closure.go

Documentation: cmd/compile/internal/walk

     1  // Copyright 2009 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 walk
     6  
     7  import (
     8  	"cmd/compile/internal/base"
     9  	"cmd/compile/internal/ir"
    10  	"cmd/compile/internal/typecheck"
    11  	"cmd/compile/internal/types"
    12  	"cmd/internal/src"
    13  )
    14  
    15  // directClosureCall rewrites a direct call of a function literal into
    16  // a normal function call with closure variables passed as arguments.
    17  // This avoids allocation of a closure object.
    18  //
    19  // For illustration, the following call:
    20  //
    21  //	func(a int) {
    22  //		println(byval)
    23  //		byref++
    24  //	}(42)
    25  //
    26  // becomes:
    27  //
    28  //	func(byval int, &byref *int, a int) {
    29  //		println(byval)
    30  //		(*&byref)++
    31  //	}(byval, &byref, 42)
    32  func directClosureCall(n *ir.CallExpr) {
    33  	clo := n.Fun.(*ir.ClosureExpr)
    34  	clofn := clo.Func
    35  
    36  	if ir.IsTrivialClosure(clo) {
    37  		return // leave for walkClosure to handle
    38  	}
    39  
    40  	// We are going to insert captured variables before input args.
    41  	var params []*types.Field
    42  	var decls []*ir.Name
    43  	for _, v := range clofn.ClosureVars {
    44  		if !v.Byval() {
    45  			// If v of type T is captured by reference,
    46  			// we introduce function param &v *T
    47  			// and v remains PAUTOHEAP with &v heapaddr
    48  			// (accesses will implicitly deref &v).
    49  
    50  			addr := ir.NewNameAt(clofn.Pos(), typecheck.Lookup("&"+v.Sym().Name), types.NewPtr(v.Type()))
    51  			addr.Curfn = clofn
    52  			v.Heapaddr = addr
    53  			v = addr
    54  		}
    55  
    56  		v.Class = ir.PPARAM
    57  		decls = append(decls, v)
    58  
    59  		fld := types.NewField(src.NoXPos, v.Sym(), v.Type())
    60  		fld.Nname = v
    61  		params = append(params, fld)
    62  	}
    63  
    64  	// f is ONAME of the actual function.
    65  	f := clofn.Nname
    66  	typ := f.Type()
    67  
    68  	// Create new function type with parameters prepended, and
    69  	// then update type and declarations.
    70  	typ = types.NewSignature(nil, append(params, typ.Params()...), typ.Results())
    71  	f.SetType(typ)
    72  	clofn.Dcl = append(decls, clofn.Dcl...)
    73  
    74  	// Rewrite call.
    75  	n.Fun = f
    76  	n.Args.Prepend(closureArgs(clo)...)
    77  
    78  	// Update the call expression's type. We need to do this
    79  	// because typecheck gave it the result type of the OCLOSURE
    80  	// node, but we only rewrote the ONAME node's type. Logically,
    81  	// they're the same, but the stack offsets probably changed.
    82  	if typ.NumResults() == 1 {
    83  		n.SetType(typ.Result(0).Type)
    84  	} else {
    85  		n.SetType(typ.ResultsTuple())
    86  	}
    87  
    88  	// Add to Closures for enqueueFunc. It's no longer a proper
    89  	// closure, but we may have already skipped over it in the
    90  	// functions list as a non-trivial closure, so this just
    91  	// ensures it's compiled.
    92  	ir.CurFunc.Closures = append(ir.CurFunc.Closures, clofn)
    93  }
    94  
    95  func walkClosure(clo *ir.ClosureExpr, init *ir.Nodes) ir.Node {
    96  	clofn := clo.Func
    97  
    98  	// If no closure vars, don't bother wrapping.
    99  	if ir.IsTrivialClosure(clo) {
   100  		if base.Debug.Closure > 0 {
   101  			base.WarnfAt(clo.Pos(), "closure converted to global")
   102  		}
   103  		return clofn.Nname
   104  	}
   105  
   106  	// The closure is not trivial or directly called, so it's going to stay a closure.
   107  	ir.ClosureDebugRuntimeCheck(clo)
   108  	clofn.SetNeedctxt(true)
   109  
   110  	// The closure expression may be walked more than once if it appeared in composite
   111  	// literal initialization (e.g, see issue #49029).
   112  	//
   113  	// Don't add the closure function to compilation queue more than once, since when
   114  	// compiling a function twice would lead to an ICE.
   115  	if !clofn.Walked() {
   116  		clofn.SetWalked(true)
   117  		ir.CurFunc.Closures = append(ir.CurFunc.Closures, clofn)
   118  	}
   119  
   120  	typ := typecheck.ClosureType(clo)
   121  
   122  	clos := ir.NewCompLitExpr(base.Pos, ir.OCOMPLIT, typ, nil)
   123  	clos.SetEsc(clo.Esc())
   124  	clos.List = append([]ir.Node{ir.NewUnaryExpr(base.Pos, ir.OCFUNC, clofn.Nname)}, closureArgs(clo)...)
