deadstore.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/ir"
     9  	"cmd/compile/internal/types"
    10  	"cmd/internal/obj"
    11  )
    12  
    13  // maxShadowRanges bounds the number of disjoint byte intervals
    14  // we track per pointer to avoid quadratic behaviour.
    15  const maxShadowRanges = 64
    16  
    17  // dse does dead-store elimination on the Function.
    18  // Dead stores are those which are unconditionally followed by
    19  // another store to the same location, with no intervening load.
    20  // This implementation only works within a basic block. TODO: use something more global.
    21  func dse(f *Func) {
    22  	var stores []*Value
    23  	loadUse := f.newSparseSet(f.NumValues())
    24  	defer f.retSparseSet(loadUse)
    25  	storeUse := f.newSparseSet(f.NumValues())
    26  	defer f.retSparseSet(storeUse)
    27  	shadowed := f.newSparseMap(f.NumValues())
    28  	defer f.retSparseMap(shadowed)
    29  	// localAddrs maps from a local variable (the Aux field of a LocalAddr value) to an instance of a LocalAddr value for that variable in the current block.
    30  	localAddrs := map[any]*Value{}
    31  
    32  	// shadowedRanges stores the actual range data. The 'shadowed' sparseMap stores a 1-based index into this slice.
    33  	var shadowedRanges []*shadowRanges
    34  
    35  	for _, b := range f.Blocks {
    36  		// Find all the stores in this block. Categorize their uses:
    37  		//  loadUse contains stores which are used by a subsequent load.
    38  		//  storeUse contains stores which are used by a subsequent store.
    39  		loadUse.clear()
    40  		storeUse.clear()
    41  		clear(localAddrs)
    42  		stores = stores[:0]
    43  		for _, v := range b.Values {
    44  			if v.Op == OpPhi {
    45  				// Ignore phis - they will always be first and can't be eliminated
    46  				continue
    47  			}
    48  			if v.Type.IsMemory() {
    49  				stores = append(stores, v)
    50  				for _, a := range v.Args {
    51  					if a.Block == b && a.Type.IsMemory() {
    52  						storeUse.add(a.ID)
    53  						if v.Op != OpStore && v.Op != OpZero && v.Op != OpVarDef {
    54  							// CALL, DUFFCOPY, etc. are both
    55  							// reads and writes.
    56  							loadUse.add(a.ID)
    57  						}
    58  					}
    59  				}
    60  			} else {
    61  				if v.Op == OpLocalAddr {
    62  					if _, ok := localAddrs[v.Aux]; !ok {
    63  						localAddrs[v.Aux] = v
    64  					}
    65  					continue
    66  				}
    67  				if v.Op == OpInlMark || v.Op == OpConvert {
    68  					// Not really a use of the memory. See #67957.
    69  					continue
    70  				}
    71  				for _, a := range v.Args {
    72  					if a.Block == b && a.Type.IsMemory() {
    73  						loadUse.add(a.ID)
    74  					}
    75  				}
    76  			}
    77  		}
    78  		if len(stores) == 0 {
    79  			continue
    80  		}
    81  
    82  		// find last store in the block
    83  		var last *Value
    84  		for _, v := range stores {
    85  			if storeUse.contains(v.ID) {
    86  				continue
    87  			}
    88  			if last != nil {
    89  				b.Fatalf("two final stores - simultaneous live stores %s %s", last.LongString(), v.LongString())
    90  			}
    91  			last = v
    92  		}
    93  		if last == nil {
    94  			b.Fatalf("no last store found - cycle?")
    95  		}
    96  
    97  		// Walk backwards looking for dead stores. Keep track of shadowed addresses.
    98  		// A "shadowed address" is a pointer, offset, and size describing a memory region that
    99  		// is known to be written. We keep track of shadowed addresses in the shadowed map,
   100  		// mapping the ID of the address to a shadowRanges where future writes will happen.
   101  		// Since we're walking backwards, writes to a shadowed region are useless,
   102  		// as they will be immediately overwritten.
   103  		shadowed.clear()
   104  		shadowedRanges = shadowedRanges[:0]
   105  		v := last
   106  
   107  	walkloop:
   108  		if loadUse.contains(v.ID) {
   109  			// Someone might be reading this memory state.
   110  			// Clear all shadowed addresses.
