likelyadjust.go

     1  // Copyright 2016 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package ssa
     6  
     7  import (
     8  	"fmt"
     9  )
    10  
    11  type loop struct {
    12  	header *Block // The header node of this (reducible) loop
    13  	outer  *loop  // loop containing this loop
    14  
    15  	// By default, children, exits, and depth are not initialized.
    16  	children []*loop  // loops nested directly within this loop. Initialized by assembleChildren().
    17  	exits    []*Block // exits records blocks reached by exits from this loop. Initialized by findExits().
    18  
    19  	// Next three fields used by regalloc and/or
    20  	// aid in computation of inner-ness and list of blocks.
    21  	nBlocks int32 // Number of blocks in this loop but not within inner loops
    22  	depth   int16 // Nesting depth of the loop; 1 is outermost. Initialized by calculateDepths().
    23  	isInner bool  // True if never discovered to contain a loop
    24  
    25  	// register allocation uses this.
    26  	containsUnavoidableCall bool // True if all paths through the loop have a call
    27  }
    28  
    29  // outerinner records that outer contains inner
    30  func (sdom SparseTree) outerinner(outer, inner *loop) {
    31  	// There could be other outer loops found in some random order,
    32  	// locate the new outer loop appropriately among them.
    33  
    34  	// Outer loop headers dominate inner loop headers.
    35  	// Use this to put the "new" "outer" loop in the right place.
    36  	oldouter := inner.outer
    37  	for oldouter != nil && sdom.isAncestor(outer.header, oldouter.header) {
    38  		inner = oldouter
    39  		oldouter = inner.outer
    40  	}
    41  	if outer == oldouter {
    42  		return
    43  	}
    44  	if oldouter != nil {
    45  		sdom.outerinner(oldouter, outer)
    46  	}
    47  
    48  	inner.outer = outer
    49  	outer.isInner = false
    50  }
    51  
    52  func checkContainsCall(bb *Block) bool {
    53  	if bb.Kind == BlockDefer {
    54  		return true
    55  	}
    56  	for _, v := range bb.Values {
    57  		if opcodeTable[v.Op].call {
    58  			return true
    59  		}
    60  	}
    61  	return false
    62  }
    63  
    64  type loopnest struct {
    65  	f              *Func
    66  	b2l            []*loop
    67  	po             []*Block
    68  	sdom           SparseTree
    69  	loops          []*loop
    70  	hasIrreducible bool // TODO current treatment of irreducible loops is very flaky, if accurate loops are needed, must punt at function level.
    71  
    72  	// Record which of the lazily initialized fields have actually been initialized.
    73  	initializedChildren, initializedDepth, initializedExits bool
    74  }
    75  
    76  const (
    77  	blDEFAULT = 0
    78  	blMin     = blDEFAULT
    79  	blCALL    = 1
    80  	blRET     = 2
    81  	blEXIT    = 3
    82  )
    83  
    84  var bllikelies = [4]string{"default", "call", "ret", "exit"}
    85  
    86  func describePredictionAgrees(b *Block, prediction BranchPrediction) string {
    87  	s := ""
    88  	if prediction == b.Likely {
    89  		s = " (agrees with previous)"
    90  	} else if b.Likely != BranchUnknown {
    91  		s = " (disagrees with previous, ignored)"
    92  	}
    93  	return s
    94  }
    95  
    96  func describeBranchPrediction(f *Func, b *Block, likely, not int8, prediction BranchPrediction) {
    97  	f.Warnl(b.Pos, "Branch prediction rule %s < %s%s",
    98  		bllikelies[likely-blMin], bllikelies[not-blMin], describePredictionAgrees(b, prediction))
    99  }
   100  
   101  func likelyadjust(f *Func) {
   102  	// The values assigned to certain and local only matter
   103  	// in their rank order.  0 is default, more positive
   104  	// is less likely. It's possible to assign a negative
   105  	// unlikeliness (though not currently the case).
   106  	certain := f.Cache.allocInt8Slice(f.NumBlocks()) // In the long run, all outcomes are at least this bad. Mainly for Exit
   107  	defer f.Cache.freeInt8Slice(certain)
   108  	local := f.Cache.allocInt8Slice(f.NumBlocks()) // for our immediate predecessors.
   109  	defer f.Cache.freeInt8Slice(local)
   110  
   111  	po := f.postorder()
   112  	nest := f.loopnest()
   113  	b2l := nest.b2l
   114  
   115  	for _, b := range po {
   116  		switch b.Kind {
   117  		case BlockExit:
   118  			// Very unlikely.
