runtime/internal/math: add multiplication with overflow check

This CL adds a new internal math package for use by the runtime.
The new package exports a MulUintptr function with uintptr arguments
a and b and returns uintptr(a*b) and whether the full-width product
x*y does overflow the uintptr value range (uintptr(x*y) != x*y).

Uses of MulUinptr in the runtime and intrinsics for performance
will be added in followup CLs.

Updates #21588

Change-Id: Ia5a02eeabc955249118e4edf68c67d9fc0858058
Reviewed-on: https://go-review.googlesource.com/c/91755
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/inl_test.go

@@ -96,6 +96,9 @@ func TestIntendedInlining(t *testing.T) {
 			"(*puintptr).set",
 		},
 		"runtime/internal/sys": {},
+		"runtime/internal/math": {
+			"MulUintptr",
+		},
 		"bytes": {
 			"(*Buffer).Bytes",
 			"(*Buffer).Cap",

src/cmd/compile/internal/gc/racewalk.go

@@ -32,7 +32,15 @@ import (
 
 // Do not instrument the following packages at all,
 // at best instrumentation would cause infinite recursion.
-var omit_pkgs = []string{"runtime/internal/atomic", "runtime/internal/sys", "runtime", "runtime/race", "runtime/msan", "internal/cpu"}
+var omit_pkgs = []string{
+	"runtime/internal/atomic",
+	"runtime/internal/sys",
+	"runtime/internal/math",
+	"runtime",
+	"runtime/race",
+	"runtime/msan",
+	"internal/cpu",
+}
 
 // Only insert racefuncenterfp/racefuncexit into the following packages.
 // Memory accesses in the packages are either uninteresting or will cause false positives.

src/go/build/deps_test.go

@@ -36,9 +36,10 @@ var pkgDeps = map[string][]string{
 	// L0 is the lowest level, core, nearly unavoidable packages.
 	"errors":                  {},
 	"io":                      {"errors", "sync", "sync/atomic"},
-	"runtime":                 {"unsafe", "runtime/internal/atomic", "runtime/internal/sys", "internal/cpu", "internal/bytealg"},
+	"runtime":                 {"unsafe", "runtime/internal/atomic", "runtime/internal/sys", "runtime/internal/math", "internal/cpu", "internal/bytealg"},
 	"runtime/internal/sys":    {},
 	"runtime/internal/atomic": {"unsafe", "internal/cpu"},
+	"runtime/internal/math":   {"runtime/internal/sys"},
 	"internal/race":           {"runtime", "unsafe"},
 	"sync":                    {"internal/race", "runtime", "sync/atomic", "unsafe"},
 	"sync/atomic":             {"unsafe"},

src/runtime/internal/math/math.go

@@ -0,0 +1,19 @@
+// Copyright 2018 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 math
+
+import "runtime/internal/sys"
+
+const MaxUintptr = ^uintptr(0)
+
+// MulUintptr returns a * b and whether the multiplication overflowed.
+// On supported platforms this is an intrinsic lowered by the compiler.
+func MulUintptr(a, b uintptr) (uintptr, bool) {
+	if a|b < 1<<(4*sys.PtrSize) || a == 0 {
+		return a * b, false
+	}
+	overflow := b > MaxUintptr/a
+	return a * b, overflow
+}

src/runtime/internal/math/math_test.go

@@ -0,0 +1,51 @@
+// Copyright 2018 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 math_test
+
+import (
+	. "runtime/internal/math"
+	"testing"
+)
+
+const (
+	UintptrSize = 32 << (^uintptr(0) >> 63)
+)
+
+type mulUintptrTest struct {
+	a        uintptr
+	b        uintptr
+	overflow bool
+}
+
+var mulUintptrTests = []mulUintptrTest{
+	{0, 0, false},
+	{1000, 1000, false},
+	{MaxUintptr, 0, false},
+	{MaxUintptr, 1, false},
+	{MaxUintptr / 2, 2, false},
+	{MaxUintptr / 2, 3, true},
+	{MaxUintptr, 10, true},
+	{MaxUintptr, 100, true},
+	{MaxUintptr / 100, 100, false},
+	{MaxUintptr / 1000, 1001, true},
+	{1<<(UintptrSize/2) - 1, 1<<(UintptrSize/2) - 1, false},
+	{1 << (UintptrSize / 2), 1 << (UintptrSize / 2), true},
+	{MaxUintptr >> 32, MaxUintptr >> 32, false},
+	{MaxUintptr, MaxUintptr, true},
+}
+
+func TestMulUintptr(t *testing.T) {
+	for _, test := range mulUintptrTests {
+		a, b := test.a, test.b
+		for i := 0; i < 2; i++ {
+			mul, overflow := MulUintptr(a, b)
+			if mul != a*b || overflow != test.overflow {
+				t.Errorf("MulUintptr(%v, %v) = %v, %v want %v, %v",
+					a, b, mul, overflow, a*b, test.overflow)
+			}
+			a, b = b, a
+		}
+	}
+}