runtime/internal/math: add Add64

This makes the intrinsic available on 64-bit platforms,
since the runtime cannot import math/bits.

Change-Id: I5296cc6a97d1cb4756ab369d96dc9605df9f8247
Reviewed-on: https://go-review.googlesource.com/c/go/+/516861
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Bypass: Russ Cox <rsc@golang.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/runtime/internal/math/math.go b/src/runtime/internal/math/math.go
index c3fac366be..b2e5508651 100644
--- a/src/runtime/internal/math/math.go
+++ b/src/runtime/internal/math/math.go
@@ -38,3 +38,18 @@ func Mul64(x, y uint64) (hi, lo uint64) {
 	lo = x * y
 	return
 }
+
+// Add64 returns the sum with carry of x, y and carry: sum = x + y + carry.
+// The carry input must be 0 or 1; otherwise the behavior is undefined.
+// The carryOut output is guaranteed to be 0 or 1.
+//
+// This function's execution time does not depend on the inputs.
+// On supported platforms this is an intrinsic lowered by the compiler.
+func Add64(x, y, carry uint64) (sum, carryOut uint64) {
+	sum = x + y + carry
+	// The sum will overflow if both top bits are set (x & y) or if one of them
+	// is (x | y), and a carry from the lower place happened. If such a carry
+	// happens, the top bit will be 1 + 0 + 1 = 0 (&^ sum).
+	carryOut = ((x & y) | ((x | y) &^ sum)) >> 63
+	return
+}