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>

2 files changed, 70 insertions, 0 deletions

diff --git a/src/runtime/internal/math/math.go b/src/runtime/internal/math/math.go
new file mode 100644
index 0000000000..5385f5dd86
--- /dev/null
+++ b/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
+}
diff --git a/src/runtime/internal/math/math_test.go b/src/runtime/internal/math/math_test.go
new file mode 100644
index 0000000000..9447bd23f9
--- /dev/null
+++ b/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
+		}
+	}
+}