   111  			shadowed.clear()
   112  			shadowedRanges = shadowedRanges[:0]
   113  		}
   114  		if v.Op == OpStore || v.Op == OpZero {
   115  			ptr := v.Args[0]
   116  			var off int64
   117  			for ptr.Op == OpOffPtr { // Walk to base pointer
   118  				off += ptr.AuxInt
   119  				ptr = ptr.Args[0]
   120  			}
   121  			var sz int64
   122  			if v.Op == OpStore {
   123  				sz = v.Aux.(*types.Type).Size()
   124  			} else { // OpZero
   125  				sz = v.AuxInt
   126  			}
   127  			if ptr.Op == OpLocalAddr {
   128  				if la, ok := localAddrs[ptr.Aux]; ok {
   129  					ptr = la
   130  				}
   131  			}
   132  			var si *shadowRanges
   133  			idx, ok := shadowed.get(ptr.ID)
   134  			if ok {
   135  				// The sparseMap stores a 1-based index, so we subtract 1.
   136  				si = shadowedRanges[idx-1]
   137  			}
   138  
   139  			if si != nil && si.contains(off, off+sz) {
   140  				// Modify the store/zero into a copy of the memory state,
   141  				// effectively eliding the store operation.
   142  				if v.Op == OpStore {
   143  					// store addr value mem
   144  					v.SetArgs1(v.Args[2])
   145  				} else {
   146  					// zero addr mem
   147  					v.SetArgs1(v.Args[1])
   148  				}
   149  				v.Aux = nil
   150  				v.AuxInt = 0
   151  				v.Op = OpCopy
   152  			} else {
   153  				// Extend shadowed region.
   154  				if si == nil {
   155  					si = &shadowRanges{}
   156  					shadowedRanges = append(shadowedRanges, si)
   157  					// Store a 1-based index in the sparseMap.
   158  					shadowed.set(ptr.ID, int32(len(shadowedRanges)))
   159  				}
   160  				si.add(off, off+sz)
   161  			}
   162  		}
   163  		// walk to previous store
   164  		if v.Op == OpPhi {
   165  			// At start of block.  Move on to next block.
   166  			// The memory phi, if it exists, is always
   167  			// the first logical store in the block.
   168  			// (Even if it isn't the first in the current b.Values order.)
   169  			continue
   170  		}
   171  		for _, a := range v.Args {
   172  			if a.Block == b && a.Type.IsMemory() {
   173  				v = a
   174  				goto walkloop
   175  			}
   176  		}
   177  	}
   178  }
   179  
   180  // shadowRange represents a single byte range [lo,hi] that will be written.
   181  type shadowRange struct {
   182  	lo, hi uint16
   183  }
   184  
   185  // shadowRanges stores an unordered collection of disjoint byte ranges.
   186  type shadowRanges struct {
   187  	ranges []shadowRange
   188  }
   189  
   190  // contains reports whether [lo:hi] is completely within sr.
   191  func (sr *shadowRanges) contains(lo, hi int64) bool {
   192  	for _, r := range sr.ranges {
   193  		if lo >= int64(r.lo) && hi <= int64(r.hi) {
   194  			return true
   195  		}
   196  	}
   197  	return false
   198  }
   199  
   200  func (sr *shadowRanges) add(lo, hi int64) {
   201  	// Ignore the store if:
   202  	// - the range doesn't fit in 16 bits, or
   203  	// - we already track maxShadowRanges intervals.
   204  	// The cap prevents a theoretical O(n^2) blow-up.
   205  	if lo < 0 || hi > 0xffff || len(sr.ranges) >= maxShadowRanges {
   206  		return
   207  	}
   208  	nlo := lo
   209  	nhi := hi
   210  	out := sr.ranges[:0]
   211  
   212  	for _, r := range sr.ranges {
   213  		if nhi < int64(r.lo) || nlo > int64(r.hi) {
   214  			out = append(out, r)
   215  			continue
   216  		}
   217  		if int64(r.lo) < nlo {
   218  			nlo = int64(r.lo)
   219  		}
   220  		if int64(r.hi) > nhi {
   221  			nhi = int64(r.hi)
   222  		}
   223  	}
   224  	sr.ranges = append(out, shadowRange{uint16(nlo), uint16(nhi)})
   225  }
   226  
   227  // elimDeadAutosGeneric deletes autos that are never accessed. To achieve this
   228  // we track the operations that the address of each auto reaches and if it only
   229  // reaches stores then we delete all the stores. The other operations will then
   230  // be eliminated by the dead code elimination pass.
