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Source file src/cmd/compile/internal/ssa/likelyadjust.go

Documentation: cmd/compile/internal/ssa

     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  func min8(a, b int8) int8 {
    77  	if a < b {
    78  		return a
    79  	}
    80  	return b
    81  }
    82  
    83  func max8(a, b int8) int8 {
    84  	if a > b {
    85  		return a
    86  	}
    87  	return b
    88  }
    89  
    90  const (
    91  	blDEFAULT = 0
    92  	blMin     = blDEFAULT
    93  	blCALL    = 1
    94  	blRET     = 2
    95  	blEXIT    = 3
    96  )
    97  
    98  var bllikelies = [4]string{"default", "call", "ret", "exit"}
    99  
   100  func describePredictionAgrees(b *Block, prediction BranchPrediction) string {
   101  	s := ""
   102  	if prediction == b.Likely {
   103  		s = " (agrees with previous)"
   104  	} else if b.Likely != BranchUnknown {
   105  		s = " (disagrees with previous, ignored)"
   106  	}
   107  	return s
   108  }
   109  
   110  func describeBranchPrediction(f *Func, b *Block, likely, not int8, prediction BranchPrediction) {
   111  	f.Warnl(b.Pos, "Branch prediction rule %s < %s%s",
   112  		bllikelies[likely-blMin], bllikelies[not-blMin], describePredictionAgrees(b, prediction))
   113  }
   114  
   115  func likelyadjust(f *Func) {
   116  	// The values assigned to certain and local only matter
   117  	// in their rank order.  0 is default, more positive
   118  	// is less likely. It's possible to assign a negative
   119  	// unlikeliness (though not currently the case).
   120  	certain := make([]int8, f.NumBlocks()) // In the long run, all outcomes are at least this bad. Mainly for Exit
   121  	local := make([]int8, f.NumBlocks())   // for our immediate predecessors.
   122  
   123  	po := f.postorder()
   124  	nest := f.loopnest()
   125  	b2l := nest.b2l
   126  
   127  	for _, b := range po {
   128  		switch b.Kind {
   129  		case BlockExit:
   130  			// Very unlikely.
   131  			local[b.ID] = blEXIT
   132  			certain[b.ID] = blEXIT
   133  
   134  			// Ret, it depends.
   135  		case BlockRet, BlockRetJmp:
   136  			local[b.ID] = blRET
   137  			certain[b.ID] = blRET
   138  
   139  			// Calls. TODO not all calls are equal, names give useful clues.
   140  			// Any name-based heuristics are only relative to other calls,
   141  			// and less influential than inferences from loop structure.
   142  		case BlockDefer:
   143  			local[b.ID] = blCALL
   144  			certain[b.ID] = max8(blCALL, certain[b.Succs[0].b.ID])
   145  
   146  		default:
   147  			if len(b.Succs) == 1 {
   148  				certain[b.ID] = certain[b.Succs[0].b.ID]
   149  			} else if len(b.Succs) == 2 {
   150  				// If successor is an unvisited backedge, it's in loop and we don't care.
   151  				// Its default unlikely is also zero which is consistent with favoring loop edges.
   152  				// Notice that this can act like a "reset" on unlikeliness at loops; the
   153  				// default "everything returns" unlikeliness is erased by min with the
   154  				// backedge likeliness; however a loop with calls on every path will be
   155  				// tagged with call cost. Net effect is that loop entry is favored.
   156  				b0 := b.Succs[0].b.ID
   157  				b1 := b.Succs[1].b.ID
   158  				certain[b.ID] = min8(certain[b0], certain[b1])
   159  
   160  				l := b2l[b.ID]
   161  				l0 := b2l[b0]
   162  				l1 := b2l[b1]
   163  
   164  				prediction := b.Likely
   165  				// Weak loop heuristic -- both source and at least one dest are in loops,
   166  				// and there is a difference in the destinations.
