From: Robert Griesemer <gri@golang.org>
Date: Wed, 14 Mar 2018 18:19:42 +0000 (-0700)
Subject: math/big: add comment about internal assumptions on nat values
X-Git-Tag: go1.11beta1~1208
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=7d4d2cb686d2068a9a07dbde9f2a72c237c1f4a4;p=gostls13.git

math/big: add comment about internal assumptions on nat values

Change-Id: I7ed40507a019c0bf521ba748fc22c03d74bb17b7
Reviewed-on: https://go-review.googlesource.com/100719
Reviewed-by: Ian Lance Taylor <iant@golang.org>
---

diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index c1812e3544..436c108c96 100644
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -5,6 +5,11 @@
 // This file implements unsigned multi-precision integers (natural
 // numbers). They are the building blocks for the implementation
 // of signed integers, rationals, and floating-point numbers.
+//
+// Caution: This implementation relies on the function "alias"
+//          which assumes that (nat) slice capacities are never
+//          changed (no 3-operand slice expressions). If that
+//          changes, alias needs to be updated for correctness.
 
 package big
 
@@ -352,6 +357,10 @@ func karatsuba(z, x, y nat) {
 }
 
 // alias reports whether x and y share the same base array.
+// Note: alias assumes that the capacity of underlying arrays
+//       is never changed for nat values; i.e. that there are
+//       no 3-operand slice expressions in this code (or worse,
+//       reflect-based operations to the same effect).
 func alias(x, y nat) bool {
 	return cap(x) > 0 && cap(y) > 0 && &x[0:cap(x)][cap(x)-1] == &y[0:cap(y)][cap(y)-1]
 }