cse.go

     1  // Copyright 2015 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  	"cmd/compile/internal/types"
     9  	"cmd/internal/src"
    10  	"cmp"
    11  	"fmt"
    12  	"slices"
    13  )
    14  
    15  // cse does common-subexpression elimination on the Function.
    16  // Values are just relinked, nothing is deleted. A subsequent deadcode
    17  // pass is required to actually remove duplicate expressions.
    18  func cse(f *Func) {
    19  	// Two values are equivalent if they satisfy the following definition:
    20  	// equivalent(v, w):
    21  	//   v.op == w.op
    22  	//   v.type == w.type
    23  	//   v.aux == w.aux
    24  	//   v.auxint == w.auxint
    25  	//   len(v.args) == len(w.args)
    26  	//   v.block == w.block if v.op == OpPhi
    27  	//   equivalent(v.args[i], w.args[i]) for i in 0..len(v.args)-1
    28  
    29  	// The algorithm searches for a partition of f's values into
    30  	// equivalence classes using the above definition.
    31  	// It starts with a coarse partition and iteratively refines it
    32  	// until it reaches a fixed point.
    33  
    34  	// Make initial coarse partitions by using a subset of the conditions above.
    35  	a := f.Cache.allocValueSlice(f.NumValues())
    36  	defer func() { f.Cache.freeValueSlice(a) }() // inside closure to use final value of a
    37  	a = a[:0]
    38  	o := f.Cache.allocInt32Slice(f.NumValues()) // the ordering score for stores
    39  	defer func() { f.Cache.freeInt32Slice(o) }()
    40  	if f.auxmap == nil {
    41  		f.auxmap = auxmap{}
    42  	}
    43  	for _, b := range f.Blocks {
    44  		for _, v := range b.Values {
    45  			if v.Type.IsMemory() {
    46  				continue // memory values can never cse
    47  			}
    48  			if f.auxmap[v.Aux] == 0 {
    49  				f.auxmap[v.Aux] = int32(len(f.auxmap)) + 1
    50  			}
    51  			a = append(a, v)
    52  		}
    53  	}
    54  	partition := partitionValues(a, f.auxmap)
    55  
    56  	// map from value id back to eqclass id
    57  	valueEqClass := f.Cache.allocIDSlice(f.NumValues())
    58  	defer f.Cache.freeIDSlice(valueEqClass)
    59  	for _, b := range f.Blocks {
    60  		for _, v := range b.Values {
    61  			// Use negative equivalence class #s for unique values.
    62  			valueEqClass[v.ID] = -v.ID
    63  		}
    64  	}
    65  	var pNum ID = 1
    66  	for _, e := range partition {
    67  		if f.pass.debug > 1 && len(e) > 500 {
    68  			fmt.Printf("CSE.large partition (%d): ", len(e))
    69  			for j := 0; j < 3; j++ {
    70  				fmt.Printf("%s ", e[j].LongString())
    71  			}
    72  			fmt.Println()
    73  		}
    74  
    75  		for _, v := range e {
    76  			valueEqClass[v.ID] = pNum
    77  		}
    78  		if f.pass.debug > 2 && len(e) > 1 {
    79  			fmt.Printf("CSE.partition #%d:", pNum)
    80  			for _, v := range e {
    81  				fmt.Printf(" %s", v.String())
    82  			}
    83  			fmt.Printf("\n")
    84  		}
    85  		pNum++
    86  	}
    87  
    88  	// Split equivalence classes at points where they have
    89  	// non-equivalent arguments.  Repeat until we can't find any
    90  	// more splits.
    91  	var splitPoints []int
    92  	for {
    93  		changed := false
    94  
    95  		// partition can grow in the loop. By not using a range loop here,
    96  		// we process new additions as they arrive, avoiding O(n^2) behavior.
    97  		for i := 0; i < len(partition); i++ {
    98  			e := partition[i]
    99  
   100  			if opcodeTable[e[0].Op].commutative {
   101  				// Order the first two args before comparison.
   102  				for _, v := range e {
   103  					if valueEqClass[v.Args[0].ID] > valueEqClass[v.Args[1].ID] {
   104  						v.Args[0], v.Args[1] = v.Args[1], v.Args[0]
   105  					}
   106  				}
   107  			}
   108  
   109  			// Sort by eq class of arguments.
   110  			slices.SortFunc(e, func(v, w *Value) int {
   111  				for i, a := range v.Args {
   112  					b := w.Args[i]
   113  					if valueEqClass[a.ID] < valueEqClass[b.ID] {
   114  						return -1
   115  					}
   116  					if valueEqClass[a.ID] > valueEqClass[b.ID] {
   117  						return +1
   118  					}
   119  				}
   120  				return 0
   121  			})
   122  
   123  			// Find split points.
   124  			splitPoints = append(splitPoints[:0], 0)
   125  			for j := 1; j < len(e); j++ {
   126  				v, w := e[j-1], e[j]
   127  				// Note: commutative args already correctly ordered by byArgClass.