   167  				// TODO what is best arrangement for nested loops?
   168  				if l != nil && l0 != l1 {
   169  					noprediction := false
   170  					switch {
   171  					// prefer not to exit loops
   172  					case l1 == nil:
   173  						prediction = BranchLikely
   174  					case l0 == nil:
   175  						prediction = BranchUnlikely
   176  
   177  						// prefer to stay in loop, not exit to outer.
   178  					case l == l0:
   179  						prediction = BranchLikely
   180  					case l == l1:
   181  						prediction = BranchUnlikely
   182  					default:
   183  						noprediction = true
   184  					}
   185  					if f.pass.debug > 0 && !noprediction {
   186  						f.Warnl(b.Pos, "Branch prediction rule stay in loop%s",
   187  							describePredictionAgrees(b, prediction))
   188  					}
   189  
   190  				} else {
   191  					// Lacking loop structure, fall back on heuristics.
   192  					if certain[b1] > certain[b0] {
   193  						prediction = BranchLikely
   194  						if f.pass.debug > 0 {
   195  							describeBranchPrediction(f, b, certain[b0], certain[b1], prediction)
   196  						}
   197  					} else if certain[b0] > certain[b1] {
   198  						prediction = BranchUnlikely
   199  						if f.pass.debug > 0 {
   200  							describeBranchPrediction(f, b, certain[b1], certain[b0], prediction)
   201  						}
   202  					} else if local[b1] > local[b0] {
   203  						prediction = BranchLikely
   204  						if f.pass.debug > 0 {
   205  							describeBranchPrediction(f, b, local[b0], local[b1], prediction)
   206  						}
   207  					} else if local[b0] > local[b1] {
   208  						prediction = BranchUnlikely
   209  						if f.pass.debug > 0 {
   210  							describeBranchPrediction(f, b, local[b1], local[b0], prediction)
   211  						}
   212  					}
   213  				}
   214  				if b.Likely != prediction {
   215  					if b.Likely == BranchUnknown {
   216  						b.Likely = prediction
   217  					}
   218  				}
   219  			}
   220  			// Look for calls in the block.  If there is one, make this block unlikely.
   221  			for _, v := range b.Values {
   222  				if opcodeTable[v.Op].call {
   223  					local[b.ID] = blCALL
   224  					certain[b.ID] = max8(blCALL, certain[b.Succs[0].b.ID])
   225  				}
   226  			}
   227  		}
   228  		if f.pass.debug > 2 {
   229  			f.Warnl(b.Pos, "BP: Block %s, local=%s, certain=%s", b, bllikelies[local[b.ID]-blMin], bllikelies[certain[b.ID]-blMin])
   230  		}
   231  
   232  	}
   233  }
   234  
   235  func (l *loop) String() string {
   236  	return fmt.Sprintf("hdr:%s", l.header)
   237  }
   238  
   239  func (l *loop) LongString() string {
   240  	i := ""
   241  	o := ""
   242  	if l.isInner {
   243  		i = ", INNER"
   244  	}
   245  	if l.outer != nil {
   246  		o = ", o=" + l.outer.header.String()
   247  	}
   248  	return fmt.Sprintf("hdr:%s%s%s", l.header, i, o)
   249  }
   250  
   251  func (l *loop) isWithinOrEq(ll *loop) bool {
   252  	if ll == nil { // nil means whole program
   253  		return true
   254  	}
   255  	for ; l != nil; l = l.outer {
   256  		if l == ll {
   257  			return true
   258  		}
   259  	}
   260  	return false
   261  }
   262  
   263  // nearestOuterLoop returns the outer loop of loop most nearly
   264  // containing block b; the header must dominate b.  loop itself
   265  // is assumed to not be that loop. For acceptable performance,
   266  // we're relying on loop nests to not be terribly deep.
