bloop.go

     1  // Copyright 2025 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 bloop
     6  
     7  // This file contains support routines for keeping
     8  // statements alive
     9  // in such loops (example):
    10  //
    11  //	for b.Loop() {
    12  //		var a, b int
    13  //		a = 5
    14  //		b = 6
    15  //		f(a, b)
    16  //	}
    17  //
    18  // The results of a, b and f(a, b) will be kept alive.
    19  //
    20  // Formally, the lhs (if they are [ir.Name]-s) of
    21  // [ir.AssignStmt], [ir.AssignListStmt],
    22  // [ir.AssignOpStmt], and the results of [ir.CallExpr]
    23  // or its args if it doesn't return a value will be kept
    24  // alive.
    25  //
    26  // The keep alive logic is implemented with as wrapping a
    27  // runtime.KeepAlive around the Name.
    28  //
    29  // TODO: currently this is implemented with KeepAlive
    30  // because it will prevent DSE and DCE which is probably
    31  // what we want right now. And KeepAlive takes an ssa
    32  // value instead of a symbol, which is easier to manage.
    33  // But since KeepAlive's context was mainly in the runtime
    34  // and GC, should we implement a new intrinsic that lowers
    35  // to OpVarLive? Peeling out the symbols is a bit tricky
    36  // and also VarLive seems to assume that there exists a
    37  // VarDef on the same symbol that dominates it.
    38  
    39  import (
    40  	"cmd/compile/internal/base"
    41  	"cmd/compile/internal/ir"
    42  	"cmd/compile/internal/reflectdata"
    43  	"cmd/compile/internal/typecheck"
    44  	"cmd/compile/internal/types"
    45  	"cmd/internal/src"
    46  )
    47  
    48  // getNameFromNode tries to iteratively peel down the node to
    49  // get the name.
    50  func getNameFromNode(n ir.Node) *ir.Name {
    51  	// Tries to iteratively peel down the node to get the names.
    52  	for n != nil {
    53  		switch n.Op() {
    54  		case ir.ONAME:
    55  			// Found the name, stop the loop.
    56  			return n.(*ir.Name)
    57  		case ir.OSLICE, ir.OSLICE3:
    58  			n = n.(*ir.SliceExpr).X
    59  		case ir.ODOT:
    60  			n = n.(*ir.SelectorExpr).X
    61  		case ir.OCONV, ir.OCONVIFACE, ir.OCONVNOP:
    62  			n = n.(*ir.ConvExpr).X
    63  		case ir.OADDR:
    64  			n = n.(*ir.AddrExpr).X
    65  		case ir.ODOTPTR:
    66  			n = n.(*ir.SelectorExpr).X
    67  		case ir.OINDEX, ir.OINDEXMAP:
    68  			n = n.(*ir.IndexExpr).X
    69  		default:
    70  			n = nil
    71  		}
    72  	}
    73  	return nil
    74  }
    75  
    76  // keepAliveAt returns a statement that is either curNode, or a
    77  // block containing curNode followed by a call to runtime.KeepAlive for each
    78  // node in ns. These calls ensure that nodes in ns will be live until
    79  // after curNode's execution.
    80  func keepAliveAt(ns []ir.Node, curNode ir.Node) ir.Node {
    81  	if len(ns) == 0 {
    82  		return curNode
    83  	}
    84  
    85  	pos := curNode.Pos()
    86  	calls := []ir.Node{curNode}
    87  	for _, n := range ns {
    88  		if n == nil {
    89  			continue
    90  		}
    91  		if n.Sym() == nil {
    92  			continue
    93  		}
    94  		if n.Sym().IsBlank() {
    95  			continue
    96  		}
    97  		arg := ir.NewConvExpr(pos, ir.OCONV, types.Types[types.TUNSAFEPTR], typecheck.NodAddr(n))
    98  		if !n.Type().IsInterface() {
    99  			srcRType0 := reflectdata.TypePtrAt(pos, n.Type())
   100  			arg.TypeWord = srcRType0
   101  			arg.SrcRType = srcRType0
   102  		}
   103  		callExpr := typecheck.Call(pos,
   104  			typecheck.LookupRuntime("KeepAlive"),
   105  			[]ir.Node{arg}, false).(*ir.CallExpr)
   106  		callExpr.IsCompilerVarLive = true
   107  		callExpr.NoInline = true
   108  		calls = append(calls, callExpr)
   109  	}
   110  
   111  	return ir.NewBlockStmt(pos, calls)
   112  }
   113  
   114  func debugName(name *ir.Name, pos src.XPos) {
   115  	if base.Flag.LowerM > 1 {
   116  		if name.Linksym() != nil {
   117  			base.WarnfAt(pos, "%s will be kept alive", name.Linksym().Name)
   118  		} else {
   119  			base.WarnfAt(pos, "expr will be kept alive")
   120  		}
   121  	}
   122  }
   123  
   124  // preserveStmt transforms stmt so that any names defined/assigned within it
   125  // are used after stmt's execution, preventing their dead code elimination
   126  // and dead store elimination. The return value is the transformed statement.
