src/reflect/makefunc.go

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// MakeFunc implementation.

package reflect

import (
	"internal/abi"
	"internal/goarch"
	"unsafe"
)

// makeFuncImpl is the closure value implementing the function
// returned by MakeFunc.
// The first three words of this type must be kept in sync with
// methodValue and runtime.reflectMethodValue.
// Any changes should be reflected in all three.
type makeFuncImpl struct {
	makeFuncCtxt
	ftyp *funcType
	fn   func([]Value) []Value
}

// MakeFunc returns a new function of the given [Type]
// that wraps the function fn. When called, that new function
// does the following:
//
//   - converts its arguments to a slice of Values.
//   - runs results := fn(args).
//   - returns the results as a slice of Values, one per formal result.
//
// The implementation fn can assume that the argument [Value] slice
// has the number and type of arguments given by typ.
// If typ describes a variadic function, the final Value is itself
// a slice representing the variadic arguments, as in the
// body of a variadic function. The result Value slice returned by fn
// must have the number and type of results given by typ.
//
// The [Value.Call] method allows the caller to invoke a typed function
// in terms of Values; in contrast, MakeFunc allows the caller to implement
// a typed function in terms of Values.
//
// The Examples section of the documentation includes an illustration
// of how to use MakeFunc to build a swap function for different types.
func MakeFunc(typ Type, fn func(args []Value) (results []Value)) Value {
	t := typ.common()
	typ = toType(t) // for #80332, ensure t's exported methods are not shadowed
	if typ.Kind() != Func {
		panic("reflect: call of MakeFunc with non-Func type")
	}

	ftyp := (*funcType)(unsafe.Pointer(t))

	code := abi.FuncPCABI0(makeFuncStub)

	// makeFuncImpl contains a stack map for use by the runtime
	_, _, abid := funcLayout(ftyp, nil)

	impl := &makeFuncImpl{
		makeFuncCtxt: makeFuncCtxt{
			fn:      code,
			stack:   abid.stackPtrs,
			argLen:  abid.stackCallArgsSize,
			regPtrs: abid.inRegPtrs,
		},
		ftyp: ftyp,
		fn:   fn,
	}

	return Value{t, unsafe.Pointer(impl), flag(Func)}
}

// makeFuncStub is an assembly function that is the code half of
// the function returned from MakeFunc. It expects a *callReflectFunc
// as its context register, and its job is to invoke callReflect(ctxt, frame)
// where ctxt is the context register and frame is a pointer to the first
// word in the passed-in argument frame.
func makeFuncStub()

// The first 3 words of this type must be kept in sync with
// makeFuncImpl and runtime.reflectMethodValue.
// Any changes should be reflected in all three.
type methodValue struct {
	makeFuncCtxt
	method int
	rcvr   Value
}

// makeMethodValue implements Value.Method(i). It creates a method func
// value closure containing the receiver v and method index i, with the
// flagMethod bit set so that direct calls via reflect can use a fast path.
func makeMethodValue(v Value, i int) Value {
	var mtyp *abi.Type
	typ := v.typ()
	if typ.Kind() == abi.Interface {
		tt := (*interfaceType)(unsafe.Pointer(typ))
		if uint(i) >= uint(len(tt.Methods)) {
			panic("reflect: internal error: invalid method index")
		}
		m := &tt.Methods[i]
		mtyp = typeOffFor(typ, m.Typ)
	} else {
		ms := typ.ExportedMethods()
		if uint(i) >= uint(len(ms)) {
			panic("reflect: internal error: invalid method index")
		}
		m := ms[i]
		mtyp = typeOffFor(typ, m.Mtyp)
	}
	ftyp := mtyp.FuncType()

	code := methodValueCallCodePtr()

	// methodValue contains a stack map for use by the runtime
	_, _, abid := funcLayout(ftyp, nil)
	fv := &methodValue{
		makeFuncCtxt: makeFuncCtxt{
			fn:      code,
			stack:   abid.stackPtrs,
			argLen:  abid.stackCallArgsSize,
			regPtrs: abid.inRegPtrs,
		},
		method: i,
		rcvr:   v,
	}

	return Value{ftyp.Common(), unsafe.Pointer(fv), v.flag.ro() | flagMethod | flag(Func)}
}

func methodValueCallCodePtr() uintptr {
	return abi.FuncPCABI0(methodValueCall)
}

// methodValueCall is an assembly function that is the code half of
// the function returned from makeMethodValue. It expects a *methodValue
// as its context register, and its job is to invoke callMethod(ctxt, frame)
// where ctxt is the context register and frame is a pointer to the first
// word in the passed-in argument frame.
func methodValueCall()

// This structure must be kept in sync with runtime.reflectMethodValue.
// Any changes should be reflected in all both.
type makeFuncCtxt struct {
	fn      uintptr
	stack   *bitVector // ptrmap for both stack args and results
	argLen  uintptr    // just args
	regPtrs abi.IntArgRegBitmap
}

// moveMakeFuncArgPtrs uses ctxt.regPtrs to copy integer pointer arguments
// in args.Ints to args.Ptrs where the GC can see them.
//
// This is similar to what reflectcallmove does in the runtime, except
// that happens on the return path, whereas this happens on the call path.
//
// nosplit because pointers are being held in uintptr slots in args, so
// having our stack scanned now could lead to accidentally freeing
// memory.
//
//go:nosplit
func moveMakeFuncArgPtrs(ctxt *makeFuncCtxt, args *abi.RegArgs) {
	for i, arg := range args.Ints {
		// Avoid write barriers! Because our write barrier enqueues what
		// was there before, we might enqueue garbage.
		// Also avoid bounds checks, we don't have the stack space for it.
		// (Normally the prove pass removes them, but for -N builds we
		// use too much stack.)
		// ptr := &args.Ptrs[i] (but cast from *unsafe.Pointer to *uintptr)
		ptr := (*uintptr)(add(unsafe.Pointer(unsafe.SliceData(args.Ptrs[:])), uintptr(i)*goarch.PtrSize, "always in [0:IntArgRegs]"))
		if ctxt.regPtrs.Get(i) {
			*ptr = arg
		} else {
			// We *must* zero this space ourselves because it's defined in
			// assembly code and the GC will scan these pointers. Otherwise,
			// there will be garbage here.
			*ptr = 0
		}
	}
}