src/cmd/compile/internal/types/size.go

// Copyright 2009 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.

package types

import (
	"math"
	"sort"

	"cmd/compile/internal/base"
	"cmd/internal/src"
	"internal/types/errors"
)

var PtrSize int

var RegSize int

// Slices in the runtime are represented by three components:
//
//	type slice struct {
//		ptr unsafe.Pointer
//		len int
//		cap int
//	}
//
// Strings in the runtime are represented by two components:
//
//	type string struct {
//		ptr unsafe.Pointer
//		len int
//	}
//
// These variables are the offsets of fields and sizes of these structs.
var (
	SlicePtrOffset int64
	SliceLenOffset int64
	SliceCapOffset int64

	SliceSize  int64
	StringSize int64
)

var SkipSizeForTracing bool

// typePos returns the position associated with t.
// This is where t was declared or where it appeared as a type expression.
func typePos(t *Type) src.XPos {
	if pos := t.Pos(); pos.IsKnown() {
		return pos
	}
	base.Fatalf("bad type: %v", t)
	panic("unreachable")
}

// MaxWidth is the maximum size of a value on the target architecture.
var MaxWidth int64

// CalcSizeDisabled indicates whether it is safe
// to calculate Types' widths and alignments. See CalcSize.
var CalcSizeDisabled bool

// machine size and rounding alignment is dictated around
// the size of a pointer, set in gc.Main (see ../gc/main.go).
var defercalc int

// RoundUp rounds o to a multiple of r, r is a power of 2.
func RoundUp(o int64, r int64) int64 {
	if r < 1 || r > 8 || r&(r-1) != 0 {
		base.Fatalf("Round %d", r)
	}
	return (o + r - 1) &^ (r - 1)
}

// expandiface computes the method set for interface type t by
// expanding embedded interfaces.
func expandiface(t *Type) {
	seen := make(map[*Sym]*Field)
	var methods []*Field

	addMethod := func(m *Field, explicit bool) {
		switch prev := seen[m.Sym]; {
		case prev == nil:
			seen[m.Sym] = m
		case !explicit && Identical(m.Type, prev.Type):
			return
		default:
			base.ErrorfAt(m.Pos, errors.DuplicateDecl, "duplicate method %s", m.Sym.Name)
		}
		methods = append(methods, m)
	}

	{
		methods := t.Methods()
		sort.SliceStable(methods, func(i, j int) bool {
			mi, mj := methods[i], methods[j]

			// Sort embedded types by type name (if any).
			if mi.Sym == nil && mj.Sym == nil {
				return mi.Type.Sym().Less(mj.Type.Sym())
			}

			// Sort methods before embedded types.
			if mi.Sym == nil || mj.Sym == nil {
				return mi.Sym != nil
			}

			// Sort methods by symbol name.
			return mi.Sym.Less(mj.Sym)
		})
	}

	for _, m := range t.Methods() {
		if m.Sym == nil {
			continue
		}

		CheckSize(m.Type)
		addMethod(m, true)
	}

	for _, m := range t.Methods() {
		if m.Sym != nil || m.Type == nil {
			continue
		}

		// In 1.18, embedded types can be anything. In Go 1.17, we disallow
		// embedding anything other than interfaces. This requirement was caught
		// by types2 already, so allow non-interface here.
		if !m.Type.IsInterface() {
			continue
		}

		// Embedded interface: duplicate all methods
		// and add to t's method set.
		for _, t1 := range m.Type.AllMethods() {
			f := NewField(m.Pos, t1.Sym, t1.Type)
			addMethod(f, false)

			// Clear position after typechecking, for consistency with types2.
			f.Pos = src.NoXPos
		}

		// Clear position after typechecking, for consistency with types2.
		m.Pos = src.NoXPos
	}

	sort.Sort(MethodsByName(methods))

	if int64(len(methods)) >= MaxWidth/int64(PtrSize) {
		base.ErrorfAt(typePos(t), 0, "interface too large")
	}
	for i, m := range methods {
		m.Offset = int64(i) * int64(PtrSize)
	}

	t.SetAllMethods(methods)
}

// calcStructOffset computes the offsets of a sequence of fields,
// starting at the given offset. It returns the resulting offset and
// maximum field alignment.
func calcStructOffset(t *Type, fields []*Field, offset int64) int64 {
	for _, f := range fields {
		CalcSize(f.Type)
		offset = RoundUp(offset, int64(f.Type.align))

		if t.IsStruct() { // param offsets depend on ABI
			f.Offset = offset

			// If type T contains a field F marked as not-in-heap,
			// then T must also be a not-in-heap type. Otherwise,
			// you could heap allocate T and then get a pointer F,
			// which would be a heap pointer to a not-in-heap type.
			if f.Type.NotInHeap() {
				t.SetNotInHeap(true)
			}
		}