   127  func preserveStmt(curFn *ir.Func, stmt ir.Node) (ret ir.Node) {
   128  	ret = stmt
   129  	switch n := stmt.(type) {
   130  	case *ir.AssignStmt:
   131  		// Peel down struct and slice indexing to get the names
   132  		name := getNameFromNode(n.X)
   133  		if name != nil {
   134  			debugName(name, n.Pos())
   135  			ret = keepAliveAt([]ir.Node{name}, n)
   136  		} else if deref := n.X.(*ir.StarExpr); deref != nil {
   137  			ret = keepAliveAt([]ir.Node{deref}, n)
   138  			if base.Flag.LowerM > 1 {
   139  				base.WarnfAt(n.Pos(), "dereference will be kept alive")
   140  			}
   141  		} else if base.Flag.LowerM > 1 {
   142  			base.WarnfAt(n.Pos(), "expr is unknown to bloop pass")
   143  		}
   144  	case *ir.AssignListStmt:
   145  		ns := []ir.Node{}
   146  		for _, lhs := range n.Lhs {
   147  			name := getNameFromNode(lhs)
   148  			if name != nil {
   149  				debugName(name, n.Pos())
   150  				ns = append(ns, name)
   151  			} else if deref := lhs.(*ir.StarExpr); deref != nil {
   152  				ns = append(ns, deref)
   153  				if base.Flag.LowerM > 1 {
   154  					base.WarnfAt(n.Pos(), "dereference will be kept alive")
   155  				}
   156  			} else if base.Flag.LowerM > 1 {
   157  				base.WarnfAt(n.Pos(), "expr is unknown to bloop pass")
   158  			}
   159  		}
   160  		ret = keepAliveAt(ns, n)
   161  	case *ir.AssignOpStmt:
   162  		name := getNameFromNode(n.X)
   163  		if name != nil {
   164  			debugName(name, n.Pos())
   165  			ret = keepAliveAt([]ir.Node{name}, n)
   166  		} else if deref := n.X.(*ir.StarExpr); deref != nil {
   167  			ret = keepAliveAt([]ir.Node{deref}, n)
   168  			if base.Flag.LowerM > 1 {
   169  				base.WarnfAt(n.Pos(), "dereference will be kept alive")
   170  			}
   171  		} else if base.Flag.LowerM > 1 {
   172  			base.WarnfAt(n.Pos(), "expr is unknown to bloop pass")
   173  		}
   174  	case *ir.CallExpr:
   175  		curNode := stmt
   176  		if n.Fun != nil && n.Fun.Type() != nil && n.Fun.Type().NumResults() != 0 {
   177  			ns := []ir.Node{}
   178  			// This function's results are not assigned, assign them to
   179  			// auto tmps and then keepAliveAt these autos.
   180  			// Note: markStmt assumes the context that it's called - this CallExpr is
   181  			// not within another OAS2, which is guaranteed by the case above.
   182  			results := n.Fun.Type().Results()
   183  			lhs := make([]ir.Node, len(results))
   184  			for i, res := range results {
   185  				tmp := typecheck.TempAt(n.Pos(), curFn, res.Type)
   186  				lhs[i] = tmp
   187  				ns = append(ns, tmp)
   188  			}
   189  
   190  			// Create an assignment statement.
   191  			assign := typecheck.AssignExpr(
   192  				ir.NewAssignListStmt(n.Pos(), ir.OAS2, lhs,
   193  					[]ir.Node{n})).(*ir.AssignListStmt)
   194  			assign.Def = true
   195  			curNode = assign
   196  			plural := ""
   197  			if len(results) > 1 {
   198  				plural = "s"
   199  			}
   200  			if base.Flag.LowerM > 1 {
   201  				base.WarnfAt(n.Pos(), "function result%s will be kept alive", plural)
   202  			}
   203  			ret = keepAliveAt(ns, curNode)
   204  		} else {
   205  			// This function probably doesn't return anything, keep its args alive.
   206  			argTmps := []ir.Node{}
   207  			names := []ir.Node{}
   208  			for i, a := range n.Args {
   209  				if name := getNameFromNode(a); name != nil {
   210  					// If they are name, keep them alive directly.
   211  					debugName(name, n.Pos())
   212  					names = append(names, name)
   213  				} else if a.Op() == ir.OSLICELIT {
   214  					// variadic args are encoded as slice literal.
   215  					s := a.(*ir.CompLitExpr)
   216  					ns := []ir.Node{}
   217  					for i, elem := range s.List {
   218  						if name := getNameFromNode(elem); name != nil {
   219  							debugName(name, n.Pos())
   220  							ns = append(ns, name)
   221  						} else {
   222  							// We need a temporary to save this arg.