   231  func elimDeadAutosGeneric(f *Func) {
   232  	addr := make(map[*Value]*ir.Name)     // values that the address of the auto reaches
   233  	elim := make(map[*Value]*ir.Name)     // values that could be eliminated if the auto is
   234  	move := make(map[*ir.Name]ir.NameSet) // for a (Move &y &x _) and y is unused, move[y].Add(x)
   235  	var used ir.NameSet                   // used autos that must be kept
   236  
   237  	// Adds a name to used and, when it is the target of a move, also
   238  	// propagates the used state to its source.
   239  	var usedAdd func(n *ir.Name) bool
   240  	usedAdd = func(n *ir.Name) bool {
   241  		if used.Has(n) {
   242  			return false
   243  		}
   244  		used.Add(n)
   245  		if s := move[n]; s != nil {
   246  			delete(move, n)
   247  			for n := range s {
   248  				usedAdd(n)
   249  			}
   250  		}
   251  		return true
   252  	}
   253  
   254  	// visit the value and report whether any of the maps are updated
   255  	visit := func(v *Value) (changed bool) {
   256  		args := v.Args
   257  		switch v.Op {
   258  		case OpAddr, OpLocalAddr:
   259  			// Propagate the address if it points to an auto.
   260  			n, ok := v.Aux.(*ir.Name)
   261  			if !ok || (n.Class != ir.PAUTO && !isABIInternalParam(f, n)) {
   262  				return
   263  			}
   264  			if addr[v] == nil {
   265  				addr[v] = n
   266  				changed = true
   267  			}
   268  			return
   269  		case OpVarDef:
   270  			// v should be eliminated if we eliminate the auto.
   271  			n, ok := v.Aux.(*ir.Name)
   272  			if !ok || (n.Class != ir.PAUTO && !isABIInternalParam(f, n)) {
   273  				return
   274  			}
   275  			if elim[v] == nil {
   276  				elim[v] = n
   277  				changed = true
   278  			}
   279  			return
   280  		case OpVarLive:
   281  			// Don't delete the auto if it needs to be kept alive.
   282  
   283  			// We depend on this check to keep the autotmp stack slots
   284  			// for open-coded defers from being removed (since they
   285  			// may not be used by the inline code, but will be used by
   286  			// panic processing).
   287  			n, ok := v.Aux.(*ir.Name)
   288  			if !ok || (n.Class != ir.PAUTO && !isABIInternalParam(f, n)) {
   289  				return
   290  			}
   291  			changed = usedAdd(n) || changed
   292  			return
   293  		case OpStore, OpMove, OpZero:
   294  			// v should be eliminated if we eliminate the auto.
   295  			n, ok := addr[args[0]]
   296  			if ok && elim[v] == nil {
   297  				elim[v] = n
   298  				changed = true
   299  			}
   300  			// Other args might hold pointers to autos.
   301  			args = args[1:]
   302  		}
   303  
   304  		// The code below assumes that we have handled all the ops
   305  		// with sym effects already. Sanity check that here.
   306  		// Ignore Args since they can't be autos.
   307  		if v.Op.SymEffect() != SymNone && v.Op != OpArg {
   308  			panic("unhandled op with sym effect")
   309  		}
   310  
   311  		if v.Uses == 0 && v.Op != OpNilCheck && !v.Op.IsCall() && !v.Op.HasSideEffects() || len(args) == 0 {
   312  			// We need to keep nil checks even if they have no use.
   313  			// Also keep calls and values that have side effects.
   314  			return
   315  		}
   316  
   317  		// If the address of the auto reaches a memory or control
   318  		// operation not covered above then we probably need to keep it.
   319  		// We also need to keep autos if they reach Phis (issue #26153).
   320  		if v.Type.IsMemory() || v.Type.IsFlags() || v.Op == OpPhi || v.MemoryArg() != nil {
   321  			for _, a := range args {
   322  				if n, ok := addr[a]; ok {
   323  					// If the addr of n is used by an OpMove as its source arg,
   324  					// and the OpMove's target arg is the addr of a unused name,
   325  					// then temporarily treat n as unused, and record in move map.
   326  					if nam, ok := elim[v]; ok && v.Op == OpMove && !used.Has(nam) {
   327  						if used.Has(n) {
   328  							continue
   329  						}
   330  						s := move[nam]
   331  						if s == nil {
   332  							s = ir.NameSet{}
   333  							move[nam] = s
   334  						}
   335  						s.Add(n)
   336  						continue
   337  					}
   338  					changed = usedAdd(n) || changed
   339  				}
   340  			}
   341  			return
   342  		}
   343  
   344  		// Propagate any auto addresses through v.
