[dev.regabi] internal/abi: add new internal/abi package for ABI constants

This change creates a new internal std package internal/abi which is
intended to hold constants with platform-specific values related to
our ABI that is useful to different std packages, such as runtime and
reflect.

For #40724.

Change-Id: Ie7ae7f687629cd3d613ba603e9371f0887601fe6
Reviewed-on: https://go-review.googlesource.com/c/go/+/272567
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/go/build/deps_test.go

@@ -71,13 +71,13 @@ var depsRules = `
 	# No dependencies allowed for any of these packages.
 	NONE
 	< container/list, container/ring,
-	  internal/cfg, internal/cpu,
+	  internal/abi, internal/cfg, internal/cpu,
 	  internal/goversion, internal/nettrace,
 	  unicode/utf8, unicode/utf16, unicode,
 	  unsafe;
 
 	# RUNTIME is the core runtime group of packages, all of them very light-weight.
-	internal/cpu, unsafe
+	internal/abi, internal/cpu, unsafe
 	< internal/bytealg
 	< internal/unsafeheader
 	< runtime/internal/sys

src/internal/abi/abi.go

@@ -0,0 +1,12 @@
+// Copyright 2020 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 abi
+
+// RegArgs is a struct that has space for each argument
+// and return value register on the current architecture.
+type RegArgs struct {
+	Ints   [IntArgRegs]uintptr
+	Floats [FloatArgRegs]uint64
+}

src/internal/abi/abi_amd64.go

@@ -0,0 +1,20 @@
+// Copyright 2020 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.
+
+// +build goexperiment.regabi
+
+package abi
+
+const (
+	// See abi_generic.go.
+
+	// RAX, RBX, RCX, RDI, RSI, R8, R9, R10, R11.
+	IntArgRegs = 9
+
+	// X0 -> X14.
+	FloatArgRegs = 15
+
+	// We use SSE2 registers which support 64-bit float operations.
+	EffectiveFloatRegSize = 8
+)

src/internal/abi/abi_generic.go

@@ -0,0 +1,38 @@
+// Copyright 2020 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.
+
+// +build !goexperiment.regabi
+
+package abi
+
+const (
+	// ABI-related constants.
+	//
+	// In the generic case, these are all zero
+	// which lets them gracefully degrade to ABI0.
+
+	// IntArgRegs is the number of registers dedicated
+	// to passing integer argument values. Result registers are identical
+	// to argument registers, so this number is used for those too.
+	IntArgRegs = 0
+
+	// FloatArgRegs is the number of registers dedicated
+	// to passing floating-point argument values. Result registers are
+	// identical to argument registers, so this number is used for
+	// those too.
+	FloatArgRegs = 0
+
+	// EffectiveFloatRegSize describes the width of floating point
+	// registers on the current platform from the ABI's perspective.
+	//
+	// Since Go only supports 32-bit and 64-bit floating point primitives,
+	// this number should be either 0, 4, or 8. 0 indicates no floating
+	// point registers for the ABI or that floating point values will be
+	// passed via the softfloat ABI.
+	//
+	// For platforms that support larger floating point register widths,
+	// such as x87's 80-bit "registers" (not that we support x87 currently),
+	// use 8.
+	EffectiveFloatRegSize = 0
+)