   128  				eqArgs := true
   129  				for k, a := range v.Args {
   130  					if v.Op == OpLocalAddr && k == 1 {
   131  						continue
   132  					}
   133  					b := w.Args[k]
   134  					if valueEqClass[a.ID] != valueEqClass[b.ID] {
   135  						eqArgs = false
   136  						break
   137  					}
   138  				}
   139  				if !eqArgs {
   140  					splitPoints = append(splitPoints, j)
   141  				}
   142  			}
   143  			if len(splitPoints) == 1 {
   144  				continue // no splits, leave equivalence class alone.
   145  			}
   146  
   147  			// Move another equivalence class down in place of e.
   148  			partition[i] = partition[len(partition)-1]
   149  			partition = partition[:len(partition)-1]
   150  			i--
   151  
   152  			// Add new equivalence classes for the parts of e we found.
   153  			splitPoints = append(splitPoints, len(e))
   154  			for j := 0; j < len(splitPoints)-1; j++ {
   155  				f := e[splitPoints[j]:splitPoints[j+1]]
   156  				if len(f) == 1 {
   157  					// Don't add singletons.
   158  					valueEqClass[f[0].ID] = -f[0].ID
   159  					continue
   160  				}
   161  				for _, v := range f {
   162  					valueEqClass[v.ID] = pNum
   163  				}
   164  				pNum++
   165  				partition = append(partition, f)
   166  			}
   167  			changed = true
   168  		}
   169  
   170  		if !changed {
   171  			break
   172  		}
   173  	}
   174  
   175  	sdom := f.Sdom()
   176  
   177  	// Compute substitutions we would like to do. We substitute v for w
   178  	// if v and w are in the same equivalence class and v dominates w.
   179  	rewrite := f.Cache.allocValueSlice(f.NumValues())
   180  	defer f.Cache.freeValueSlice(rewrite)
   181  	for _, e := range partition {
   182  		slices.SortFunc(e, func(v, w *Value) int {
   183  			c := cmp.Compare(sdom.domorder(v.Block), sdom.domorder(w.Block))
   184  			if v.Op != OpLocalAddr || c != 0 {
   185  				return c
   186  			}
   187  			// compare the memory args for OpLocalAddrs in the same block
   188  			vm := v.Args[1]
   189  			wm := w.Args[1]
   190  			if vm == wm {
   191  				return 0
   192  			}
   193  			// if the two OpLocalAddrs are in the same block, and one's memory
   194  			// arg also in the same block, but the other one's memory arg not,
   195  			// the latter must be in an ancestor block
   196  			if vm.Block != v.Block {
   197  				return -1
   198  			}
   199  			if wm.Block != w.Block {
   200  				return +1
   201  			}
   202  			// use store order if the memory args are in the same block
   203  			vs := storeOrdering(vm, o)
   204  			ws := storeOrdering(wm, o)
   205  			if vs <= 0 {
   206  				f.Fatalf("unable to determine the order of %s", vm.LongString())
   207  			}
   208  			if ws <= 0 {
   209  				f.Fatalf("unable to determine the order of %s", wm.LongString())
   210  			}
   211  			return cmp.Compare(vs, ws)
   212  		})
   213  
   214  		for i := 0; i < len(e)-1; i++ {
   215  			// e is sorted by domorder, so a maximal dominant element is first in the slice
   216  			v := e[i]
   217  			if v == nil {
   218  				continue
   219  			}
   220  
   221  			e[i] = nil
   222  			// Replace all elements of e which v dominates
   223  			for j := i + 1; j < len(e); j++ {
   224  				w := e[j]
   225  				if w == nil {
   226  					continue
   227  				}
   228  				if sdom.IsAncestorEq(v.Block, w.Block) {
   229  					rewrite[w.ID] = v
   230  					e[j] = nil
   231  				} else {
   232  					// e is sorted by domorder, so v.Block doesn't dominate any subsequent blocks in e
   233  					break
   234  				}
   235  			}
   236  		}
   237  	}
   238  
   239  	rewrites := int64(0)
   240  
   241  	// Apply substitutions
   242  	for _, b := range f.Blocks {
   243  		for _, v := range b.Values {
   244  			for i, w := range v.Args {
   245  				if x := rewrite[w.ID]; x != nil {
   246  					if w.Pos.IsStmt() == src.PosIsStmt {
   247  						// about to lose a statement marker, w
   248  						// w is an input to v; if they're in the same block
   249  						// and the same line, v is a good-enough new statement boundary.
   250  						if w.Block == v.Block && w.Pos.Line() == v.Pos.Line() {
   251  							v.Pos = v.Pos.WithIsStmt()
   252  							w.Pos = w.Pos.WithNotStmt()
   253  						} // TODO and if this fails?
   254  					}
   255  					v.SetArg(i, x)
   256  					rewrites++
   257  				}
   258  			}
   259  		}
   260  		for i, v := range b.ControlValues() {
   261  			if x := rewrite[v.ID]; x != nil {
   262  				if v.Op == OpNilCheck {
   263  					// nilcheck pass will remove the nil checks and log
   264  					// them appropriately, so don't mess with them here.
