tinygo/compiler/reflect.go

package compiler

// This file has some compiler support for run-time reflection using the reflect
// package. In particular, it encodes type information in type codes in such a
// way that the reflect package can decode the type from this information.
// Where needed, it also adds some side tables for looking up more information
// about a type, when that information cannot be stored directly in the type
// code.
//
// Go has 26 different type kinds.
//
// Type kinds are subdivided in basic types (see the list of basicTypes below)
// that are mostly numeric literals and non-basic (or "complex") types that are
// more difficult to encode. These non-basic types come in two forms:
//   * Prefix types (pointer, slice, interface, channel): these just add
//     something to an existing type. For example, a pointer like *int just adds
//     the fact that it's a pointer to an existing type (int).
//     These are encoded efficiently by adding a prefix to a type code.
//   * Types with multiple fields (struct, array, func, map). All of these have
//     multiple fields contained within. Most obviously structs can contain many
//     types as fields. Also arrays contain not just the element type but also
//     the length parameter which can be any arbitrary number and thus may not
//     fit in a type code.
//     These types are encoded using side tables.
//
// This distinction is also important for how named types are encoded. At the
// moment, named basic type just get a unique number assigned while named
// non-basic types have their underlying type stored in a sidetable.

import (
	"math/big"
	"strings"

	"tinygo.org/x/go-llvm"
)

// A list of basic types and their numbers. This list should be kept in sync
// with the list of Kind constants of type.go in the runtime package.
var basicTypes = map[string]int64{
	"bool":       1,
	"int":        2,
	"int8":       3,
	"int16":      4,
	"int32":      5,
	"int64":      6,
	"uint":       7,
	"uint8":      8,
	"uint16":     9,
	"uint32":     10,
	"uint64":     11,
	"uintptr":    12,
	"float32":    13,
	"float64":    14,
	"complex64":  15,
	"complex128": 16,
	"string":     17,
	"unsafeptr":  18,
}

// typeCodeAssignmentState keeps some global state around for type code
// assignments, used to assign one unique type code to each Go type.
type typeCodeAssignmentState struct {
	// An integer that's incremented each time it's used to give unique IDs to
	// type codes that are not yet fully supported otherwise by the reflect
	// package (or are simply unused in the compiled program).
	fallbackIndex int

	// Map of named types to their type code. It is important that named types
	// get unique IDs for each type.
	namedBasicTypes    map[string]int
	namedNonBasicTypes map[string]int

	// This byte array is stored in reflect.namedNonBasicTypesSidetable and is
	// used at runtime to get details about a named non-basic type.
	// Entries are varints (see makeVarint below and readVarint in
	// reflect/sidetables.go for the encoding): one varint per entry. The
	// integers in namedNonBasicTypes are indices into this array. Because these
	// are varints, most type codes are really small (just one byte).
	//
	// Note that this byte buffer is not created when it is not needed
	// (reflect.namedNonBasicTypesSidetable has no uses), see
	// needsNamedTypesSidetable.
	namedNonBasicTypesSidetable []byte

	// This is the length of an uintptr. Only used occasionally to know whether
	// a given number can be encoded as a varint.
	uintptrLen int

	// This indicates whether namedNonBasicTypesSidetable needs to be created at
	// all. If it is false, namedNonBasicTypesSidetable will contain simple
	// monotonically increasing numbers.
	needsNamedNonBasicTypesSidetable bool
}

// assignTypeCodes is used to assign a type code to each type in the program
// that is ever stored in an interface. It tries to use the smallest possible
// numbers to make the code that works with interfaces as small as possible.
func (c *Compiler) assignTypeCodes(typeSlice typeInfoSlice) {
	fn := c.mod.NamedFunction("reflect.ValueOf")
	if fn.IsNil() {
		// reflect.ValueOf is never used, so we can use the most efficient
		// encoding possible.
		for i, t := range typeSlice {
			t.num = uint64(i + 1)
		}
		return
	}

