reflect,runtime: assume register ABI with GOEXPERIMENT=regabiargs

This change causes finalizers, reflect calls, and Windows syscall
callbacks to assume the register ABI when GOEXPERIMENT=regabiargs is
set. That is, when all Go functions are using the new ABI by default,
these features should assume the new ABI too.

For #40724.

Change-Id: Ie4ee66b8085b692e1ff675f01134c9a4703ae1b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/306571
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/reflect/abi.go

@@ -28,9 +28,9 @@ import (
 // commented out there should be the actual values once
 // we're ready to use the register ABI everywhere.
 var (
-	intArgRegs   = 0          // abi.IntArgRegs
+	intArgRegs   = abi.IntArgRegs * experimentRegabiArgs
-	floatArgRegs = 0          // abi.FloatArgRegs
+	floatArgRegs = abi.FloatArgRegs * experimentRegabiArgs
-	floatRegSize = uintptr(0) // uintptr(abi.EffectiveFloatRegSize)
+	floatRegSize = uintptr(abi.EffectiveFloatRegSize * experimentRegabiArgs)
 )
 
 // abiStep represents an ABI "instruction." Each instruction

src/reflect/regabiargs_off.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.
+
+//go:build !goexperiment.regabiargs
+// +build !goexperiment.regabiargs
+
+package reflect
+
+const experimentRegabiArgs = 0

src/reflect/regabiargs_on.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.
+
+//go:build goexperiment.regabiargs
+// +build goexperiment.regabiargs
+
+package reflect
+
+const experimentRegabiArgs = 1

src/runtime/regabiargs_off.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.
+
+//go:build !goexperiment.regabiargs
+// +build !goexperiment.regabiargs
+
+package runtime
+
+const experimentRegabiArgs = 0

src/runtime/regabiargs_on.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.
+
+//go:build goexperiment.regabiargs
+// +build goexperiment.regabiargs
+
+package runtime
+
+const experimentRegabiArgs = 1

src/runtime/stubs.go

@@ -423,4 +423,4 @@ func sigpanic0()
 // everywhere.
 //
 // Protected by finlock.
-var intArgRegs = 0 // abi.IntArgRegs
+var intArgRegs = abi.IntArgRegs * experimentRegabiArgs