cmd/compile: add structs.HostLayout

This is for the proposal, plus a few bug fixes
that would/will be necessary when this is put into
actual use.

Fixes #66408.
Updates #63131.

Change-Id: I3a66e09d707dd579c59f155e7f53367f41214c30
Reviewed-on: https://go-review.googlesource.com/c/go/+/578355
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: David Chase <drchase@google.com>

Author: dr2chase

api/next/66408.txt

@@ -0,0 +1 @@
+pkg structs, type HostLayout struct #66408

doc/next/6-stdlib/3-structs.md

@@ -0,0 +1,12 @@
+### New structs package
+
+
+The new [structs](/pkg/structs) package provides
+types for struct fields that modify properties of
+the containing struct type such as memory layout.
+
+In this release, the only such type is
+[`HostLayout`](/pkg/structs#HostLayout)
+which indicates that a structure with a field of that
+type has a layout that conforms to host platform
+expectations.

doc/next/6-stdlib/99-minor/structs/66408.md

@@ -0,0 +1 @@
+<!-- This is a new package; covered in 6-stdlib/3-structs.md. -->

src/cmd/compile/internal/abi/abiutils.go

@@ -141,7 +141,7 @@ func (pa *ABIParamAssignment) RegisterTypesAndOffsets() ([]*types.Type, []int64)
 	}
 	typs := make([]*types.Type, 0, l)
 	offs := make([]int64, 0, l)
-	offs, _ = appendParamOffsets(offs, 0, pa.Type)
+	offs, _ = appendParamOffsets(offs, 0, pa.Type) // 0 is aligned for everything.
 	return appendParamTypes(typs, pa.Type), offs
 }
 
@@ -193,8 +193,8 @@ func appendParamTypes(rts []*types.Type, t *types.Type) []*types.Type {
 
 // appendParamOffsets appends the offset(s) of type t, starting from "at",
 // to input offsets, and returns the longer slice and the next unused offset.
+// at should already be aligned for t.
 func appendParamOffsets(offsets []int64, at int64, t *types.Type) ([]int64, int64) {
-	at = align(at, t)
 	w := t.Size()
 	if w == 0 {
 		return offsets, at
@@ -210,11 +210,15 @@ func appendParamOffsets(offsets []int64, at int64, t *types.Type) ([]int64, int6
 		typ := t.Kind()
 		switch typ {
 		case types.TARRAY:
+			te := t.Elem()
 			for i := int64(0); i < t.NumElem(); i++ {
-				offsets, at = appendParamOffsets(offsets, at, t.Elem())
+				at = align(at, te)
+				offsets, at = appendParamOffsets(offsets, at, te)
 			}
 		case types.TSTRUCT:
+			at0 := at
 			for i, f := range t.Fields() {
+				at = at0 + f.Offset // Fields may be over-aligned, see wasm32.
 				offsets, at = appendParamOffsets(offsets, at, f.Type)
 				if f.Type.Size() == 0 && i == t.NumFields()-1 {
 					at++ // last field has zero width
@@ -668,12 +672,13 @@ func (pa *ABIParamAssignment) ComputePadding(storage []uint64) []uint64 {
 	if len(types) != nr {
 		panic("internal error")
 	}
+	offsets, _ := appendParamOffsets([]int64{}, 0, pa.Type)
 	off := int64(0)
 	for idx, t := range types {
 		ts := t.Size()
 		off += int64(ts)
 		if idx < len(types)-1 {
-			noff := align(off, types[idx+1])
+			noff := offsets[idx+1]
 			if noff != off {
 				padding[idx] = uint64(noff - off)
 			}

src/cmd/compile/internal/compare/compare_test.go

@@ -23,8 +23,8 @@ func init() {
 	types.PtrSize = 8
 	types.RegSize = 8
 	types.MaxWidth = 1 << 50
-	typecheck.InitUniverse()
 	base.Ctxt = &obj.Link{Arch: &obj.LinkArch{Arch: &sys.Arch{Alignment: 1, CanMergeLoads: true}}}
+	typecheck.InitUniverse()
 }
 
 func TestEqStructCost(t *testing.T) {

src/cmd/compile/internal/devirtualize/pgo_test.go

@@ -13,6 +13,7 @@ import (
 	"cmd/internal/obj"
 	"cmd/internal/pgo"
 	"cmd/internal/src"
+	"cmd/internal/sys"
 	"testing"
 )
 