   267  func (l *loop) nearestOuterLoop(sdom SparseTree, b *Block) *loop {
   268  	var o *loop
   269  	for o = l.outer; o != nil && !sdom.isAncestorEq(o.header, b); o = o.outer {
   270  	}
   271  	return o
   272  }
   273  
   274  func loopnestfor(f *Func) *loopnest {
   275  	po := f.postorder()
   276  	sdom := f.sdom()
   277  	b2l := make([]*loop, f.NumBlocks())
   278  	loops := make([]*loop, 0)
   279  	visited := make([]bool, f.NumBlocks())
   280  	sawIrred := false
   281  
   282  	if f.pass.debug > 2 {
   283  		fmt.Printf("loop finding in %s\n", f.Name)
   284  	}
   285  
   286  	// Reducible-loop-nest-finding.
   287  	for _, b := range po {
   288  		if f.pass != nil && f.pass.debug > 3 {
   289  			fmt.Printf("loop finding at %s\n", b)
   290  		}
   291  
   292  		var innermost *loop // innermost header reachable from this block
   293  
   294  		// IF any successor s of b is in a loop headed by h
   295  		// AND h dominates b
   296  		// THEN b is in the loop headed by h.
   297  		//
   298  		// Choose the first/innermost such h.
   299  		//
   300  		// IF s itself dominates b, then s is a loop header;
   301  		// and there may be more than one such s.
   302  		// Since there's at most 2 successors, the inner/outer ordering
   303  		// between them can be established with simple comparisons.
   304  		for _, e := range b.Succs {
   305  			bb := e.b
   306  			l := b2l[bb.ID]
   307  
   308  			if sdom.isAncestorEq(bb, b) { // Found a loop header
   309  				if f.pass != nil && f.pass.debug > 4 {
   310  					fmt.Printf("loop finding    succ %s of %s is header\n", bb.String(), b.String())
   311  				}
   312  				if l == nil {
   313  					l = &loop{header: bb, isInner: true}
   314  					loops = append(loops, l)
   315  					b2l[bb.ID] = l
   316  				}
   317  			} else if !visited[bb.ID] { // Found an irreducible loop
   318  				sawIrred = true
   319  				if f.pass != nil && f.pass.debug > 4 {
   320  					fmt.Printf("loop finding    succ %s of %s is IRRED, in %s\n", bb.String(), b.String(), f.Name)
   321  				}
   322  			} else if l != nil {
   323  				// TODO handle case where l is irreducible.
   324  				// Perhaps a loop header is inherited.
   325  				// is there any loop containing our successor whose
   326  				// header dominates b?
   327  				if !sdom.isAncestorEq(l.header, b) {
   328  					l = l.nearestOuterLoop(sdom, b)
   329  				}
   330  				if f.pass != nil && f.pass.debug > 4 {
   331  					if l == nil {
   332  						fmt.Printf("loop finding    succ %s of %s has no loop\n", bb.String(), b.String())
   333  					} else {
   334  						fmt.Printf("loop finding    succ %s of %s provides loop with header %s\n", bb.String(), b.String(), l.header.String())
   335  					}
   336  				}
   337  			} else { // No loop
   338  				if f.pass != nil && f.pass.debug > 4 {
   339  					fmt.Printf("loop finding    succ %s of %s has no loop\n", bb.String(), b.String())
   340  				}
   341  
   342  			}
   343  
   344  			if l == nil || innermost == l {
   345  				continue
   346  			}
   347  
   348  			if innermost == nil {
   349  				innermost = l
   350  				continue
   351  			}
   352  
   353  			if sdom.isAncestor(innermost.header, l.header) {
   354  				sdom.outerinner(innermost, l)
   355  				innermost = l
   356  			} else if sdom.isAncestor(l.header, innermost.header) {
   357  				sdom.outerinner(l, innermost)
   358  			}
   359  		}
   360  
   361  		if innermost != nil {
   362  			b2l[b.ID] = innermost
   363  			innermost.nBlocks++
   364  		}
   365  		visited[b.ID] = true
   366  	}
   367  
   368  	ln := &loopnest{f: f, b2l: b2l, po: po, sdom: sdom, loops: loops, hasIrreducible: sawIrred}
   369  
   370  	// Calculate containsUnavoidableCall for regalloc
   371  	dominatedByCall := make([]bool, f.NumBlocks())
   372  	for _, b := range po {
   373  		if checkContainsCall(b) {
   374  			dominatedByCall[b.ID] = true
   375  		}
   376  	}
   377  	// Run dfs to find path through the loop that avoids all calls.