	// Assign typecodes the way the reflect package expects.
	state := typeCodeAssignmentState{
		fallbackIndex:                    1,
		namedBasicTypes:                  make(map[string]int),
		namedNonBasicTypes:               make(map[string]int),
		uintptrLen:                       c.uintptrType.IntTypeWidth(),
		needsNamedNonBasicTypesSidetable: len(getUses(c.mod.NamedGlobal("reflect.namedNonBasicTypesSidetable"))) != 0,
	}
	for _, t := range typeSlice {
		num := c.getTypeCodeNum(t.typecode, &state)
		if num.BitLen() > c.uintptrType.IntTypeWidth() || !num.IsUint64() {
			// TODO: support this in some way, using a side table for example.
			// That's less efficient but better than not working at all.
			// Particularly important on systems with 16-bit pointers (e.g.
			// AVR).
			panic("compiler: could not store type code number inside interface type code")
		}
		t.num = num.Uint64()
	}

	// Only create this sidetable when it is necessary.
	if state.needsNamedNonBasicTypesSidetable {
		// Create the sidetable and replace the old dummy global with this value.
		globalType := llvm.ArrayType(c.ctx.Int8Type(), len(state.namedNonBasicTypesSidetable))
		global := llvm.AddGlobal(c.mod, globalType, "reflect.namedNonBasicTypesSidetable.tmp")
		value := llvm.Undef(globalType)
		for i, ch := range state.namedNonBasicTypesSidetable {
			value = llvm.ConstInsertValue(value, llvm.ConstInt(c.ctx.Int8Type(), uint64(ch), false), []uint32{uint32(i)})
		}
		global.SetInitializer(value)
		oldGlobal := c.mod.NamedGlobal("reflect.namedNonBasicTypesSidetable")
		gep := llvm.ConstGEP(global, []llvm.Value{
			llvm.ConstInt(c.ctx.Int32Type(), 0, false),
			llvm.ConstInt(c.ctx.Int32Type(), 0, false),
		})
		oldGlobal.ReplaceAllUsesWith(gep)
		oldGlobal.EraseFromParentAsGlobal()
		global.SetName("reflect.namedNonBasicTypesSidetable")
	}
}

// getTypeCodeNum returns the typecode for a given type as expected by the
// reflect package. Also see getTypeCodeName, which serializes types to a string
// based on a types.Type value for this function.
func (c *Compiler) getTypeCodeNum(typecode llvm.Value, state *typeCodeAssignmentState) *big.Int {
	// Note: see src/reflect/type.go for bit allocations.
	class, value := getClassAndValueFromTypeCode(typecode)
	name := ""
	if class == "named" {
		name = value
		typecode = llvm.ConstExtractValue(typecode.Initializer(), []uint32{0})
		class, value = getClassAndValueFromTypeCode(typecode)
	}
	if class == "basic" {
		// Basic types follow the following bit pattern:
		//    ...xxxxx0
		// where xxxxx is allocated for the 18 possible basic types and all the
		// upper bits are used to indicate the named type.
		num, ok := basicTypes[value]
		if !ok {
			panic("invalid basic type: " + value)
		}
		if name != "" {
			// This type is named, set the upper bits to the name ID.
			num |= int64(state.getBasicNamedTypeNum(name)) << 5
		}
		return big.NewInt(num << 1)
	} else {
		// Non-baisc types use the following bit pattern:
		//    ...nxxx1
		// where xxx indicates the non-basic type. The upper bits contain
		// whatever the type contains. Types that wrap a single other type
		// (channel, interface, pointer, slice) just contain the bits of the
		// wrapped type. Other types (like struct) need more fields and thus
		// cannot be encoded as a simple prefix.
		var num *big.Int
		var classNumber int64
		switch class {
		case "chan":
			sub := llvm.ConstExtractValue(typecode.Initializer(), []uint32{0})
			num = c.getTypeCodeNum(sub, state)
			classNumber = 0
		case "interface":
			num = big.NewInt(int64(state.fallbackIndex))
			state.fallbackIndex++
			classNumber = 1
		case "pointer":
			sub := llvm.ConstExtractValue(typecode.Initializer(), []uint32{0})
			num = c.getTypeCodeNum(sub, state)
			classNumber = 2
		case "slice":
			sub := llvm.ConstExtractValue(typecode.Initializer(), []uint32{0})
			num = c.getTypeCodeNum(sub, state)
			classNumber = 3
		case "array":
			num = big.NewInt(int64(state.fallbackIndex))
			state.fallbackIndex++
			classNumber = 4
		case "func":
			num = big.NewInt(int64(state.fallbackIndex))
			state.fallbackIndex++
			classNumber = 5
		case "map":
			num = big.NewInt(int64(state.fallbackIndex))
			state.fallbackIndex++
			classNumber = 6
		case "struct":
			num = big.NewInt(int64(state.fallbackIndex))
			state.fallbackIndex++
			classNumber = 7
		default:
			panic("unknown type kind: " + class)
		}
		if name == "" {
			num.Lsh(num, 5).Or(num, big.NewInt((classNumber<<1)+1))
		} else {
			num = big.NewInt(int64(state.getNonBasicNamedTypeNum(name, num))<<1 | 1)
			num.Lsh(num, 4).Or(num, big.NewInt((classNumber<<1)+1))
		}
		return num
	}
}