   223  							tmp := typecheck.TempAt(elem.Pos(), curFn, elem.Type())
   224  							argTmps = append(argTmps, typecheck.AssignExpr(ir.NewAssignStmt(elem.Pos(), tmp, elem)))
   225  							names = append(names, tmp)
   226  							s.List[i] = tmp
   227  							if base.Flag.LowerM > 1 {
   228  								base.WarnfAt(n.Pos(), "function arg will be kept alive")
   229  							}
   230  						}
   231  					}
   232  					names = append(names, ns...)
   233  				} else {
   234  					// expressions, we need to assign them to temps and change the original arg to reference
   235  					// them.
   236  					tmp := typecheck.TempAt(n.Pos(), curFn, a.Type())
   237  					argTmps = append(argTmps, typecheck.AssignExpr(ir.NewAssignStmt(n.Pos(), tmp, a)))
   238  					names = append(names, tmp)
   239  					n.Args[i] = tmp
   240  					if base.Flag.LowerM > 1 {
   241  						base.WarnfAt(n.Pos(), "function arg will be kept alive")
   242  					}
   243  				}
   244  			}
   245  			if len(argTmps) > 0 {
   246  				argTmps = append(argTmps, n)
   247  				curNode = ir.NewBlockStmt(n.Pos(), argTmps)
   248  			}
   249  			ret = keepAliveAt(names, curNode)
   250  		}
   251  	}
   252  	return
   253  }
   254  
   255  func preserveStmts(curFn *ir.Func, list ir.Nodes) {
   256  	for i := range list {
   257  		list[i] = preserveStmt(curFn, list[i])
   258  	}
   259  }
   260  
   261  // isTestingBLoop returns true if it matches the node as a
   262  // testing.(*B).Loop. See issue #61515.
   263  func isTestingBLoop(t ir.Node) bool {
   264  	if t.Op() != ir.OFOR {
   265  		return false
   266  	}
   267  	nFor, ok := t.(*ir.ForStmt)
   268  	if !ok || nFor.Cond == nil || nFor.Cond.Op() != ir.OCALLFUNC {
   269  		return false
   270  	}
   271  	n, ok := nFor.Cond.(*ir.CallExpr)
   272  	if !ok || n.Fun == nil || n.Fun.Op() != ir.OMETHEXPR {
   273  		return false
   274  	}
   275  	name := ir.MethodExprName(n.Fun)
   276  	if name == nil {
   277  		return false
   278  	}
   279  	if fSym := name.Sym(); fSym != nil && name.Class == ir.PFUNC && fSym.Pkg != nil &&
   280  		fSym.Name == "(*B).Loop" && fSym.Pkg.Path == "testing" {
   281  		// Attempting to match a function call to testing.(*B).Loop
   282  		return true
   283  	}
   284  	return false
   285  }
   286  
   287  type editor struct {
   288  	inBloop bool
   289  	curFn   *ir.Func
   290  }
   291  
   292  func (e editor) edit(n ir.Node) ir.Node {
   293  	e.inBloop = isTestingBLoop(n) || e.inBloop
   294  	// It's in bloop, mark the stmts with bodies.
   295  	ir.EditChildren(n, e.edit)
   296  	if e.inBloop {
   297  		switch n := n.(type) {
   298  		case *ir.ForStmt:
   299  			preserveStmts(e.curFn, n.Body)
   300  		case *ir.IfStmt:
   301  			preserveStmts(e.curFn, n.Body)
   302  			preserveStmts(e.curFn, n.Else)
   303  		case *ir.BlockStmt:
   304  			preserveStmts(e.curFn, n.List)
   305  		case *ir.CaseClause:
   306  			preserveStmts(e.curFn, n.List)
   307  			preserveStmts(e.curFn, n.Body)
   308  		case *ir.CommClause:
   309  			preserveStmts(e.curFn, n.Body)
   310  		case *ir.RangeStmt:
   311  			preserveStmts(e.curFn, n.Body)
   312  		}
   313  	}
   314  	return n
   315  }
   316  
   317  // BloopWalk performs a walk on all functions in the package
   318  // if it imports testing and wrap the results of all qualified
   319  // statements in a runtime.KeepAlive intrinsic call. See package
   320  // doc for more details.
   321  //
   322  //	for b.Loop() {...}
   323  //
   324  // loop's body.
   325  func BloopWalk(pkg *ir.Package) {
   326  	hasTesting := false
   327  	for _, i := range pkg.Imports {
   328  		if i.Path == "testing" {
   329  			hasTesting = true
   330  			break
   331  		}
   332  	}
   333  	if !hasTesting {
   334  		return
   335  	}
   336  	for _, fn := range pkg.Funcs {
   337  		e := editor{false, fn}
   338  		ir.EditChildren(fn, e.edit)
   339  	}
   340  }
   341
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