   378  	// Such path either escapes loop or return back to header.
   379  	// It isn't enough to have exit not dominated by any call, for example:
   380  	// ... some loop
   381  	// call1   call2
   382  	//   \      /
   383  	//     exit
   384  	// ...
   385  	// exit is not dominated by any call, but we don't have call-free path to it.
   386  	for _, l := range loops {
   387  		// Header contains call.
   388  		if dominatedByCall[l.header.ID] {
   389  			l.containsUnavoidableCall = true
   390  			continue
   391  		}
   392  		callfreepath := false
   393  		tovisit := make([]*Block, 0, len(l.header.Succs))
   394  		// Push all non-loop non-exit successors of header onto toVisit.
   395  		for _, s := range l.header.Succs {
   396  			nb := s.Block()
   397  			// This corresponds to loop with zero iterations.
   398  			if !l.iterationEnd(nb, b2l) {
   399  				tovisit = append(tovisit, nb)
   400  			}
   401  		}
   402  		for len(tovisit) > 0 {
   403  			cur := tovisit[len(tovisit)-1]
   404  			tovisit = tovisit[:len(tovisit)-1]
   405  			if dominatedByCall[cur.ID] {
   406  				continue
   407  			}
   408  			// Record visited in dominatedByCall.
   409  			dominatedByCall[cur.ID] = true
   410  			for _, s := range cur.Succs {
   411  				nb := s.Block()
   412  				if l.iterationEnd(nb, b2l) {
   413  					callfreepath = true
   414  				}
   415  				if !dominatedByCall[nb.ID] {
   416  					tovisit = append(tovisit, nb)
   417  				}
   418  
   419  			}
   420  			if callfreepath {
   421  				break
   422  			}
   423  		}
   424  		if !callfreepath {
   425  			l.containsUnavoidableCall = true
   426  		}
   427  	}
   428  
   429  	// Curious about the loopiness? "-d=ssa/likelyadjust/stats"
   430  	if f.pass != nil && f.pass.stats > 0 && len(loops) > 0 {
   431  		ln.assembleChildren()
   432  		ln.calculateDepths()
   433  		ln.findExits()
   434  
   435  		// Note stats for non-innermost loops are slightly flawed because
   436  		// they don't account for inner loop exits that span multiple levels.
