// 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
// By default, children, exits, and depth are not initialized.
children []*loop // loops nested directly within this loop. Initialized by assembleChildren().
exits []*Block // exits records blocks reached by exits from this loop. Initialized by findExits().
// Next three fields used by regalloc and/or
// aid in computation of inner-ness and list of blocks.
nBlocks int32 // Number of blocks in this loop but not within inner loops
depth int16 // Nesting depth of the loop; 1 is outermost. Initialized by calculateDepths().
isInner bool // True if never discovered to contain a loop
// register allocation uses this.
containsUnavoidableCall bool // True if all paths through the loop have a call
}
// outerinner records that outer contains inner
func (sdom SparseTree) outerinner(outer, inner *loop) {
// There could be other outer loops found in some random order,
// locate the new outer loop appropriately among them.
// Outer loop headers dominate inner loop headers.
// Use this to put the "new" "outer" loop in the right place.
oldouter := inner.outer
for oldouter != nil && sdom.isAncestor(outer.header, oldouter.header) {
inner = oldouter
oldouter = inner.outer
}
if outer == oldouter {
return
}
if oldouter != nil {
sdom.outerinner(oldouter, outer)
}
inner.outer = outer
outer.isInner = false
}
func checkContainsCall(bb *Block) bool {
if bb.Kind == BlockDefer {
return true
}
for _, v := range bb.Values {
if opcodeTable[v.Op].call {
return true
}
}
return false
}
type loopnest struct {
f *Func
b2l []*loop
po []*Block
sdom SparseTree
loops []*loop
hasIrreducible bool // TODO current treatment of irreducible loops is very flaky, if accurate loops are needed, must punt at function level.
// Record which of the lazily initialized fields have actually been initialized.
initializedChildren, initializedDepth, initializedExits bool
}
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{"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.Warnl(b.Pos, "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.
po := f.postorder()
nest := f.loopnest()
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 BlockDefer:
local[b.ID] = blCALL
certain[b.ID] = max8(blCALL, certain[b.Succs[0].b.ID])
default:
if len(b.Succs) == 1 {
certain[b.ID] = certain[b.Succs[0].b.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].b.ID
b1 := b.Succs[1].b.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.Warnl(b.Pos, "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
}
}
}
// Look for calls in the block. If there is one, make this block unlikely.
for _, v := range b.Values {
if opcodeTable[v.Op].call {
local[b.ID] = blCALL
certain[b.ID] = max8(blCALL, certain[b.Succs[0].b.ID])
}
}
}
if f.pass.debug > 2 {
f.Warnl(b.Pos, "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)
}
func (l *loop) isWithinOrEq(ll *loop) bool {
if ll == nil { // nil means whole program
return true
}
for ; l != nil; l = l.outer {
if l == ll {
return true
}
}
return false
}
// 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 := f.postorder()
sdom := f.sdom()
b2l := make([]*loop, f.NumBlocks())
loops := make([]*loop, 0)
visited := make([]bool, f.NumBlocks())
sawIrred := false
if f.pass.debug > 2 {
fmt.Printf("loop finding in %s\n", f.Name)
}
// Reducible-loop-nest-finding.
for _, b := range po {
if f.pass != nil && f.pass.debug > 3 {
fmt.Printf("loop finding 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, then 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 _, e := range b.Succs {
bb := e.b
l := b2l[bb.ID]
if sdom.isAncestorEq(bb, b) { // Found a loop header
if f.pass != nil && f.pass.debug > 4 {
fmt.Printf("loop finding succ %s of %s is header\n", bb.String(), b.String())
}
if l == nil {
l = &loop{header: bb, isInner: true}
loops = append(loops, l)
b2l[bb.ID] = l
}
} else if !visited[bb.ID] { // Found an irreducible loop
sawIrred = true
if f.pass != nil && f.pass.debug > 4 {
fmt.Printf("loop finding succ %s of %s is IRRED, in %s\n", bb.String(), b.String(), f.Name)
}
} else if l != nil {
// TODO handle case where l is irreducible.
// Perhaps a loop header is inherited.
// is there any loop containing our successor whose
// header dominates b?
if !sdom.isAncestorEq(l.header, b) {
l = l.nearestOuterLoop(sdom, b)
}
if f.pass != nil && f.pass.debug > 4 {
if l == nil {
fmt.Printf("loop finding succ %s of %s has no loop\n", bb.String(), b.String())
} else {
fmt.Printf("loop finding succ %s of %s provides loop with header %s\n", bb.String(), b.String(), l.header.String())
}
}
} else { // No loop
if f.pass != nil && f.pass.debug > 4 {
fmt.Printf("loop finding succ %s of %s has no loop\n", bb.String(), b.String())
}
}
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++
}
visited[b.ID] = true
}
ln := &loopnest{f: f, b2l: b2l, po: po, sdom: sdom, loops: loops, hasIrreducible: sawIrred}
// Calculate containsUnavoidableCall for regalloc
dominatedByCall := make([]bool, f.NumBlocks())
for _, b := range po {
if checkContainsCall(b) {
dominatedByCall[b.ID] = true
}
}
// Run dfs to find path through the loop that avoids all calls.
