assign.go

Documentation: cmd/compile/internal/escape

     1  // Copyright 2018 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 escape
     6  
     7  import (
     8  	"cmd/compile/internal/base"
     9  	"cmd/compile/internal/ir"
    10  )
    11  
    12  // addr evaluates an addressable expression n and returns a hole
    13  // that represents storing into the represented location.
    14  func (e *escape) addr(n ir.Node) hole {
    15  	if n == nil || ir.IsBlank(n) {
    16  		// Can happen in select case, range, maybe others.
    17  		return e.discardHole()
    18  	}
    19  
    20  	k := e.heapHole()
    21  
    22  	switch n.Op() {
    23  	default:
    24  		base.Fatalf("unexpected addr: %v", n)
    25  	case ir.ONAME:
    26  		n := n.(*ir.Name)
    27  		if n.Class == ir.PEXTERN {
    28  			break
    29  		}
    30  		k = e.oldLoc(n).asHole()
    31  	case ir.OLINKSYMOFFSET:
    32  		break
    33  	case ir.ODOT:
    34  		n := n.(*ir.SelectorExpr)
    35  		k = e.addr(n.X)
    36  	case ir.OINDEX:
    37  		n := n.(*ir.IndexExpr)
    38  		e.discard(n.Index)
    39  		if n.X.Type().IsArray() {
    40  			k = e.addr(n.X)
    41  		} else {
    42  			e.mutate(n.X)
    43  		}
    44  	case ir.ODEREF:
    45  		n := n.(*ir.StarExpr)
    46  		e.mutate(n.X)
    47  	case ir.ODOTPTR:
    48  		n := n.(*ir.SelectorExpr)
    49  		e.mutate(n.X)
    50  	case ir.OINDEXMAP:
    51  		n := n.(*ir.IndexExpr)
    52  		e.discard(n.X)
    53  		e.assignHeap(n.Index, "key of map put", n)
    54  	}
    55  
    56  	return k
    57  }
    58  
    59  func (e *escape) mutate(n ir.Node) {
    60  	e.expr(e.mutatorHole(), n)
    61  }
    62  
    63  func (e *escape) addrs(l ir.Nodes) []hole {
    64  	var ks []hole
    65  	for _, n := range l {
    66  		ks = append(ks, e.addr(n))
    67  	}
    68  	return ks
    69  }
    70  
    71  func (e *escape) assignHeap(src ir.Node, why string, where ir.Node) {
    72  	e.expr(e.heapHole().note(where, why), src)
    73  }
    74  
    75  // assignList evaluates the assignment dsts... = srcs....
    76  func (e *escape) assignList(dsts, srcs []ir.Node, why string, where ir.Node) {
    77  	ks := e.addrs(dsts)
    78  	for i, k := range ks {
    79  		var src ir.Node
    80  		if i < len(srcs) {
    81  			src = srcs[i]
    82  		}
    83  
    84  		if dst := dsts[i]; dst != nil {
    85  			// Detect implicit conversion of uintptr to unsafe.Pointer when
    86  			// storing into reflect.{Slice,String}Header.
    87  			if dst.Op() == ir.ODOTPTR && ir.IsReflectHeaderDataField(dst) {
    88  				e.unsafeValue(e.heapHole().note(where, why), src)
    89  				continue
    90  			}
    91  
    92  			// Filter out some no-op assignments for escape analysis.
    93  			if src != nil && isSelfAssign(dst, src) {
    94  				if base.Flag.LowerM != 0 {
    95  					base.WarnfAt(where.Pos(), "%v ignoring self-assignment in %v", e.curfn, where)
    96  				}
    97  				k = e.discardHole()
    98  			}
    99  		}
   100  
   101  		e.expr(k.note(where, why), src)
   102  	}
   103  
   104  	e.reassigned(ks, where)
   105  }
   106  
   107  // reassigned marks the locations associated with the given holes as
   108  // reassigned, unless the location represents a variable declared and
   109  // assigned exactly once by where.
   110  func (e *escape) reassigned(ks []hole, where ir.Node) {
   111  	if as, ok := where.(*ir.AssignStmt); ok && as.Op() == ir.OAS && as.Y == nil {
   112  		if dst, ok := as.X.(*ir.Name); ok && dst.Op() == ir.ONAME && dst.Defn == nil {
   113  			// Zero-value assignment for variable declared without an
   114  			// explicit initial value. Assume this is its initialization
   115  			// statement.
   116  			return
   117  		}
   118  	}
   119  
   120  	for _, k := range ks {
   121  		loc := k.dst
   122  		// Variables declared by range statements are assigned on every iteration.
   123  		if n, ok := loc.n.(*ir.Name); ok && n.Defn == where && where.Op() != ir.ORANGE {
   124  			continue
   125  		}
   126  		loc.reassigned = true
   127  	}
   128  }
   129
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