   437  
   438  		for _, l := range loops {
   439  			x := len(l.exits)
   440  			cf := 0
   441  			if !l.containsUnavoidableCall {
   442  				cf = 1
   443  			}
   444  			inner := 0
   445  			if l.isInner {
   446  				inner++
   447  			}
   448  
   449  			f.LogStat("loopstats:",
   450  				l.depth, "depth", x, "exits",
   451  				inner, "is_inner", cf, "always_calls", l.nBlocks, "n_blocks")
   452  		}
   453  	}
   454  
   455  	if f.pass != nil && f.pass.debug > 1 && len(loops) > 0 {
   456  		fmt.Printf("Loops in %s:\n", f.Name)
   457  		for _, l := range loops {
   458  			fmt.Printf("%s, b=", l.LongString())
   459  			for _, b := range f.Blocks {
   460  				if b2l[b.ID] == l {
   461  					fmt.Printf(" %s", b)
   462  				}
   463  			}
   464  			fmt.Print("\n")
   465  		}
   466  		fmt.Printf("Nonloop blocks in %s:", f.Name)
   467  		for _, b := range f.Blocks {
   468  			if b2l[b.ID] == nil {
   469  				fmt.Printf(" %s", b)
   470  			}
   471  		}
   472  		fmt.Print("\n")
   473  	}
   474  	return ln
   475  }
   476  
   477  // assembleChildren initializes the children field of each
   478  // loop in the nest.  Loop A is a child of loop B if A is
   479  // directly nested within B (based on the reducible-loops
   480  // detection above)
   481  func (ln *loopnest) assembleChildren() {
   482  	if ln.initializedChildren {
   483  		return
   484  	}
   485  	for _, l := range ln.loops {
   486  		if l.outer != nil {
   487  			l.outer.children = append(l.outer.children, l)
   488  		}
   489  	}
   490  	ln.initializedChildren = true
   491  }
   492  
   493  // calculateDepths uses the children field of loops
   494  // to determine the nesting depth (outer=1) of each
   495  // loop.  This is helpful for finding exit edges.
   496  func (ln *loopnest) calculateDepths() {
   497  	if ln.initializedDepth {
   498  		return
   499  	}
   500  	ln.assembleChildren()
   501  	for _, l := range ln.loops {
   502  		if l.outer == nil {
   503  			l.setDepth(1)
   504  		}
   505  	}
   506  	ln.initializedDepth = true
   507  }
   508  
   509  // findExits uses loop depth information to find the
   510  // exits from a loop.
   511  func (ln *loopnest) findExits() {
   512  	if ln.initializedExits {
   513  		return
   514  	}
   515  	ln.calculateDepths()
   516  	b2l := ln.b2l
   517  	for _, b := range ln.po {
   518  		l := b2l[b.ID]
   519  		if l != nil && len(b.Succs) == 2 {
   520  			sl := b2l[b.Succs[0].b.ID]
   521  			if recordIfExit(l, sl, b.Succs[0].b) {
   522  				continue
   523  			}
   524  			sl = b2l[b.Succs[1].b.ID]
   525  			if recordIfExit(l, sl, b.Succs[1].b) {
   526  				continue
   527  			}
   528  		}
   529  	}
   530  	ln.initializedExits = true
   531  }
   532  
   533  // depth returns the loop nesting level of block b.
   534  func (ln *loopnest) depth(b ID) int16 {
   535  	if l := ln.b2l[b]; l != nil {
   536  		return l.depth
   537  	}
   538  	return 0
   539  }
   540  
   541  // recordIfExit checks sl (the loop containing b) to see if it
   542  // is outside of loop l, and if so, records b as an exit block
   543  // from l and returns true.
   544  func recordIfExit(l, sl *loop, b *Block) bool {
   545  	if sl != l {
   546  		if sl == nil || sl.depth <= l.depth {
   547  			l.exits = append(l.exits, b)
   548  			return true
   549  		}
   550  		// sl is not nil, and is deeper than l
   551  		// it's possible for this to be a goto into an irreducible loop made from gotos.
   552  		for sl.depth > l.depth {
   553  			sl = sl.outer
   554  		}
   555  		if sl != l {
   556  			l.exits = append(l.exits, b)
   557  			return true
   558  		}
   559  	}
   560  	return false
   561  }
   562  
   563  func (l *loop) setDepth(d int16) {
   564  	l.depth = d
   565  	for _, c := range l.children {
   566  		c.setDepth(d + 1)
   567  	}
   568  }
   569  
   570  // iterationEnd checks if block b ends iteration of loop l.
   571  // Ending iteration means either escaping to outer loop/code or
   572  // going back to header
   573  func (l *loop) iterationEnd(b *Block, b2l []*loop) bool {
   574  	return b == l.header || b2l[b.ID] == nil || (b2l[b.ID] != l && b2l[b.ID].depth <= l.depth)
   575  }
   576  

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