[dev.typeparams] cmd/compile: add types2.Sizes implementation

This CL adds an implementation of types2.Sizes that calculates sizes
using the same sizing algorithm as cmd/compile. In particular, it
matches how cmd/compile pads structures and includes padding in size
calculations.

Change-Id: I4dd8e51f95c90f9d7bd1e7463e40edcd3955a219
Reviewed-on: https://go-review.googlesource.com/c/go/+/282915
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/noder/noder.go

@@ -133,6 +133,7 @@ func ParseFiles(filenames []string) (lines uint) {
 					return os.Open(file)
 				},
 			},
+			Sizes: &gcSizes{},
 		}
 		info := types2.Info{
 			Types:      make(map[syntax.Expr]types2.TypeAndValue),

src/cmd/compile/internal/noder/sizes.go

@@ -0,0 +1,150 @@
+// Copyright 2021 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 noder
+
+import (
+	"fmt"
+
+	"cmd/compile/internal/types"
+	"cmd/compile/internal/types2"
+)
+
+// Code below based on go/types.StdSizes.
+// Intentional differences are marked with "gc:".
+
+type gcSizes struct{}
+
+func (s *gcSizes) Alignof(T types2.Type) int64 {
+	// For arrays and structs, alignment is defined in terms
+	// of alignment of the elements and fields, respectively.
+	switch t := T.Underlying().(type) {
+	case *types2.Array:
+		// spec: "For a variable x of array type: unsafe.Alignof(x)
+		// is the same as unsafe.Alignof(x[0]), but at least 1."
+		return s.Alignof(t.Elem())
+	case *types2.Struct:
+		// spec: "For a variable x of struct type: unsafe.Alignof(x)
+		// is the largest of the values unsafe.Alignof(x.f) for each
+		// field f of x, but at least 1."
+		max := int64(1)
+		for i, nf := 0, t.NumFields(); i < nf; i++ {
+			if a := s.Alignof(t.Field(i).Type()); a > max {
+				max = a
+			}
+		}
+		return max
+	case *types2.Slice, *types2.Interface:
+		// Multiword data structures are effectively structs
+		// in which each element has size PtrSize.
+		return int64(types.PtrSize)
+	case *types2.Basic:
+		// Strings are like slices and interfaces.
+		if t.Info()&types2.IsString != 0 {
+			return int64(types.PtrSize)
+		}
+	}
+	a := s.Sizeof(T) // may be 0
+	// spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1."
+	if a < 1 {
+		return 1
+	}
+	// complex{64,128} are aligned like [2]float{32,64}.
+	if isComplex(T) {
+		a /= 2
+	}
+	if a > int64(types.RegSize) {
+		return int64(types.RegSize)
+	}
+	return a
+}
+
+func isComplex(T types2.Type) bool {
+	basic, ok := T.Underlying().(*types2.Basic)
+	return ok && basic.Info()&types2.IsComplex != 0
+}
+
+func (s *gcSizes) Offsetsof(fields []*types2.Var) []int64 {
+	offsets := make([]int64, len(fields))
+	var o int64
+	for i, f := range fields {
+		typ := f.Type()
+		a := s.Alignof(typ)
+		o = types.Rnd(o, a)
+		offsets[i] = o
+		o += s.Sizeof(typ)
+	}
+	return offsets
+}
+
+func (s *gcSizes) Sizeof(T types2.Type) int64 {
+	switch t := T.Underlying().(type) {
+	case *types2.Basic:
+		k := t.Kind()
+		if int(k) < len(basicSizes) {
+			if s := basicSizes[k]; s > 0 {
+				return int64(s)
+			}
+		}
+		switch k {
+		case types2.String:
+			return int64(types.PtrSize) * 2
+		case types2.Int, types2.Uint, types2.Uintptr, types2.UnsafePointer:
+			return int64(types.PtrSize)
+		}
+		panic(fmt.Sprintf("unimplemented basic: %v (kind %v)", T, k))
+	case *types2.Array:
+		n := t.Len()
+		if n <= 0 {
+			return 0
+		}
+		// n > 0
+		// gc: Size includes alignment padding.
+		return s.Sizeof(t.Elem()) * n
+	case *types2.Slice:
+		return int64(types.PtrSize) * 3
+	case *types2.Struct:
+		n := t.NumFields()
+		if n == 0 {
+			return 0
+		}
+		fields := make([]*types2.Var, n)
+		for i := range fields {
+			fields[i] = t.Field(i)
+		}
+		offsets := s.Offsetsof(fields)
+
+		// gc: The last field of a struct is not allowed to
+		// have size 0.
+		last := s.Sizeof(fields[n-1].Type())
+		if last == 0 {
+			last = 1
+		}
+
+		// gc: Size includes alignment padding.
+		return types.Rnd(offsets[n-1]+last, s.Alignof(t))
+	case *types2.Interface:
+		return int64(types.PtrSize) * 2
+	case *types2.Chan, *types2.Map, *types2.Pointer, *types2.Signature:
+		return int64(types.PtrSize)
+	default:
+		panic(fmt.Sprintf("unimplemented type: %T", t))
+	}
+}
+
+var basicSizes = [...]byte{
+	types2.Bool:       1,
+	types2.Int8:       1,
+	types2.Int16:      2,
+	types2.Int32:      4,
+	types2.Int64:      8,
+	types2.Uint8:      1,
+	types2.Uint16:     2,
+	types2.Uint32:     4,
+	types2.Uint64:     8,
+	types2.Float32:    4,
+	types2.Float64:    8,
+	types2.Complex64:  8,
+	types2.Complex128: 16,
+}