   125  	for i, value := range clos.List {
   126  		clos.List[i] = ir.NewStructKeyExpr(base.Pos, typ.Field(i), value)
   127  	}
   128  
   129  	addr := typecheck.NodAddr(clos)
   130  	addr.SetEsc(clo.Esc())
   131  
   132  	// Force type conversion from *struct to the func type.
   133  	cfn := typecheck.ConvNop(addr, clo.Type())
   134  
   135  	// non-escaping temp to use, if any.
   136  	if x := clo.Prealloc; x != nil {
   137  		if !types.Identical(typ, x.Type()) {
   138  			panic("closure type does not match order's assigned type")
   139  		}
   140  		addr.Prealloc = x
   141  		clo.Prealloc = nil
   142  	}
   143  
   144  	return walkExpr(cfn, init)
   145  }
   146  
   147  // closureArgs returns a slice of expressions that can be used to
   148  // initialize the given closure's free variables. These correspond
   149  // one-to-one with the variables in clo.Func.ClosureVars, and will be
   150  // either an ONAME node (if the variable is captured by value) or an
   151  // OADDR-of-ONAME node (if not).
   152  func closureArgs(clo *ir.ClosureExpr) []ir.Node {
   153  	fn := clo.Func
   154  
   155  	args := make([]ir.Node, len(fn.ClosureVars))
   156  	for i, v := range fn.ClosureVars {
   157  		var outer ir.Node
   158  		outer = v.Outer
   159  		if !v.Byval() {
   160  			outer = typecheck.NodAddrAt(fn.Pos(), outer)
   161  		}
   162  		args[i] = typecheck.Expr(outer)
   163  	}
   164  	return args
   165  }
   166  
   167  func walkMethodValue(n *ir.SelectorExpr, init *ir.Nodes) ir.Node {
   168  	// Create closure in the form of a composite literal.
   169  	// For x.M with receiver (x) type T, the generated code looks like:
   170  	//
   171  	//	clos = &struct{F uintptr; R T}{T.M·f, x}
   172  	//
   173  	// Like walkClosure above.
   174  
   175  	if n.X.Type().IsInterface() {
   176  		// Trigger panic for method on nil interface now.
   177  		// Otherwise it happens in the wrapper and is confusing.
   178  		n.X = cheapExpr(n.X, init)
   179  		n.X = walkExpr(n.X, nil)
   180  
   181  		tab := ir.NewUnaryExpr(base.Pos, ir.OITAB, n.X)
   182  		check := ir.NewUnaryExpr(base.Pos, ir.OCHECKNIL, tab)
   183  		init.Append(typecheck.Stmt(check))
   184  	}
   185  
   186  	typ := typecheck.MethodValueType(n)
   187  
   188  	clos := ir.NewCompLitExpr(base.Pos, ir.OCOMPLIT, typ, nil)
   189  	clos.SetEsc(n.Esc())
   190  	clos.List = []ir.Node{ir.NewUnaryExpr(base.Pos, ir.OCFUNC, methodValueWrapper(n)), n.X}
   191  
   192  	addr := typecheck.NodAddr(clos)
   193  	addr.SetEsc(n.Esc())
   194  
   195  	// Force type conversion from *struct to the func type.
   196  	cfn := typecheck.ConvNop(addr, n.Type())
   197  
   198  	// non-escaping temp to use, if any.
   199  	if x := n.Prealloc; x != nil {
   200  		if !types.Identical(typ, x.Type()) {
   201  			panic("partial call type does not match order's assigned type")
   202  		}
   203  		addr.Prealloc = x
   204  		n.Prealloc = nil
   205  	}
   206  
   207  	return walkExpr(cfn, init)
   208  }
   209  
   210  // methodValueWrapper returns the ONAME node representing the
   211  // wrapper function (*-fm) needed for the given method value. If the
   212  // wrapper function hasn't already been created yet, it's created and
   213  // added to typecheck.Target.Decls.
   214  func methodValueWrapper(dot *ir.SelectorExpr) *ir.Name {
   215  	if dot.Op() != ir.OMETHVALUE {
   216  		base.Fatalf("methodValueWrapper: unexpected %v (%v)", dot, dot.Op())
   217  	}
   218  
   219  	meth := dot.Sel
   220  	rcvrtype := dot.X.Type()
   221  	sym := ir.MethodSymSuffix(rcvrtype, meth, "-fm")
   222  
   223  	if sym.Uniq() {
   224  		return sym.Def.(*ir.Name)
   225  	}
   226  	sym.SetUniq(true)
   227  
   228  	base.FatalfAt(dot.Pos(), "missing wrapper for %v", meth)
   229  	panic("unreachable")
   230  }
   231
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