   119  			local[b.ID] = blEXIT
   120  			certain[b.ID] = blEXIT
   121  
   122  			// Ret, it depends.
   123  		case BlockRet, BlockRetJmp:
   124  			local[b.ID] = blRET
   125  			certain[b.ID] = blRET
   126  
   127  			// Calls. TODO not all calls are equal, names give useful clues.
   128  			// Any name-based heuristics are only relative to other calls,
   129  			// and less influential than inferences from loop structure.
   130  		case BlockDefer:
   131  			local[b.ID] = blCALL
   132  			certain[b.ID] = max(blCALL, certain[b.Succs[0].b.ID])
   133  
   134  		default:
   135  			if len(b.Succs) == 1 {
   136  				certain[b.ID] = certain[b.Succs[0].b.ID]
   137  			} else if len(b.Succs) == 2 {
   138  				// If successor is an unvisited backedge, it's in loop and we don't care.
   139  				// Its default unlikely is also zero which is consistent with favoring loop edges.
   140  				// Notice that this can act like a "reset" on unlikeliness at loops; the
   141  				// default "everything returns" unlikeliness is erased by min with the
   142  				// backedge likeliness; however a loop with calls on every path will be
   143  				// tagged with call cost. Net effect is that loop entry is favored.
   144  				b0 := b.Succs[0].b.ID
   145  				b1 := b.Succs[1].b.ID
   146  				certain[b.ID] = min(certain[b0], certain[b1])
   147  
   148  				l := b2l[b.ID]
   149  				l0 := b2l[b0]
   150  				l1 := b2l[b1]
   151  
   152  				prediction := b.Likely
   153  				// Weak loop heuristic -- both source and at least one dest are in loops,
   154  				// and there is a difference in the destinations.
   155  				// TODO what is best arrangement for nested loops?
   156  				if l != nil && l0 != l1 {
   157  					noprediction := false
   158  					switch {
   159  					// prefer not to exit loops
   160  					case l1 == nil:
   161  						prediction = BranchLikely
   162  					case l0 == nil:
   163  						prediction = BranchUnlikely
   164  
   165  						// prefer to stay in loop, not exit to outer.
   166  					case l == l0:
   167  						prediction = BranchLikely
   168  					case l == l1:
   169  						prediction = BranchUnlikely
   170  					default:
   171  						noprediction = true
   172  					}
   173  					if f.pass.debug > 0 && !noprediction {
   174  						f.Warnl(b.Pos, "Branch prediction rule stay in loop%s",
   175  							describePredictionAgrees(b, prediction))
   176  					}
   177  
   178  				} else {
   179  					// Lacking loop structure, fall back on heuristics.
   180  					if certain[b1] > certain[b0] {
   181  						prediction = BranchLikely
   182  						if f.pass.debug > 0 {
   183  							describeBranchPrediction(f, b, certain[b0], certain[b1], prediction)
   184  						}
   185  					} else if certain[b0] > certain[b1] {
   186  						prediction = BranchUnlikely
   187  						if f.pass.debug > 0 {
   188  							describeBranchPrediction(f, b, certain[b1], certain[b0], prediction)
   189  						}
   190  					} else if local[b1] > local[b0] {
   191  						prediction = BranchLikely
   192  						if f.pass.debug > 0 {
   193  							describeBranchPrediction(f, b, local[b0], local[b1], prediction)
   194  						}
   195  					} else if local[b0] > local[b1] {
   196  						prediction = BranchUnlikely
   197  						if f.pass.debug > 0 {
   198  							describeBranchPrediction(f, b, local[b1], local[b0], prediction)
   199  						}
   200  					}
   201  				}
   202  				if b.Likely != prediction {
   203  					if b.Likely == BranchUnknown {
   204  						b.Likely = prediction
   205  					}
   206  				}
   207  			}
   208  			// Look for calls in the block.  If there is one, make this block unlikely.
