[System] Ensure we use logical shift and not arithmetic shift

diff --git a/mcs/class/System/System.Collections.Concurrent/ConcurrentBag.cs b/mcs/class/System/System.Collections.Concurrent/ConcurrentBag.cs
index 827bc74b83cdca80c0bf7127051d4b99d64f6bb1..5998c061ab0302690b8f07f5328a0919759de805 100644 (file)
--- a/mcs/class/System/System.Collections.Concurrent/ConcurrentBag.cs
+++ b/mcs/class/System/System.Collections.Concurrent/ConcurrentBag.cs
@@ -153,15 +153,19 @@ namespace System.Collections.Concurrent
                                return;
                        var hs = hints;
                        // If cas failed then we don't retry
-                       Interlocked.CompareExchange (ref hints, (hs << 4) | (uint)index, hs);
+                       Interlocked.CompareExchange (ref hints, (long)(((ulong)hs) << 4 | (uint)index), (long)hs);
                }
 
                bool TryGetHint (out int index)
                {
+                       /* Funny little thing to know, since hints is a long (because CAS has no ulong overload),
+                        * a shift-right operation is an arithmetic shift which might set high-order right bits
+                        * to 1 instead of 0 if the number turns negative.
+                        */
                        var hs = hints;
                        index = 0;
 
-                       if (Interlocked.CompareExchange (ref hints, hs >> 4, hs) == hs)
+                       if (Interlocked.CompareExchange (ref hints, (long)(((ulong)hs) >> 4), hs) == hs)
                                index = (int)(hs & 0xF);
 
                        return index > 0;