ""
 	o := ""
 	if l.isInner {
 		i = ", INNER"
 	}
 	if l.outer != nil {
 		o = ", o=" + l.outer.header.String()
 	}
 	return fmt.Sprintf("hdr:%s%s%s", l.header, i, o)
 }

 // nearestOuterLoop returns the outer loop of loop most nearly
 // containing block b; the header must dominate b.  loop itself
 // is assumed to not be that loop.  For acceptable performance,
 // we're relying on loop nests to not be terribly deep.
 func (l *loop) nearestOuterLoop(sdom sparseTree, b *Block) *loop {
 	var o *loop
 	for o = l.outer; o != nil && !sdom.isAncestorEq(o.header, b); o = o.outer {
 	}
 	return o
 }

 func loopnestfor(f *Func) *loopnest {
 	po := postorder(f)
 	dom := dominators(f)
 	sdom := newSparseTree(f, dom)
 	b2l := make([]*loop, f.NumBlocks())
 	loops := make([]*loop, 0)

 	// Reducible-loop-nest-finding.
 	for _, b := range po {
 		if f.pass.debug > 3 {
 			fmt.Printf("loop finding (0) at %s\n", b)
 		}

 		var innermost *loop // innermost header reachable from this block

 		// IF any successor s of b is in a loop headed by h
 		// AND h dominates b
 		// THEN b is in the loop headed by h.
 		//
 		// Choose the first/innermost such h.
 		//
 		// IF s itself dominates b, the s is a loop header;
 		// and there may be more than one such s.
 		// Since there's at most 2 successors, the inner/outer ordering
 		// between them can be established with simple comparisons.
 		for _, bb := range b.Succs {
 			l := b2l[bb.ID]

 			if sdom.isAncestorEq(bb, b) { // Found a loop header
 				if l == nil {
 					l = &loop{header: bb, isInner: true}
 					loops = append(loops, l)
 					b2l[bb.ID] = l
 				}
 			} else { // Perhaps a loop header is inherited.
 				// is there any loop containing our successor whose
 				// header dominates b?
 				if l != nil && !sdom.isAncestorEq(l.header, b) {
 					l = l.nearestOuterLoop(sdom, b)
 				}
 			}

 			if l == nil || innermost == l {
 				continue
 			}

 			if innermost == nil {
 				innermost = l
 				continue
 			}

 			if sdom.isAncestor(innermost.header, l.header) {
 				sdom.outerinner(innermost, l)
 				innermost = l
 			} else if sdom.isAncestor(l.header, innermost.header) {
 				sdom.outerinner(l, innermost)
 			}
 		}

 		if innermost != nil {
 			b2l[b.ID] = innermost
 			innermost.nBlocks++
 		}
 	}
 	if f.pass.debug > 1 && len(loops) > 0 {
 		fmt.Printf("Loops in %s:\n", f.Name)
 		for _, l := range loops {
 			fmt.Printf("%s, b=", l.LongString())
 			for _, b := range f.Blocks {
 				if b2l[b.ID] == l {
 					fmt.Printf(" %s", b)
 				}
 			}
 			fmt.Print("\n")
 		}
 		fmt.Printf("Nonloop blocks in %s:", f.Name)
 		for _, b := range f.Blocks {
 			if b2l[b.ID] == nil {
 				fmt.Printf(" %s", b)
 			}
 		}
 		fmt.Print("\n")
 	}
 	return &loopnest{f, b2l, po, sdom, loops}
 }
	// Copyright 2016 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 (
	"fmt"
	)

	type loop struct {
	header *Block // The header node of this (reducible) loop
	outer *loop // loop containing this loop
	// Next two fields not currently used, but cheap to maintain,
	// and aid in computation of inner-ness and list of blocks.
	nBlocks int32 // Number of blocks in this loop but not within inner loops
	isInner bool // True if never discovered to contain a loop
	}

