makefunc.go

     1  // Copyright 2012 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  // MakeFunc implementation.
     6  
     7  package reflect
     8  
     9  import (
    10  	"internal/abi"
    11  	"unsafe"
    12  )
    13  
    14  // makeFuncImpl is the closure value implementing the function
    15  // returned by MakeFunc.
    16  // The first three words of this type must be kept in sync with
    17  // methodValue and runtime.reflectMethodValue.
    18  // Any changes should be reflected in all three.
    19  type makeFuncImpl struct {
    20  	makeFuncCtxt
    21  	ftyp *funcType
    22  	fn   func([]Value) []Value
    23  }
    24  
    25  // MakeFunc returns a new function of the given [Type]
    26  // that wraps the function fn. When called, that new function
    27  // does the following:
    28  //
    29  //   - converts its arguments to a slice of Values.
    30  //   - runs results := fn(args).
    31  //   - returns the results as a slice of Values, one per formal result.
    32  //
    33  // The implementation fn can assume that the argument [Value] slice
    34  // has the number and type of arguments given by typ.
    35  // If typ describes a variadic function, the final Value is itself
    36  // a slice representing the variadic arguments, as in the
    37  // body of a variadic function. The result Value slice returned by fn
    38  // must have the number and type of results given by typ.
    39  //
    40  // The [Value.Call] method allows the caller to invoke a typed function
    41  // in terms of Values; in contrast, MakeFunc allows the caller to implement
    42  // a typed function in terms of Values.
    43  //
    44  // The Examples section of the documentation includes an illustration
    45  // of how to use MakeFunc to build a swap function for different types.
    46  func MakeFunc(typ Type, fn func(args []Value) (results []Value)) Value {
    47  	if typ.Kind() != Func {
    48  		panic("reflect: call of MakeFunc with non-Func type")
    49  	}
    50  
    51  	t := typ.common()
    52  	ftyp := (*funcType)(unsafe.Pointer(t))
    53  
    54  	code := abi.FuncPCABI0(makeFuncStub)
    55  
    56  	// makeFuncImpl contains a stack map for use by the runtime
    57  	_, _, abid := funcLayout(ftyp, nil)
    58  
    59  	impl := &makeFuncImpl{
    60  		makeFuncCtxt: makeFuncCtxt{
    61  			fn:      code,
    62  			stack:   abid.stackPtrs,
    63  			argLen:  abid.stackCallArgsSize,
    64  			regPtrs: abid.inRegPtrs,
    65  		},
    66  		ftyp: ftyp,
    67  		fn:   fn,
    68  	}
    69  
    70  	return Value{t, unsafe.Pointer(impl), flag(Func)}
    71  }
    72  
    73  // makeFuncStub is an assembly function that is the code half of
    74  // the function returned from MakeFunc. It expects a *callReflectFunc
    75  // as its context register, and its job is to invoke callReflect(ctxt, frame)
    76  // where ctxt is the context register and frame is a pointer to the first
    77  // word in the passed-in argument frame.
    78  func makeFuncStub()
    79  
    80  // The first 3 words of this type must be kept in sync with
    81  // makeFuncImpl and runtime.reflectMethodValue.
    82  // Any changes should be reflected in all three.
    83  type methodValue struct {
    84  	makeFuncCtxt
    85  	method int
    86  	rcvr   Value
    87  }
    88  
    89  // makeMethodValue converts v from the rcvr+method index representation
    90  // of a method value to an actual method func value, which is
    91  // basically the receiver value with a special bit set, into a true
    92  // func value - a value holding an actual func. The output is
    93  // semantically equivalent to the input as far as the user of package
    94  // reflect can tell, but the true func representation can be handled
    95  // by code like Convert and Interface and Assign.
    96  func makeMethodValue(op string, v Value) Value {
    97  	if v.flag&flagMethod == 0 {
    98  		panic("reflect: internal error: invalid use of makeMethodValue")
    99  	}
   100  
   101  	// Ignoring the flagMethod bit, v describes the receiver, not the method type.
   102  	fl := v.flag & (flagRO | flagAddr | flagIndir)
   103  	fl |= flag(v.typ().Kind())
   104  	rcvr := Value{v.typ(), v.ptr, fl}
   105  
   106  	// v.Type returns the actual type of the method value.
   107  	ftyp := (*funcType)(unsafe.Pointer(v.Type().(*rtype)))
   108  
   109  	code := methodValueCallCodePtr()
   110  
   111  	// methodValue contains a stack map for use by the runtime
   112  	_, _, abid := funcLayout(ftyp, nil)
   113  	fv := &methodValue{
   114  		makeFuncCtxt: makeFuncCtxt{
   115  			fn:      code,
   116  			stack:   abid.stackPtrs,
   117  			argLen:  abid.stackCallArgsSize,
   118  			regPtrs: abid.inRegPtrs,
   119  		},
   120  		method: int(v.flag) >> flagMethodShift,
   121  		rcvr:   rcvr,
   122  	}
   123  
   124  	// Cause panic if method is not appropriate.
   125  	// The panic would still happen during the call if we omit this,
   126  	// but we want Interface() and other operations to fail early.
   127  	methodReceiver(op, fv.rcvr, fv.method)
   128  
   129  	return Value{ftyp.Common(), unsafe.Pointer(fv), v.flag&flagRO | flag(Func)}
   130  }
   131  
   132  func methodValueCallCodePtr() uintptr {
   133  	return abi.FuncPCABI0(methodValueCall)
   134  }
   135  
   136  // methodValueCall is an assembly function that is the code half of
   137  // the function returned from makeMethodValue. It expects a *methodValue
   138  // as its context register, and its job is to invoke callMethod(ctxt, frame)
   139  // where ctxt is the context register and frame is a pointer to the first
   140  // word in the passed-in argument frame.
   141  func methodValueCall()
   142  
   143  // This structure must be kept in sync with runtime.reflectMethodValue.
   144  // Any changes should be reflected in all both.
   145  type makeFuncCtxt struct {
   146  	fn      uintptr
   147  	stack   *bitVector // ptrmap for both stack args and results
   148  	argLen  uintptr    // just args
   149  	regPtrs abi.IntArgRegBitmap
   150  }
   151  
   152  // moveMakeFuncArgPtrs uses ctxt.regPtrs to copy integer pointer arguments
   153  // in args.Ints to args.Ptrs where the GC can see them.
   154  //
   155  // This is similar to what reflectcallmove does in the runtime, except
   156  // that happens on the return path, whereas this happens on the call path.
   157  //
   158  // nosplit because pointers are being held in uintptr slots in args, so
   159  // having our stack scanned now could lead to accidentally freeing
   160  // memory.
   161  //
   162  //go:nosplit
   163  func moveMakeFuncArgPtrs(ctxt *makeFuncCtxt, args *abi.RegArgs) {
   164  	for i, arg := range args.Ints {
   165  		// Avoid write barriers! Because our write barrier enqueues what
   166  		// was there before, we might enqueue garbage.
   167  		if ctxt.regPtrs.Get(i) {
   168  			*(*uintptr)(unsafe.Pointer(&args.Ptrs[i])) = arg
   169  		} else {
   170  			// We *must* zero this space ourselves because it's defined in
   171  			// assembly code and the GC will scan these pointers. Otherwise,
   172  			// there will be garbage here.
   173  			*(*uintptr)(unsafe.Pointer(&args.Ptrs[i])) = 0
   174  		}
   175  	}
   176  }
   177
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