[dev.typeparams] cmd/compile: put shape types in their own package

Put shape types in the top level package called ".shape".
Name them using the serialization of the shape name, instead of
the .shapeN names.

This allows the linker to deduplicate instantiations across packages.

Not sure that this is entirely correct, as shapes in this package
may reference other packages (e.g. a field of a struct). But it seems
to work for now.

For the added test, when you look at the resulting binary (use the -k
option with run.go) it has only one instantiation of F, and 4 call sites:

$ objdump -d a.exe | grep _a\.F
 1053cb0:	e8 8b 00 00 00 	callq	139 <_a.F[.shape.*uint8]>
 1053ce9:	e8 52 00 00 00 	callq	82 <_a.F[.shape.*uint8]>
_a.F[.shape.*uint8]:
 1053d90:	e8 ab ff ff ff 	callq	-85 <_a.F[.shape.*uint8]>
 1053dc9:	e8 72 ff ff ff 	callq	-142 <_a.F[.shape.*uint8]>

Change-Id: I627f7e50210aabe4a10d0e2717d87b75ac82e99b
Reviewed-on: https://go-review.googlesource.com/c/go/+/339595
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -947,7 +947,7 @@ func writeType(t *types.Type) *obj.LSym {
 	}
 
 	dupok := 0
-	if tbase.Sym() == nil { // TODO(mdempsky): Probably need DUPOK for instantiated types too.
+	if tbase.Sym() == nil || tbase.HasShape() { // TODO(mdempsky): Probably need DUPOK for instantiated types too.
 		dupok = obj.DUPOK
 	}
 
@@ -1738,6 +1738,11 @@ func NeedEmit(typ *types.Type) bool {
 		// Need to emit to be safe (however, see TODO above).
 		return true
 
+	case typ.HasShape():
+		// Shape type; need to emit even though it lives in the .shape package.
+		// TODO: make sure the linker deduplicates them (see dupok in writeType above).
+		return true
+
 	default:
 		// Should have been emitted by an imported package.
 		return false

src/cmd/compile/internal/typecheck/subr.go

@@ -1362,8 +1362,7 @@ func Shapify(t *types.Type) *types.Type {
 		return s
 	}
 
-	sym := Lookup(fmt.Sprintf(".shape%d", snum))
-	snum++
+	sym := shapePkg.Lookup(u.LinkString())
 	name := ir.NewDeclNameAt(u.Pos(), ir.OTYPE, sym)
 	s := types.NewNamed(name)
 	s.SetUnderlying(u)
@@ -1375,6 +1374,6 @@ func Shapify(t *types.Type) *types.Type {
 	return s
 }
 
-var snum int
-
 var shaped = map[*types.Type]*types.Type{}
+
+var shapePkg = types.NewPkg(".shape", ".shape")

test/typeparam/dedup.dir/a.go

@@ -0,0 +1,10 @@
+// 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 a
+
+//go:noinline
+func F[T comparable](a, b T) bool {
+	return a == b
+}

test/typeparam/dedup.dir/b.go

@@ -0,0 +1,14 @@
+// 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 b
+
+import "a"
+
+func B() {
+	var x int64
+	println(a.F(&x, &x))
+	var y int32
+	println(a.F(&y, &y))
+}

test/typeparam/dedup.dir/c.go

@@ -0,0 +1,14 @@
+// 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 c
+
+import "a"
+
+func C() {
+	var x int64
+	println(a.F(&x, &x))
+	var y int32
+	println(a.F(&y, &y))
+}

test/typeparam/dedup.dir/main.go

@@ -0,0 +1,15 @@
+// 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 main
+
+import (
+	"b"
+	"c"
+)
+
+func main() {
+	b.B()
+	c.C()
+}

test/typeparam/dedup.go

@@ -0,0 +1,12 @@
+// rundir -G=3
+
+// 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.
+
+// Note: this doesn't really test the deduplication of
+// instantiations. It just provides an easy mechanism to build a
+// binary that you can then check with objdump manually to make sure
+// deduplication is happening. TODO: automate this somehow?
+
+package ignored

test/typeparam/dedup.out

@@ -0,0 +1,4 @@
+true
+true
+true
+true