@@ -23,8 +24,8 @@ func init() {
 	types.PtrSize = 8
 	types.RegSize = 8
 	types.MaxWidth = 1 << 50
+	base.Ctxt = &obj.Link{Arch: &obj.LinkArch{Arch: &sys.Arch{Alignment: 1, CanMergeLoads: true}}}
 	typecheck.InitUniverse()
-	base.Ctxt = &obj.Link{}
 	base.Debug.PGODebug = 3
 }
 

src/cmd/compile/internal/inline/inlheur/texpr_classify_test.go

@@ -5,10 +5,13 @@
 package inlheur
 
 import (
+	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
+	"cmd/internal/obj"
 	"cmd/internal/src"
+	"cmd/internal/sys"
 	"go/constant"
 	"testing"
 )
@@ -21,6 +24,8 @@ func init() {
 	types.PtrSize = 8
 	types.RegSize = 8
 	types.MaxWidth = 1 << 50
+	base.Ctxt = &obj.Link{Arch: &obj.LinkArch{Arch: &sys.Arch{Alignment: 1, CanMergeLoads: true}}}
+
 	typecheck.InitUniverse()
 	local = types.NewPkg("", "")
 	fsym := &types.Sym{

src/cmd/compile/internal/ssa/export_test.go

@@ -7,6 +7,7 @@ package ssa
 import (
 	"testing"
 
+	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
@@ -15,6 +16,7 @@ import (
 	"cmd/internal/obj/s390x"
 	"cmd/internal/obj/x86"
 	"cmd/internal/src"
+	"cmd/internal/sys"
 )
 
 var CheckFunc = checkFunc
@@ -115,6 +117,7 @@ func init() {
 	types.RegSize = 8
 	types.MaxWidth = 1 << 50
 
+	base.Ctxt = &obj.Link{Arch: &obj.LinkArch{Arch: &sys.Arch{Alignment: 1, CanMergeLoads: true}}}
 	typecheck.InitUniverse()
 	testTypes.SetTypPtrs()
 }

src/go/doc/comment/std.go

@@ -0,0 +1,10 @@
+// Copyright 2024 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 structs defines marker types that can be used as struct fields
+// to modify the properties of a struct.
+//
+// By convention, a marker type should be used as the type of a field
+// named "_", placed at the beginning of a struct type definition.
+package structs

src/structs/doc.go

@@ -0,0 +1,30 @@
+// Copyright 2024 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 structs
+
+// HostLayout marks a struct as using host memory layout. A struct with a
+// field of type HostLayout will be laid out in memory according to host
+// expectations, generally following the host's C ABI.
+//
+// HostLayout does not affect layout within any other struct-typed fields
+// of the containing struct, nor does it affect layout of structs
+// containing the struct marked as host layout.
+//
+// By convention, HostLayout should be used as the type of a field
+// named "_", placed at the beginning of the struct type definition.
+type HostLayout struct {
+	_ hostLayout // prevent accidental conversion with plain struct{}
+}
+
+// We use an unexported type within the exported type to give the marker
+// type itself, rather than merely its name, a recognizable identity in
+// the type system. The main consequence of this is that a user can give
+// the type a new name and it will still have the same properties, e.g.,
+//
+//	type HL structs.HostLayout
+//
+// It also prevents unintentional conversion of struct{} to a named marker type.
+type hostLayout struct {
+}