// getClassAndValueFromTypeCode takes a typecode (a llvm.Value of type
// runtime.typecodeID), looks at the name, and extracts the typecode class and
// value from it. For example, for a typecode with the following name:
//     reflect/types.type:pointer:named:reflect.ValueError
// It extracts:
//     class = "pointer"
//     value = "named:reflect.ValueError"
func getClassAndValueFromTypeCode(typecode llvm.Value) (class, value string) {
	typecodeName := typecode.Name()
	const prefix = "reflect/types.type:"
	if !strings.HasPrefix(typecodeName, prefix) {
		panic("unexpected typecode name: " + typecodeName)
	}
	id := typecodeName[len(prefix):]
	class = id[:strings.IndexByte(id, ':')]
	value = id[len(class)+1:]
	return
}

// getBasicNamedTypeNum returns an appropriate (unique) number for the given
// named type. If the name already has a number that number is returned, else a
// new number is returned. The number is always non-zero.
func (state *typeCodeAssignmentState) getBasicNamedTypeNum(name string) int {
	if num, ok := state.namedBasicTypes[name]; ok {
		return num
	}
	num := len(state.namedBasicTypes) + 1
	state.namedBasicTypes[name] = num
	return num
}

// getNonBasicNamedTypeNum returns a number unique for this named type. It tries
// to return the smallest number possible to make encoding of this type code
// easier.
func (state *typeCodeAssignmentState) getNonBasicNamedTypeNum(name string, value *big.Int) int {
	if num, ok := state.namedNonBasicTypes[name]; ok {
		return num
	}
	if !state.needsNamedNonBasicTypesSidetable {
		// Use simple small integers in this case, to make these numbers
		// smaller.
		num := len(state.namedNonBasicTypes) + 1
		state.namedNonBasicTypes[name] = num
		return num
	}
	num := len(state.namedNonBasicTypesSidetable)
	if value.BitLen() > state.uintptrLen || !value.IsUint64() {
		panic("cannot store value in sidetable")
	}
	state.namedNonBasicTypesSidetable = append(state.namedNonBasicTypesSidetable, makeVarint(value.Uint64())...)
	state.namedNonBasicTypes[name] = num
	return num
}

// makeVarint encodes a varint in a way that should be easy to decode.
// It may need to be decoded very quickly at runtime at low-powered processors
// so should be efficient to decode.
// The current algorithm is probably not even close to efficient, but it is easy
// to change as the format is only used inside the same program.
func makeVarint(n uint64) []byte {
	// This is the reverse of what src/runtime/sidetables.go does.
	buf := make([]byte, 0, 8)
	for {
		c := byte(n & 0x7f << 1)
		n >>= 7
		if n != 0 {
			c |= 1
		}
		buf = append(buf, c)
		if n == 0 {
			break
		}
	}
	reverseBytes(buf)
	return buf
}

func reverseBytes(s []byte) {
	// Actually copied from https://blog.golang.org/why-generics
	first := 0
	last := len(s) - 1
	for first < last {
		s[first], s[last] = s[last], s[first]
		first++
		last--
	}
}