	// outerinner records that outer contains inner
	func (sdom sparseTree) outerinner(outer, inner *loop) {
	oldouter := inner.outer
	if oldouter == nil \|\| sdom.isAncestorEq(oldouter.header, outer.header) {
	inner.outer = outer
	outer.isInner = false
	}
	}

	type loopnest struct {
	f *Func
	b2l []*loop
	po []*Block
	sdom sparseTree
	loops []*loop
	}

	func min8(a, b int8) int8 {
	if a < b {
	return a
	}
	return b
	}

	func max8(a, b int8) int8 {
	if a > b {
	return a
	}
	return b
	}

	const (
	blDEFAULT = 0
	blMin = blDEFAULT
	blCALL = 1
	blRET = 2
	blEXIT = 3
	)

	var bllikelies [4]string = [4]string{"default", "call", "ret", "exit"}

	func describePredictionAgrees(b *Block, prediction BranchPrediction) string {
	s := ""
	if prediction == b.Likely {
	s = " (agrees with previous)"
	} else if b.Likely != BranchUnknown {
	s = " (disagrees with previous, ignored)"
	}
	return s
	}

	func describeBranchPrediction(f Func, b Block, likely, not int8, prediction BranchPrediction) {
	f.Config.Warnl(int(b.Line), "Branch prediction rule %s < %s%s",
	bllikelies[likely-blMin], bllikelies[not-blMin], describePredictionAgrees(b, prediction))
	}

	func likelyadjust(f *Func) {
	// The values assigned to certain and local only matter
	// in their rank order. 0 is default, more positive
	// is less likely. It's possible to assign a negative
	// unlikeliness (though not currently the case).
	certain := make([]int8, f.NumBlocks()) // In the long run, all outcomes are at least this bad. Mainly for Exit
	local := make([]int8, f.NumBlocks()) // for our immediate predecessors.

	nest := loopnestfor(f)
	po := nest.po
	b2l := nest.b2l

	for _, b := range po {
	switch b.Kind {
	case BlockExit:
	// Very unlikely.
	local[b.ID] = blEXIT
	certain[b.ID] = blEXIT

	// Ret, it depends.
	case BlockRet, BlockRetJmp:
	local[b.ID] = blRET
	certain[b.ID] = blRET

	// Calls. TODO not all calls are equal, names give useful clues.
	// Any name-based heuristics are only relative to other calls,
	// and less influential than inferences from loop structure.
	case BlockCall:
	local[b.ID] = blCALL
	certain[b.ID] = max8(blCALL, certain[b.Succs[0].ID])

	default:
	if len(b.Succs) == 1 {
	certain[b.ID] = certain[b.Succs[0].ID]
	} else if len(b.Succs) == 2 {
	// If successor is an unvisited backedge, it's in loop and we don't care.
	// Its default unlikely is also zero which is consistent with favoring loop edges.
	// Notice that this can act like a "reset" on unlikeliness at loops; the
	// default "everything returns" unlikeliness is erased by min with the
	// backedge likeliness; however a loop with calls on every path will be
	// tagged with call cost. Net effect is that loop entry is favored.
	b0 := b.Succs[0].ID
	b1 := b.Succs[1].ID
	certain[b.ID] = min8(certain[b0], certain[b1])

	l := b2l[b.ID]
	l0 := b2l[b0]
	l1 := b2l[b1]

	prediction := b.Likely
	// Weak loop heuristic -- both source and at least one dest are in loops,
	// and there is a difference in the destinations.
	// TODO what is best arrangement for nested loops?
	if l != nil && l0 != l1 {
	noprediction := false
	switch {
	// prefer not to exit loops
	case l1 == nil:
	prediction = BranchLikely
	case l0 == nil:
	prediction = BranchUnlikely

	// prefer to stay in loop, not exit to outer.
	case l == l0:
	prediction = BranchLikely
	case l == l1:
	prediction = BranchUnlikely
	default:
	noprediction = true
	}
	if f.pass.debug > 0 && !noprediction {
	f.Config.Warnl(int(b.Line), "Branch prediction rule stay in loop%s",
	describePredictionAgrees(b, prediction))
	}