   265  					continue
   266  				}
   267  				b.ReplaceControl(i, x)
   268  			}
   269  		}
   270  	}
   271  
   272  	if f.pass.stats > 0 {
   273  		f.LogStat("CSE REWRITES", rewrites)
   274  	}
   275  }
   276  
   277  // storeOrdering computes the order for stores by iterate over the store
   278  // chain, assigns a score to each store. The scores only make sense for
   279  // stores within the same block, and the first store by store order has
   280  // the lowest score. The cache was used to ensure only compute once.
   281  func storeOrdering(v *Value, cache []int32) int32 {
   282  	const minScore int32 = 1
   283  	score := minScore
   284  	w := v
   285  	for {
   286  		if s := cache[w.ID]; s >= minScore {
   287  			score += s
   288  			break
   289  		}
   290  		if w.Op == OpPhi || w.Op == OpInitMem {
   291  			break
   292  		}
   293  		a := w.MemoryArg()
   294  		if a.Block != w.Block {
   295  			break
   296  		}
   297  		w = a
   298  		score++
   299  	}
   300  	w = v
   301  	for cache[w.ID] == 0 {
   302  		cache[w.ID] = score
   303  		if score == minScore {
   304  			break
   305  		}
   306  		w = w.MemoryArg()
   307  		score--
   308  	}
   309  	return cache[v.ID]
   310  }
   311  
   312  // An eqclass approximates an equivalence class. During the
   313  // algorithm it may represent the union of several of the
   314  // final equivalence classes.
   315  type eqclass []*Value
   316  
   317  // partitionValues partitions the values into equivalence classes
   318  // based on having all the following features match:
   319  //   - opcode
   320  //   - type
   321  //   - auxint
   322  //   - aux
   323  //   - nargs
   324  //   - block # if a phi op
   325  //   - first two arg's opcodes and auxint
   326  //   - NOT first two arg's aux; that can break CSE.
   327  //
   328  // partitionValues returns a list of equivalence classes, each
   329  // being a sorted by ID list of *Values. The eqclass slices are
   330  // backed by the same storage as the input slice.
   331  // Equivalence classes of size 1 are ignored.
   332  func partitionValues(a []*Value, auxIDs auxmap) []eqclass {
   333  	slices.SortFunc(a, func(v, w *Value) int {
   334  		switch cmpVal(v, w, auxIDs) {
   335  		case types.CMPlt:
   336  			return -1
   337  		case types.CMPgt:
   338  			return +1
   339  		default:
   340  			// Sort by value ID last to keep the sort result deterministic.
   341  			return cmp.Compare(v.ID, w.ID)
   342  		}
   343  	})
   344  
   345  	var partition []eqclass
   346  	for len(a) > 0 {
   347  		v := a[0]
   348  		j := 1
   349  		for ; j < len(a); j++ {
   350  			w := a[j]
   351  			if cmpVal(v, w, auxIDs) != types.CMPeq {
   352  				break
   353  			}
   354  		}
   355  		if j > 1 {
   356  			partition = append(partition, a[:j])
   357  		}
   358  		a = a[j:]
   359  	}
   360  
   361  	return partition
   362  }
   363  func lt2Cmp(isLt bool) types.Cmp {
   364  	if isLt {
   365  		return types.CMPlt
   366  	}
   367  	return types.CMPgt
   368  }
   369  
   370  type auxmap map[Aux]int32
   371  
   372  func cmpVal(v, w *Value, auxIDs auxmap) types.Cmp {
   373  	// Try to order these comparison by cost (cheaper first)
   374  	if v.Op != w.Op {
   375  		return lt2Cmp(v.Op < w.Op)
   376  	}
   377  	if v.AuxInt != w.AuxInt {
   378  		return lt2Cmp(v.AuxInt < w.AuxInt)
   379  	}
   380  	if len(v.Args) != len(w.Args) {
   381  		return lt2Cmp(len(v.Args) < len(w.Args))
   382  	}
   383  	if v.Op == OpPhi && v.Block != w.Block {
   384  		return lt2Cmp(v.Block.ID < w.Block.ID)
   385  	}
   386  	if v.Type.IsMemory() {
   387  		// We will never be able to CSE two values
   388  		// that generate memory.
   389  		return lt2Cmp(v.ID < w.ID)
   390  	}
   391  	// OpSelect is a pseudo-op. We need to be more aggressive
   392  	// regarding CSE to keep multiple OpSelect's of the same
   393  	// argument from existing.
   394  	if v.Op != OpSelect0 && v.Op != OpSelect1 && v.Op != OpSelectN {
   395  		if tc := v.Type.Compare(w.Type); tc != types.CMPeq {
   396  			return tc
   397  		}
   398  	}
   399  
   400  	if v.Aux != w.Aux {
   401  		if v.Aux == nil {
   402  			return types.CMPlt
   403  		}
   404  		if w.Aux == nil {
   405  			return types.CMPgt
   406  		}
   407  		return lt2Cmp(auxIDs[v.Aux] < auxIDs[w.Aux])
   408  	}
   409  
   410  	return types.CMPeq
   411  }
   412
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