		offset += f.Type.width

		maxwidth := MaxWidth
		// On 32-bit systems, reflect tables impose an additional constraint
		// that each field start offset must fit in 31 bits.
		if maxwidth < 1<<32 {
			maxwidth = 1<<31 - 1
		}
		if offset >= maxwidth {
			base.ErrorfAt(typePos(t), 0, "type %L too large", t)
			offset = 8 // small but nonzero
		}
	}

	return offset
}

func isAtomicStdPkg(p *Pkg) bool {
	if p.Prefix == `""` {
		panic("bad package prefix")
	}
	return p.Prefix == "sync/atomic" || p.Prefix == "internal/runtime/atomic"
}

// CalcSize calculates and stores the size, alignment, eq/hash algorithm,
// and ptrBytes for t.
// If CalcSizeDisabled is set, and the size/alignment
// have not already been calculated, it calls Fatal.
// This is used to prevent data races in the back end.
func CalcSize(t *Type) {
	// Calling CalcSize when typecheck tracing enabled is not safe.
	// See issue #33658.
	if base.EnableTrace && SkipSizeForTracing {
		return
	}
	if PtrSize == 0 {
		// Assume this is a test.
		return
	}

	if t == nil {
		return
	}

	if t.width == -2 {
		t.width = 0
		t.align = 1
		base.Fatalf("invalid recursive type %v", t)
		return
	}

	if t.widthCalculated() {
		return
	}

	if CalcSizeDisabled {
		base.Fatalf("width not calculated: %v", t)
	}

	// defer CheckSize calls until after we're done
	DeferCheckSize()

	lno := base.Pos
	if pos := t.Pos(); pos.IsKnown() {
		base.Pos = pos
	}

	t.width = -2
	t.align = 0  // 0 means use t.Width, below
	t.alg = AMEM // default
	// default t.ptrBytes is 0.
	if t.Noalg() {
		t.setAlg(ANOALG)
	}

	et := t.Kind()
	switch et {
	case TFUNC, TCHAN, TMAP, TSTRING:
		break

	// SimType == 0 during bootstrap
	default:
		if SimType[t.Kind()] != 0 {
			et = SimType[t.Kind()]
		}
	}

	var w int64
	switch et {
	default:
		base.Fatalf("CalcSize: unknown type: %v", t)

	// compiler-specific stuff
	case TINT8, TUINT8, TBOOL:
		// bool is int8
		w = 1
		t.intRegs = 1

	case TINT16, TUINT16:
		w = 2
		t.intRegs = 1

	case TINT32, TUINT32:
		w = 4
		t.intRegs = 1

	case TINT64, TUINT64:
		w = 8
		t.align = uint8(RegSize)
		t.intRegs = uint8(8 / RegSize)

	case TFLOAT32:
		w = 4
		t.floatRegs = 1
		t.setAlg(AFLOAT32)

	case TFLOAT64:
		w = 8
		t.align = uint8(RegSize)
		t.floatRegs = 1
		t.setAlg(AFLOAT64)

	case TCOMPLEX64:
		w = 8
		t.align = 4
		t.floatRegs = 2
		t.setAlg(ACPLX64)

	case TCOMPLEX128:
		w = 16
		t.align = uint8(RegSize)
		t.floatRegs = 2
		t.setAlg(ACPLX128)

	case TPTR:
		w = int64(PtrSize)
		t.intRegs = 1
		CheckSize(t.Elem())
		t.ptrBytes = int64(PtrSize) // See PtrDataSize

	case TUNSAFEPTR:
		w = int64(PtrSize)
		t.intRegs = 1
		t.ptrBytes = int64(PtrSize)

	case TINTER: // implemented as 2 pointers
		w = 2 * int64(PtrSize)
		t.align = uint8(PtrSize)
		t.intRegs = 2
		expandiface(t)
		if len(t.allMethods.Slice()) == 0 {
			t.setAlg(ANILINTER)
		} else {
			t.setAlg(AINTER)
		}
		t.ptrBytes = int64(2 * PtrSize)

	case TCHAN: // implemented as pointer
		w = int64(PtrSize)
		t.intRegs = 1
		t.ptrBytes = int64(PtrSize)

		CheckSize(t.Elem())