	} else {
	// Lacking loop structure, fall back on heuristics.
	if certain[b1] > certain[b0] {
	prediction = BranchLikely
	if f.pass.debug > 0 {
	describeBranchPrediction(f, b, certain[b0], certain[b1], prediction)
	}
	} else if certain[b0] > certain[b1] {
	prediction = BranchUnlikely
	if f.pass.debug > 0 {
	describeBranchPrediction(f, b, certain[b1], certain[b0], prediction)
	}
	} else if local[b1] > local[b0] {
	prediction = BranchLikely
	if f.pass.debug > 0 {
	describeBranchPrediction(f, b, local[b0], local[b1], prediction)
	}
	} else if local[b0] > local[b1] {
	prediction = BranchUnlikely
	if f.pass.debug > 0 {
	describeBranchPrediction(f, b, local[b1], local[b0], prediction)
	}
	}
	}
	if b.Likely != prediction {
	if b.Likely == BranchUnknown {
	b.Likely = prediction
	}
	}
	}
	}
	if f.pass.debug > 2 {
	f.Config.Warnl(int(b.Line), "BP: Block %s, local=%s, certain=%s", b, bllikelies[local[b.ID]-blMin], bllikelies[certain[b.ID]-blMin])
	}

	}
	}

	func (l *loop) String() string {
	return fmt.Sprintf("hdr:%s", l.header)
	}

	func (l *loop) LongString() string {
	i := ""
	o := ""
	if l.isInner {
	i = ", INNER"
	}
	if l.outer != nil {
	o = ", o=" + l.outer.header.String()
	}
	return fmt.Sprintf("hdr:%s%s%s", l.header, i, o)
	}

	// nearestOuterLoop returns the outer loop of loop most nearly
	// containing block b; the header must dominate b. loop itself
	// is assumed to not be that loop. For acceptable performance,
	// we're relying on loop nests to not be terribly deep.
	func (l loop) nearestOuterLoop(sdom sparseTree, b Block) *loop {
	var o *loop
	for o = l.outer; o != nil && !sdom.isAncestorEq(o.header, b); o = o.outer {
	}
	return o
	}

	func loopnestfor(f Func) loopnest {
	po := postorder(f)
	dom := dominators(f)
	sdom := newSparseTree(f, dom)
	b2l := make([]*loop, f.NumBlocks())
	loops := make([]*loop, 0)

	// Reducible-loop-nest-finding.
	for _, b := range po {
	if f.pass.debug > 3 {
	fmt.Printf("loop finding (0) at %s\n", b)
	}

	var innermost *loop // innermost header reachable from this block

	// IF any successor s of b is in a loop headed by h
	// AND h dominates b
	// THEN b is in the loop headed by h.
	//
	// Choose the first/innermost such h.
	//
	// IF s itself dominates b, the s is a loop header;
	// and there may be more than one such s.
	// Since there's at most 2 successors, the inner/outer ordering
	// between them can be established with simple comparisons.
	for _, bb := range b.Succs {
	l := b2l[bb.ID]

	if sdom.isAncestorEq(bb, b) { // Found a loop header
	if l == nil {
	l = &loop{header: bb, isInner: true}
	loops = append(loops, l)
	b2l[bb.ID] = l
	}
	} else { // Perhaps a loop header is inherited.
	// is there any loop containing our successor whose
	// header dominates b?
	if l != nil && !sdom.isAncestorEq(l.header, b) {
	l = l.nearestOuterLoop(sdom, b)
	}
	}

	if l == nil \|\| innermost == l {
	continue
	}

	if innermost == nil {
	innermost = l
	continue
	}

	if sdom.isAncestor(innermost.header, l.header) {
	sdom.outerinner(innermost, l)
	innermost = l
	} else if sdom.isAncestor(l.header, innermost.header) {
	sdom.outerinner(l, innermost)
	}
	}

	if innermost != nil {
	b2l[b.ID] = innermost
	innermost.nBlocks++
	}
	}
	if f.pass.debug > 1 && len(loops) > 0 {
	fmt.Printf("Loops in %s:\n", f.Name)
	for _, l := range loops {
	fmt.Printf("%s, b=", l.LongString())
	for _, b := range f.Blocks {
	if b2l[b.ID] == l {
	fmt.Printf(" %s", b)
	}
	}
	fmt.Print("\n")
	}
	fmt.Printf("Nonloop blocks in %s:", f.Name)
	for _, b := range f.Blocks {
	if b2l[b.ID] == nil {
	fmt.Printf(" %s", b)
	}
	}
	fmt.Print("\n")
	}
	return &loopnest{f, b2l, po, sdom, loops}
	}