   209  			for _, v := range b.Values {
   210  				if opcodeTable[v.Op].call {
   211  					local[b.ID] = blCALL
   212  					certain[b.ID] = max(blCALL, certain[b.Succs[0].b.ID])
   213  					break
   214  				}
   215  			}
   216  		}
   217  		if f.pass.debug > 2 {
   218  			f.Warnl(b.Pos, "BP: Block %s, local=%s, certain=%s", b, bllikelies[local[b.ID]-blMin], bllikelies[certain[b.ID]-blMin])
   219  		}
   220  
   221  	}
   222  }
   223  
   224  func (l *loop) String() string {
   225  	return fmt.Sprintf("hdr:%s", l.header)
   226  }
   227  
   228  func (l *loop) LongString() string {
   229  	i := ""
   230  	o := ""
   231  	if l.isInner {
   232  		i = ", INNER"
   233  	}
   234  	if l.outer != nil {
   235  		o = ", o=" + l.outer.header.String()
   236  	}
   237  	return fmt.Sprintf("hdr:%s%s%s", l.header, i, o)
   238  }
   239  
   240  func (l *loop) isWithinOrEq(ll *loop) bool {
   241  	if ll == nil { // nil means whole program
   242  		return true
   243  	}
   244  	for ; l != nil; l = l.outer {
   245  		if l == ll {
   246  			return true
   247  		}
   248  	}
   249  	return false
   250  }
   251  
   252  // nearestOuterLoop returns the outer loop of loop most nearly
   253  // containing block b; the header must dominate b.  loop itself
   254  // is assumed to not be that loop. For acceptable performance,
   255  // we're relying on loop nests to not be terribly deep.
   256  func (l *loop) nearestOuterLoop(sdom SparseTree, b *Block) *loop {
   257  	var o *loop
   258  	for o = l.outer; o != nil && !sdom.IsAncestorEq(o.header, b); o = o.outer {
   259  	}
   260  	return o
   261  }
   262  
   263  func loopnestfor(f *Func) *loopnest {
   264  	po := f.postorder()
   265  	sdom := f.Sdom()
   266  	b2l := make([]*loop, f.NumBlocks())
   267  	loops := make([]*loop, 0)
   268  	visited := f.Cache.allocBoolSlice(f.NumBlocks())
   269  	defer f.Cache.freeBoolSlice(visited)
   270  	sawIrred := false
   271  
   272  	if f.pass.debug > 2 {
   273  		fmt.Printf("loop finding in %s\n", f.Name)
   274  	}
   275  
   276  	// Reducible-loop-nest-finding.
   277  	for _, b := range po {
   278  		if f.pass != nil && f.pass.debug > 3 {
   279  			fmt.Printf("loop finding at %s\n", b)
   280  		}
   281  
   282  		var innermost *loop // innermost header reachable from this block
   283  
   284  		// IF any successor s of b is in a loop headed by h
   285  		// AND h dominates b
   286  		// THEN b is in the loop headed by h.
   287  		//
   288  		// Choose the first/innermost such h.
   289  		//
   290  		// IF s itself dominates b, then s is a loop header;
   291  		// and there may be more than one such s.
   292  		// Since there's at most 2 successors, the inner/outer ordering
   293  		// between them can be established with simple comparisons.
   294  		for _, e := range b.Succs {
   295  			bb := e.b
   296  			l := b2l[bb.ID]
   297  
   298  			if sdom.IsAncestorEq(bb, b) { // Found a loop header
   299  				if f.pass != nil && f.pass.debug > 4 {
   300  					fmt.Printf("loop finding    succ %s of %s is header\n", bb.String(), b.String())
   301  				}
   302  				if l == nil {
   303  					l = &loop{header: bb, isInner: true}
   304  					loops = append(loops, l)
   305  					b2l[bb.ID] = l
   306  				}
   307  			} else if !visited[bb.ID] { // Found an irreducible loop
   308  				sawIrred = true
   309  				if f.pass != nil && f.pass.debug > 4 {
   310  					fmt.Printf("loop finding    succ %s of %s is IRRED, in %s\n", bb.String(), b.String(), f.Name)
   311  				}
   312  			} else if l != nil {
   313  				// TODO handle case where l is irreducible.
   314  				// Perhaps a loop header is inherited.
   315  				// is there any loop containing our successor whose
   316  				// header dominates b?