   345  		var node *ir.Name
   346  		for _, a := range args {
   347  			if n, ok := addr[a]; ok {
   348  				if node == nil {
   349  					if !used.Has(n) {
   350  						node = n
   351  					}
   352  				} else {
   353  					if node == n {
   354  						continue
   355  					}
   356  					// Most of the time we only see one pointer
   357  					// reaching an op, but some ops can take
   358  					// multiple pointers (e.g. NeqPtr, Phi etc.).
   359  					// This is rare, so just propagate the first
   360  					// value to keep things simple.
   361  					changed = usedAdd(n) || changed
   362  				}
   363  			}
   364  		}
   365  		if node == nil {
   366  			return
   367  		}
   368  		if addr[v] == nil {
   369  			// The address of an auto reaches this op.
   370  			addr[v] = node
   371  			changed = true
   372  			return
   373  		}
   374  		if addr[v] != node {
   375  			// This doesn't happen in practice, but catch it just in case.
   376  			changed = usedAdd(node) || changed
   377  		}
   378  		return
   379  	}
   380  
   381  	iterations := 0
   382  	for {
   383  		if iterations == 4 {
   384  			// give up
   385  			return
   386  		}
   387  		iterations++
   388  		changed := false
   389  		for _, b := range f.Blocks {
   390  			for _, v := range b.Values {
   391  				changed = visit(v) || changed
   392  			}
   393  			// keep the auto if its address reaches a control value
   394  			for _, c := range b.ControlValues() {
   395  				if n, ok := addr[c]; ok {
   396  					changed = usedAdd(n) || changed
   397  				}
   398  			}
   399  		}
   400  		if !changed {
   401  			break
   402  		}
   403  	}
   404  
   405  	// Eliminate stores to unread autos.
   406  	for v, n := range elim {
   407  		if used.Has(n) {
   408  			continue
   409  		}
   410  		// replace with OpCopy
   411  		v.SetArgs1(v.MemoryArg())
   412  		v.Aux = nil
   413  		v.AuxInt = 0
   414  		v.Op = OpCopy
   415  	}
   416  }
   417  
   418  // elimUnreadAutos deletes stores (and associated bookkeeping ops VarDef and VarKill)
   419  // to autos that are never read from.
   420  func elimUnreadAutos(f *Func) {
   421  	// Loop over all ops that affect autos taking note of which
   422  	// autos we need and also stores that we might be able to
   423  	// eliminate.
   424  	var seen ir.NameSet
   425  	var stores []*Value
   426  	for _, b := range f.Blocks {
   427  		for _, v := range b.Values {
   428  			n, ok := v.Aux.(*ir.Name)
   429  			if !ok {
   430  				continue
   431  			}
   432  			if n.Class != ir.PAUTO && !isABIInternalParam(f, n) {
   433  				continue
   434  			}
   435  
   436  			effect := v.Op.SymEffect()
   437  			switch effect {
   438  			case SymNone, SymWrite:
   439  				// If we haven't seen the auto yet
   440  				// then this might be a store we can
   441  				// eliminate.
   442  				if !seen.Has(n) {
   443  					stores = append(stores, v)
   444  				}
   445  			default:
   446  				// Assume the auto is needed (loaded,
   447  				// has its address taken, etc.).
   448  				// Note we have to check the uses
   449  				// because dead loads haven't been
   450  				// eliminated yet.
   451  				if v.Uses > 0 {
   452  					seen.Add(n)
   453  				}
   454  			}
   455  		}
   456  	}
   457  
   458  	// Eliminate stores to unread autos.
   459  	for _, store := range stores {
   460  		n, _ := store.Aux.(*ir.Name)
   461  		if seen.Has(n) {
   462  			continue
   463  		}
   464  
   465  		// replace store with OpCopy
   466  		store.SetArgs1(store.MemoryArg())
   467  		store.Aux = nil
   468  		store.AuxInt = 0
   469  		store.Op = OpCopy
   470  	}
   471  }
   472  
   473  // isABIInternalParam returns whether n is a parameter of an ABIInternal
   474  // function. For dead store elimination, we can treat parameters the same
   475  // way as autos. Storing to a parameter can be removed if it is not read
   476  // or address-taken.
   477  //
   478  // We check ABI here because for a cgo_unsafe_arg function (which is ABI0),
   479  // all the args are effectively address-taken, but not necessarily have
   480  // an Addr or LocalAddr op. We could probably just check for cgo_unsafe_arg,
   481  // but ABIInternal is mostly what matters.
   482  func isABIInternalParam(f *Func, n *ir.Name) bool {
   483  	return n.Class == ir.PPARAM && f.ABISelf.Which() == obj.ABIInternal
   484  }
   485
View as plain text