// Such path either escapes loop or return back to header.
// It isn't enough to have exit not dominated by any call, for example:
// ... some loop
// call1 call2
// \ /
// exit
// ...
// exit is not dominated by any call, but we don't have call-free path to it.
for _, l := range loops {
// Header contains call.
if dominatedByCall[l.header.ID] {
l.containsUnavoidableCall = true
continue
}
callfreepath := false
tovisit := make([]*Block, 0, len(l.header.Succs))
// Push all non-loop non-exit successors of header onto toVisit.
for _, s := range l.header.Succs {
nb := s.Block()
// This corresponds to loop with zero iterations.
if !l.iterationEnd(nb, b2l) {
tovisit = append(tovisit, nb)
}
}
for len(tovisit) > 0 {
cur := tovisit[len(tovisit)-1]
tovisit = tovisit[:len(tovisit)-1]
if dominatedByCall[cur.ID] {
continue
}
// Record visited in dominatedByCall.
dominatedByCall[cur.ID] = true
for _, s := range cur.Succs {
nb := s.Block()
if l.iterationEnd(nb, b2l) {
callfreepath = true
}
if !dominatedByCall[nb.ID] {
tovisit = append(tovisit, nb)
}
}
if callfreepath {
break
}
}
if !callfreepath {
l.containsUnavoidableCall = true
}
}
// Curious about the loopiness? "-d=ssa/likelyadjust/stats"
if f.pass != nil && f.pass.stats > 0 && len(loops) > 0 {
ln.assembleChildren()
ln.calculateDepths()
ln.findExits()
// Note stats for non-innermost loops are slightly flawed because
// they don't account for inner loop exits that span multiple levels.
for _, l := range loops {
x := len(l.exits)
cf := 0
if !l.containsUnavoidableCall {
cf = 1
}
inner := 0
if l.isInner {
inner++
}
f.LogStat("loopstats:",
l.depth, "depth", x, "exits",
inner, "is_inner", cf, "always_calls", l.nBlocks, "n_blocks")
}
}
if f.pass != nil && 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 ln
}
// assembleChildren initializes the children field of each
// loop in the nest. Loop A is a child of loop B if A is
// directly nested within B (based on the reducible-loops
// detection above)
func (ln *loopnest) assembleChildren() {
if ln.initializedChildren {
return
}
for _, l := range ln.loops {
if l.outer != nil {
l.outer.children = append(l.outer.children, l)
}
}
ln.initializedChildren = true
}
// calculateDepths uses the children field of loops
// to determine the nesting depth (outer=1) of each
// loop. This is helpful for finding exit edges.
func (ln *loopnest) calculateDepths() {
if ln.initializedDepth {
return
}
ln.assembleChildren()
for _, l := range ln.loops {
if l.outer == nil {
l.setDepth(1)
}
}
ln.initializedDepth = true
}
// findExits uses loop depth information to find the
// exits from a loop.
func (ln *loopnest) findExits() {
if ln.initializedExits {
return
}
ln.calculateDepths()
b2l := ln.b2l
for _, b := range ln.po {
l := b2l[b.ID]
if l != nil && len(b.Succs) == 2 {
sl := b2l[b.Succs[0].b.ID]
if recordIfExit(l, sl, b.Succs[0].b) {
continue
}
sl = b2l[b.Succs[1].b.ID]
if recordIfExit(l, sl, b.Succs[1].b) {
continue
}
}
}
ln.initializedExits = true
}
// depth returns the loop nesting level of block b.
func (ln *loopnest) depth(b ID) int16 {
if l := ln.b2l[b]; l != nil {
return l.depth
}
return 0
}
// recordIfExit checks sl (the loop containing b) to see if it
// is outside of loop l, and if so, records b as an exit block
// from l and returns true.
func recordIfExit(l, sl *loop, b *Block) bool {
if sl != l {
if sl == nil || sl.depth <= l.depth {
l.exits = append(l.exits, b)
return true
}
// sl is not nil, and is deeper than l
// it's possible for this to be a goto into an irreducible loop made from gotos.
for sl.depth > l.depth {
sl = sl.outer
}
if sl != l {
l.exits = append(l.exits, b)
return true
}
}
return false
}
func (l *loop) setDepth(d int16) {
l.depth = d
for _, c := range l.children {
c.setDepth(d + 1)
}
}
// iterationEnd checks if block b ends iteration of loop l.
// Ending iteration means either escaping to outer loop/code or
// going back to header
func (l *loop) iterationEnd(b *Block, b2l []*loop) bool {
return b == l.header || b2l[b.ID] == nil || (b2l[b.ID] != l && b2l[b.ID].depth <= l.depth)
}
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