		// Make fake type to trigger channel element size check after
		// any top-level recursive type has been completed.
		t1 := NewChanArgs(t)
		CheckSize(t1)

	case TCHANARGS:
		t1 := t.ChanArgs()
		CalcSize(t1) // just in case
		// Make sure size of t1.Elem() is calculated at this point. We can
		// use CalcSize() here rather than CheckSize(), because the top-level
		// (possibly recursive) type will have been calculated before the fake
		// chanargs is handled.
		CalcSize(t1.Elem())
		if t1.Elem().width >= 1<<16 {
			base.Errorf("channel element type too large (>64kB)")
		}
		w = 1 // anything will do

	case TMAP: // implemented as pointer
		w = int64(PtrSize)
		t.intRegs = 1
		CheckSize(t.Elem())
		CheckSize(t.Key())
		t.setAlg(ANOEQ)
		t.ptrBytes = int64(PtrSize)

	case TFORW: // should have been filled in
		base.Fatalf("invalid recursive type %v", t)

	case TANY: // not a real type; should be replaced before use.
		base.Fatalf("CalcSize any")

	case TSTRING:
		if StringSize == 0 {
			base.Fatalf("early CalcSize string")
		}
		w = StringSize
		t.align = uint8(PtrSize)
		t.intRegs = 2
		t.setAlg(ASTRING)
		t.ptrBytes = int64(PtrSize)

	case TARRAY:
		if t.Elem() == nil {
			break
		}

		CalcSize(t.Elem())
		t.SetNotInHeap(t.Elem().NotInHeap())
		if t.Elem().width != 0 {
			cap := (uint64(MaxWidth) - 1) / uint64(t.Elem().width)
			if uint64(t.NumElem()) > cap {
				base.Errorf("type %L larger than address space", t)
			}
		}
		w = t.NumElem() * t.Elem().width
		t.align = t.Elem().align

		// ABIInternal only allows "trivial" arrays (i.e., length 0 or 1)
		// to be passed by register.
		switch t.NumElem() {
		case 0:
			t.intRegs = 0
			t.floatRegs = 0
		case 1:
			t.intRegs = t.Elem().intRegs
			t.floatRegs = t.Elem().floatRegs
		default:
			t.intRegs = math.MaxUint8
			t.floatRegs = math.MaxUint8
		}
		switch a := t.Elem().alg; a {
		case AMEM, ANOEQ, ANOALG:
			t.setAlg(a)
		default:
			switch t.NumElem() {
			case 0:
				// We checked above that the element type is comparable.
				t.setAlg(AMEM)
			case 1:
				// Single-element array is same as its lone element.
				t.setAlg(a)
			default:
				t.setAlg(ASPECIAL)
			}
		}
		if t.NumElem() > 0 {
			x := PtrDataSize(t.Elem())
			if x > 0 {
				t.ptrBytes = t.Elem().width*(t.NumElem()-1) + x
			}
		}

	case TSLICE:
		if t.Elem() == nil {
			break
		}
		w = SliceSize
		CheckSize(t.Elem())
		t.align = uint8(PtrSize)
		t.intRegs = 3
		t.setAlg(ANOEQ)
		if !t.Elem().NotInHeap() {
			t.ptrBytes = int64(PtrSize)
		}

	case TSTRUCT:
		if t.IsFuncArgStruct() {
			base.Fatalf("CalcSize fn struct %v", t)
		}
		CalcStructSize(t)
		w = t.width

	// make fake type to check later to
	// trigger function argument computation.
	case TFUNC:
		t1 := NewFuncArgs(t)
		CheckSize(t1)
		w = int64(PtrSize) // width of func type is pointer
		t.intRegs = 1
		t.setAlg(ANOEQ)
		t.ptrBytes = int64(PtrSize)

	// function is 3 cated structures;
	// compute their widths as side-effect.
	case TFUNCARGS:
		t1 := t.FuncArgs()
		// TODO(mdempsky): Should package abi be responsible for computing argwid?
		w = calcStructOffset(t1, t1.Recvs(), 0)
		w = calcStructOffset(t1, t1.Params(), w)
		w = RoundUp(w, int64(RegSize))
		w = calcStructOffset(t1, t1.Results(), w)
		w = RoundUp(w, int64(RegSize))
		t1.extra.(*Func).Argwid = w
		t.align = 1
	}

	if PtrSize == 4 && w != int64(int32(w)) {
		base.Errorf("type %v too large", t)
	}

	t.width = w
	if t.align == 0 {
		if w == 0 || w > 8 || w&(w-1) != 0 {
			base.Fatalf("invalid alignment for %v", t)
		}
		t.align = uint8(w)
	}

	base.Pos = lno

	ResumeCheckSize()
}

// CalcStructSize calculates the size of t,
// filling in t.width, t.align, t.intRegs, and t.floatRegs,
// even if size calculation is otherwise disabled.
func CalcStructSize(t *Type) {
	var maxAlign uint8 = 1