   317  				if !sdom.IsAncestorEq(l.header, b) {
   318  					l = l.nearestOuterLoop(sdom, b)
   319  				}
   320  				if f.pass != nil && f.pass.debug > 4 {
   321  					if l == nil {
   322  						fmt.Printf("loop finding    succ %s of %s has no loop\n", bb.String(), b.String())
   323  					} else {
   324  						fmt.Printf("loop finding    succ %s of %s provides loop with header %s\n", bb.String(), b.String(), l.header.String())
   325  					}
   326  				}
   327  			} else { // No loop
   328  				if f.pass != nil && f.pass.debug > 4 {
   329  					fmt.Printf("loop finding    succ %s of %s has no loop\n", bb.String(), b.String())
   330  				}
   331  
   332  			}
   333  
   334  			if l == nil || innermost == l {
   335  				continue
   336  			}
   337  
   338  			if innermost == nil {
   339  				innermost = l
   340  				continue
   341  			}
   342  
   343  			if sdom.isAncestor(innermost.header, l.header) {
   344  				sdom.outerinner(innermost, l)
   345  				innermost = l
   346  			} else if sdom.isAncestor(l.header, innermost.header) {
   347  				sdom.outerinner(l, innermost)
   348  			}
   349  		}
   350  
   351  		if innermost != nil {
   352  			b2l[b.ID] = innermost
   353  			innermost.nBlocks++
   354  		}
   355  		visited[b.ID] = true
   356  	}
   357  
   358  	ln := &loopnest{f: f, b2l: b2l, po: po, sdom: sdom, loops: loops, hasIrreducible: sawIrred}
   359  
   360  	// Calculate containsUnavoidableCall for regalloc
   361  	dominatedByCall := f.Cache.allocBoolSlice(f.NumBlocks())
   362  	defer f.Cache.freeBoolSlice(dominatedByCall)
   363  	for _, b := range po {
   364  		if checkContainsCall(b) {
   365  			dominatedByCall[b.ID] = true
   366  		}
   367  	}
   368  	// Run dfs to find path through the loop that avoids all calls.
   369  	// Such path either escapes loop or return back to header.
   370  	// It isn't enough to have exit not dominated by any call, for example:
   371  	// ... some loop
   372  	// call1   call2
   373  	//   \      /
   374  	//     exit
   375  	// ...
   376  	// exit is not dominated by any call, but we don't have call-free path to it.
   377  	for _, l := range loops {
   378  		// Header contains call.
   379  		if dominatedByCall[l.header.ID] {
   380  			l.containsUnavoidableCall = true
   381  			continue
   382  		}
   383  		callfreepath := false
   384  		tovisit := make([]*Block, 0, len(l.header.Succs))
   385  		// Push all non-loop non-exit successors of header onto toVisit.
   386  		for _, s := range l.header.Succs {
   387  			nb := s.Block()
   388  			// This corresponds to loop with zero iterations.
   389  			if !l.iterationEnd(nb, b2l) {
   390  				tovisit = append(tovisit, nb)
   391  			}
   392  		}
   393  		for len(tovisit) > 0 {
   394  			cur := tovisit[len(tovisit)-1]
   395  			tovisit = tovisit[:len(tovisit)-1]
   396  			if dominatedByCall[cur.ID] {
   397  				continue
   398  			}
   399  			// Record visited in dominatedByCall.
   400  			dominatedByCall[cur.ID] = true
   401  			for _, s := range cur.Succs {
   402  				nb := s.Block()
   403  				if l.iterationEnd(nb, b2l) {
   404  					callfreepath = true
   405  				}
   406  				if !dominatedByCall[nb.ID] {
   407  					tovisit = append(tovisit, nb)
   408  				}
   409  
   410  			}
   411  			if callfreepath {
   412  				break
   413  			}
   414  		}
   415  		if !callfreepath {
   416  			l.containsUnavoidableCall = true
   417  		}
   418  	}
   419  
   420  	// Curious about the loopiness? "-d=ssa/likelyadjust/stats"
   421  	if f.pass != nil && f.pass.stats > 0 && len(loops) > 0 {
   422  		ln.assembleChildren()
   423  		ln.calculateDepths()
   424  		ln.findExits()
   425  
   426  		// Note stats for non-innermost loops are slightly flawed because
   427  		// they don't account for inner loop exits that span multiple levels.
