cmd/compile: relax reshaping condition

CL 641955 changes the Unified IR reader to not doing shapify when
reading reshaping expression. However, this condition only matters with
pointer type shaping, which will lose the original type, causes the
reshaping ends up with a completely different type.

This CL relaxes the condition, always allow non-pointer types shaping.

Updates #71184
Fixes #73947

Change-Id: Ib0bafd8932c52d99266f311b6cbfc75c00383f9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/678335
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

2 files changed, 144 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index 2c3f7161a8..38b0bc1d8a 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -1014,7 +1014,25 @@ func (pr *pkgReader) objDictIdx(sym *types.Sym, idx index, implicits, explicits
 	// arguments.
 	for i, targ := range dict.targs {
 		basic := r.Bool()
-		if dict.shaped && !pr.reshaping {
+		isPointerShape := basic && targ.IsPtr() && !targ.Elem().NotInHeap()
+		// We should not do shapify during the reshaping process, see #71184.
+		// However, this only matters for shapify a pointer type, which will
+		// lose the original underlying type.
+		//
+		// Example with a pointer type:
+		//
+		// - First, shapifying *[]T -> *uint8
+		// - During the reshaping process, *uint8 is shapified to *go.shape.uint8
+		// - This ends up with a different type with the original *[]T
+		//
+		// For a non-pointer type:
+		//
+		// - int -> go.shape.int
+		// - go.shape.int -> go.shape.int
+		//
+		// We always end up with the identical type.
+		canShapify := !pr.reshaping || !isPointerShape
+		if dict.shaped && canShapify {
 			dict.targs[i] = shapify(targ, basic)
 		}
 	}
diff --git a/src/cmd/compile/testdata/script/issue73947.txt b/src/cmd/compile/testdata/script/issue73947.txt
new file mode 100644
index 0000000000..f888ae2bfa
--- /dev/null
+++ b/src/cmd/compile/testdata/script/issue73947.txt
@@ -0,0 +1,125 @@
+go build main.go
+! stdout .
+! stderr .
+
+-- main.go --
+
+package main
+
+import (
+	"p/b"
+)
+
+func main() {
+	f()
+}
+
+func f() {
+	typ := indexedPageType{newIndexedType(nil)}
+	page := newPage(typ.indexedType)
+	page.Data()
+}
+
+func newPage(typ *indexedType) Page {
+	values := typ.NewValues(nil, nil)
+	return &indexedPage{
+		typ:         typ,
+		values:      values.Int32(),
+		columnIndex: ^0,
+	}
+}
+
+type Type interface {
+	NewPage(columnIndex, numValues int, data b.Values) Page
+	NewValues(values []byte, offsets []uint32) b.Values
+}
+
+type Page interface {
+	Type() Type
+	Data() b.Values
+}
+
+type indexedPage struct {
+	typ         *indexedType
+	values      []int32
+	columnIndex int16
+}
+
+func (page *indexedPage) Type() Type { return indexedPageType{page.typ} }
+
+func (page *indexedPage) Data() b.Values { return b.Int32Values(page.values) }
+
+type indexedType struct {
+	Type
+}
+
+func newIndexedType(typ Type) *indexedType {
+	return &indexedType{Type: typ}
+}
+
+type indexedPageType struct{ *indexedType }
+
+func (t indexedPageType) NewValues(values []byte, _ []uint32) b.Values {
+	return b.Int32ValuesFromBytes(values)
+}
+
+-- go.mod --
+module p
+
+go 1.24
+
+-- internal/a/a.go --
+package a
+
+import "unsafe"
+
+type slice struct {
+	ptr unsafe.Pointer
+	len int
+	cap int
+}
+
+func Slice[To, From any](data []From) []To {
+	// This function could use unsafe.Slice but it would drop the capacity
+	// information, so instead we implement the type conversion.
+	var zf From
+	var zt To
+	var s = slice{
+		ptr: unsafe.Pointer(unsafe.SliceData(data)),
+		len: int((uintptr(len(data)) * unsafe.Sizeof(zf)) / unsafe.Sizeof(zt)),
+		cap: int((uintptr(cap(data)) * unsafe.Sizeof(zf)) / unsafe.Sizeof(zt)),
+	}
+	return *(*[]To)(unsafe.Pointer(&s))
+}
+
+-- b/b.go --
+package b
+
+import "p/internal/a"
+
+type Kind int32
+
+const Int32 Kind = iota + 2
+
+type Values struct {
+	kind    Kind
+	size    int32
+	data    []byte
+	offsets []uint32
+}
+
+func (v *Values) Int32() []int32 {
+	return a.Slice[int32](v.data)
+}
+
+func makeValues[T any](kind Kind, values []T) Values {
+	return Values{kind: kind, data: a.Slice[byte](values)}
+}
+
+func Int32Values(values []int32) Values {
+	return makeValues(Int32, values)
+}
+
+func Int32ValuesFromBytes(values []byte) Values {
+	return Values{kind: Int32, data: values}
+}