	// Recognize special types. This logic is duplicated in go/types and
	// cmd/compile/internal/types2.
	if sym := t.Sym(); sym != nil {
		switch {
		case sym.Name == "align64" && isAtomicStdPkg(sym.Pkg):
			maxAlign = 8
		}
	}

	fields := t.Fields()
	size := calcStructOffset(t, fields, 0)

	// For non-zero-sized structs which end in a zero-sized field, we
	// add an extra byte of padding to the type. This padding ensures
	// that taking the address of a zero-sized field can't manufacture a
	// pointer to the next object in the heap. See issue 9401.
	if size > 0 && fields[len(fields)-1].Type.width == 0 {
		size++
	}

	var intRegs, floatRegs uint64
	for _, field := range fields {
		typ := field.Type

		// The alignment of a struct type is the maximum alignment of its
		// field types.
		if align := typ.align; align > maxAlign {
			maxAlign = align
		}

		// Each field needs its own registers.
		// We sum in uint64 to avoid possible overflows.
		intRegs += uint64(typ.intRegs)
		floatRegs += uint64(typ.floatRegs)
	}

	// Final size includes trailing padding.
	size = RoundUp(size, int64(maxAlign))

	if intRegs > math.MaxUint8 || floatRegs > math.MaxUint8 {
		intRegs = math.MaxUint8
		floatRegs = math.MaxUint8
	}

	t.width = size
	t.align = maxAlign
	t.intRegs = uint8(intRegs)
	t.floatRegs = uint8(floatRegs)

	// Compute eq/hash algorithm type.
	t.alg = AMEM // default
	if t.Noalg() {
		t.setAlg(ANOALG)
	}
	if len(fields) == 1 && !fields[0].Sym.IsBlank() {
		// One-field struct is same as that one field alone.
		t.setAlg(fields[0].Type.alg)
	} else {
		for i, f := range fields {
			a := f.Type.alg
			switch a {
			case ANOEQ, ANOALG:
			case AMEM:
				// Blank fields and padded fields need a special compare.
				if f.Sym.IsBlank() || IsPaddedField(t, i) {
					a = ASPECIAL
				}
			default:
				// Fields with non-memory equality need a special compare.
				a = ASPECIAL
			}
			t.setAlg(a)
		}
	}
	// Compute ptrBytes.
	for i := len(fields) - 1; i >= 0; i-- {
		f := fields[i]
		if size := PtrDataSize(f.Type); size > 0 {
			t.ptrBytes = f.Offset + size
			break
		}
	}
}

func (t *Type) widthCalculated() bool {
	return t.align > 0
}

// when a type's width should be known, we call CheckSize
// to compute it.  during a declaration like
//
//	type T *struct { next T }
//
// it is necessary to defer the calculation of the struct width
// until after T has been initialized to be a pointer to that struct.
// similarly, during import processing structs may be used
// before their definition.  in those situations, calling
// DeferCheckSize() stops width calculations until
// ResumeCheckSize() is called, at which point all the
// CalcSizes that were deferred are executed.
// CalcSize should only be called when the type's size
// is needed immediately.  CheckSize makes sure the
// size is evaluated eventually.

var deferredTypeStack []*Type

func CheckSize(t *Type) {
	if t == nil {
		return
	}

	// function arg structs should not be checked
	// outside of the enclosing function.
	if t.IsFuncArgStruct() {
		base.Fatalf("CheckSize %v", t)
	}

	if defercalc == 0 {
		CalcSize(t)
		return
	}

	// if type has not yet been pushed on deferredTypeStack yet, do it now
	if !t.Deferwidth() {
		t.SetDeferwidth(true)
		deferredTypeStack = append(deferredTypeStack, t)
	}
}

func DeferCheckSize() {
	defercalc++
}

func ResumeCheckSize() {
	if defercalc == 1 {
		for len(deferredTypeStack) > 0 {
			t := deferredTypeStack[len(deferredTypeStack)-1]
			deferredTypeStack = deferredTypeStack[:len(deferredTypeStack)-1]
			t.SetDeferwidth(false)
			CalcSize(t)
		}
	}

	defercalc--
}

// PtrDataSize returns the length in bytes of the prefix of t
// containing pointer data. Anything after this offset is scalar data.
//
// PtrDataSize is only defined for actual Go types. It's an error to
// use it on compiler-internal types (e.g., TSSA, TRESULTS).
func PtrDataSize(t *Type) int64 {
	CalcSize(t)
	x := t.ptrBytes
	if t.Kind() == TPTR && t.Elem().NotInHeap() {
		// Note: this is done here instead of when we're setting
		// the ptrBytes field, because at that time (in NewPtr, usually)
		// the NotInHeap bit of the element type might not be set yet.
		x = 0
	}
	return x
}