   428  
   429  		for _, l := range loops {
   430  			x := len(l.exits)
   431  			cf := 0
   432  			if !l.containsUnavoidableCall {
   433  				cf = 1
   434  			}
   435  			inner := 0
   436  			if l.isInner {
   437  				inner++
   438  			}
   439  
   440  			f.LogStat("loopstats:",
   441  				l.depth, "depth", x, "exits",
   442  				inner, "is_inner", cf, "always_calls", l.nBlocks, "n_blocks")
   443  		}
   444  	}
   445  
   446  	if f.pass != nil && f.pass.debug > 1 && len(loops) > 0 {
   447  		fmt.Printf("Loops in %s:\n", f.Name)
   448  		for _, l := range loops {
   449  			fmt.Printf("%s, b=", l.LongString())
   450  			for _, b := range f.Blocks {
   451  				if b2l[b.ID] == l {
   452  					fmt.Printf(" %s", b)
   453  				}
   454  			}
   455  			fmt.Print("\n")
   456  		}
   457  		fmt.Printf("Nonloop blocks in %s:", f.Name)
   458  		for _, b := range f.Blocks {
   459  			if b2l[b.ID] == nil {
   460  				fmt.Printf(" %s", b)
   461  			}
   462  		}
   463  		fmt.Print("\n")
   464  	}
   465  	return ln
   466  }
   467  
   468  // assembleChildren initializes the children field of each
   469  // loop in the nest.  Loop A is a child of loop B if A is
   470  // directly nested within B (based on the reducible-loops
   471  // detection above)
   472  func (ln *loopnest) assembleChildren() {
   473  	if ln.initializedChildren {
   474  		return
   475  	}
   476  	for _, l := range ln.loops {
   477  		if l.outer != nil {
   478  			l.outer.children = append(l.outer.children, l)
   479  		}
   480  	}
   481  	ln.initializedChildren = true
   482  }
   483  
   484  // calculateDepths uses the children field of loops
   485  // to determine the nesting depth (outer=1) of each
   486  // loop.  This is helpful for finding exit edges.
   487  func (ln *loopnest) calculateDepths() {
   488  	if ln.initializedDepth {
   489  		return
   490  	}
   491  	ln.assembleChildren()
   492  	for _, l := range ln.loops {
   493  		if l.outer == nil {
   494  			l.setDepth(1)
   495  		}
   496  	}
   497  	ln.initializedDepth = true
   498  }
   499  
   500  // findExits uses loop depth information to find the
   501  // exits from a loop.
   502  func (ln *loopnest) findExits() {
   503  	if ln.initializedExits {
   504  		return
   505  	}
   506  	ln.calculateDepths()
   507  	b2l := ln.b2l
   508  	for _, b := range ln.po {
   509  		l := b2l[b.ID]
   510  		if l != nil && len(b.Succs) == 2 {
   511  			sl := b2l[b.Succs[0].b.ID]
   512  			if recordIfExit(l, sl, b.Succs[0].b) {
   513  				continue
   514  			}
   515  			sl = b2l[b.Succs[1].b.ID]
   516  			if recordIfExit(l, sl, b.Succs[1].b) {
   517  				continue
   518  			}
   519  		}
   520  	}
   521  	ln.initializedExits = true
   522  }
   523  
   524  // depth returns the loop nesting level of block b.
   525  func (ln *loopnest) depth(b ID) int16 {
   526  	if l := ln.b2l[b]; l != nil {
   527  		return l.depth
   528  	}
   529  	return 0
   530  }
   531  
   532  // recordIfExit checks sl (the loop containing b) to see if it
   533  // is outside of loop l, and if so, records b as an exit block
   534  // from l and returns true.
   535  func recordIfExit(l, sl *loop, b *Block) bool {
   536  	if sl != l {
   537  		if sl == nil || sl.depth <= l.depth {
   538  			l.exits = append(l.exits, b)
   539  			return true
   540  		}
   541  		// sl is not nil, and is deeper than l
   542  		// it's possible for this to be a goto into an irreducible loop made from gotos.
   543  		for sl.depth > l.depth {
   544  			sl = sl.outer
   545  		}
   546  		if sl != l {
   547  			l.exits = append(l.exits, b)
   548  			return true
   549  		}
   550  	}
   551  	return false
   552  }
   553  
   554  func (l *loop) setDepth(d int16) {
   555  	l.depth = d
   556  	for _, c := range l.children {
   557  		c.setDepth(d + 1)
   558  	}
   559  }
   560  
   561  // iterationEnd checks if block b ends iteration of loop l.
   562  // Ending iteration means either escaping to outer loop/code or
   563  // going back to header
   564  func (l *loop) iterationEnd(b *Block, b2l []*loop) bool {
   565  	return b == l.header || b2l[b.ID] == nil || (b2l[b.ID] != l && b2l[b.ID].depth <= l.depth)
   566  }
   567
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