reassignment.go

Documentation: cmd/compile/internal/ir

     1  // Copyright 2023 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 ir
     6  
     7  import (
     8  	"cmd/compile/internal/base"
     9  )
    10  
    11  // A ReassignOracle efficiently answers queries about whether local
    12  // variables are reassigned. This helper works by looking for function
    13  // params and short variable declarations (e.g.
    14  // https://go.dev/ref/spec#Short_variable_declarations) that are
    15  // neither address taken nor subsequently re-assigned. It is intended
    16  // to operate much like "ir.StaticValue" and "ir.Reassigned", but in a
    17  // way that does just a single walk of the containing function (as
    18  // opposed to a new walk on every call).
    19  type ReassignOracle struct {
    20  	fn *Func
    21  	// maps candidate name to its defining assignment (or for
    22  	// for params, defining func).
    23  	singleDef map[*Name]Node
    24  }
    25  
    26  // Init initializes the oracle based on the IR in function fn, laying
    27  // the groundwork for future calls to the StaticValue and Reassigned
    28  // methods. If the fn's IR is subsequently modified, Init must be
    29  // called again.
    30  func (ro *ReassignOracle) Init(fn *Func) {
    31  	ro.fn = fn
    32  
    33  	// Collect candidate map. Start by adding function parameters
    34  	// explicitly.
    35  	ro.singleDef = make(map[*Name]Node)
    36  	sig := fn.Type()
    37  	numParams := sig.NumRecvs() + sig.NumParams()
    38  	for _, param := range fn.Dcl[:numParams] {
    39  		if IsBlank(param) {
    40  			continue
    41  		}
    42  		// For params, use func itself as defining node.
    43  		ro.singleDef[param] = fn
    44  	}
    45  
    46  	// Walk the function body to discover any locals assigned
    47  	// via ":=" syntax (e.g. "a := <expr>").
    48  	var findLocals func(n Node) bool
    49  	findLocals = func(n Node) bool {
    50  		if nn, ok := n.(*Name); ok {
    51  			if nn.Defn != nil && !nn.Addrtaken() && nn.Class == PAUTO {
    52  				ro.singleDef[nn] = nn.Defn
    53  			}
    54  		} else if nn, ok := n.(*ClosureExpr); ok {
    55  			Any(nn.Func, findLocals)
    56  		}
    57  		return false
    58  	}
    59  	Any(fn, findLocals)
    60  
    61  	outerName := func(x Node) *Name {
    62  		if x == nil {
    63  			return nil
    64  		}
    65  		n, ok := OuterValue(x).(*Name)
    66  		if ok {
    67  			return n.Canonical()
    68  		}
    69  		return nil
    70  	}
    71  
    72  	// pruneIfNeeded examines node nn appearing on the left hand side
    73  	// of assignment statement asn to see if it contains a reassignment
    74  	// to any nodes in our candidate map ro.singleDef; if a reassignment
    75  	// is found, the corresponding name is deleted from singleDef.
    76  	pruneIfNeeded := func(nn Node, asn Node) {
    77  		oname := outerName(nn)
    78  		if oname == nil {
    79  			return
    80  		}
    81  		defn, ok := ro.singleDef[oname]
    82  		if !ok {
    83  			return
    84  		}
    85  		// any assignment to a param invalidates the entry.
    86  		paramAssigned := oname.Class == PPARAM
    87  		// assignment to local ok iff assignment is its orig def.
    88  		localAssigned := (oname.Class == PAUTO && asn != defn)
    89  		if paramAssigned || localAssigned {
    90  			// We found an assignment to name N that doesn't
    91  			// correspond to its original definition; remove
    92  			// from candidates.
    93  			delete(ro.singleDef, oname)
    94  		}
    95  	}
    96  
    97  	// Prune away anything that looks assigned. This code modeled after
    98  	// similar code in ir.Reassigned; any changes there should be made
    99  	// here as well.
   100  	var do func(n Node) bool
   101  	do = func(n Node) bool {
   102  		switch n.Op() {
   103  		case OAS:
   104  			asn := n.(*AssignStmt)
   105  			pruneIfNeeded(asn.X, n)
   106  		case OAS2, OAS2FUNC, OAS2MAPR, OAS2DOTTYPE, OAS2RECV, OSELRECV2:
   107  			asn := n.(*AssignListStmt)
   108  			for _, p := range asn.Lhs {
   109  				pruneIfNeeded(p, n)
   110  			}
   111  		case OASOP:
   112  			asn := n.(*AssignOpStmt)
   113  			pruneIfNeeded(asn.X, n)
   114  		case ORANGE:
   115  			rs := n.(*RangeStmt)
   116  			pruneIfNeeded(rs.Key, n)
   117  			pruneIfNeeded(rs.Value, n)
   118  		case OCLOSURE:
   119  			n := n.(*ClosureExpr)
   120  			Any(n.Func, do)
   121  		}
   122  		return false
   123  	}
   124  	Any(fn, do)
   125  }
   126  
   127  // StaticValue method has the same semantics as the ir package function
   128  // of the same name; see comments on [StaticValue].
   129  func (ro *ReassignOracle) StaticValue(n Node) Node {
   130  	arg := n
   131  	for {
   132  		if n.Op() == OCONVNOP {
   133  			n = n.(*ConvExpr).X
   134  			continue
   135  		}
   136  
   137  		if n.Op() == OINLCALL {
   138  			n = n.(*InlinedCallExpr).SingleResult()
   139  			continue
   140  		}
   141  
   142  		n1 := ro.staticValue1(n)
   143  		if n1 == nil {
   144  			if consistencyCheckEnabled {
   145  				checkStaticValueResult(arg, n)
   146  			}
   147  			return n
   148  		}
   149  		n = n1
   150  	}
   151  }
   152  
   153  func (ro *ReassignOracle) staticValue1(nn Node) Node {
   154  	if nn.Op() != ONAME {
   155  		return nil
   156  	}
   157  	n := nn.(*Name).Canonical()
   158  	if n.Class != PAUTO {
   159  		return nil
   160  	}
   161  
   162  	defn := n.Defn
   163  	if defn == nil {
   164  		return nil
   165  	}
   166  
   167  	var rhs Node
   168  FindRHS:
   169  	switch defn.Op() {
   170  	case OAS:
   171  		defn := defn.(*AssignStmt)
   172  		rhs = defn.Y
   173  	case OAS2:
   174  		defn := defn.(*AssignListStmt)
   175  		for i, lhs := range defn.Lhs {
   176  			if lhs == n {
   177  				rhs = defn.Rhs[i]
   178  				break FindRHS
   179  			}
   180  		}
   181  		base.Fatalf("%v missing from LHS of %v", n, defn)
   182  	default:
   183  		return nil
   184  	}
   185  	if rhs == nil {
   186  		base.Fatalf("RHS is nil: %v", defn)
   187  	}
   188  
   189  	if _, ok := ro.singleDef[n]; !ok {
   190  		return nil
   191  	}
   192  
   193  	return rhs
   194  }
   195  
   196  // Reassigned method has the same semantics as the ir package function
   197  // of the same name; see comments on [Reassigned] for more info.
   198  func (ro *ReassignOracle) Reassigned(n *Name) bool {
   199  	_, ok := ro.singleDef[n]
   200  	result := !ok
   201  	if consistencyCheckEnabled {
   202  		checkReassignedResult(n, result)
   203  	}
   204  	return result
   205  }
   206
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