On 30/10/2019 17.19, Ilya Dryomov wrote:

On Thu, Oct 24, 2019 at 11:49 AM David Howells [off-list ref] wrote:

quoted

 /*
- * We use a start+len construction, which provides full use of the
- * allocated memory.
- * -- Florian Coosmann (FGC)
- *
+ * We use head and tail indices that aren't masked off, except at the point of
+ * dereference, but rather they're allowed to wrap naturally.  This means there
+ * isn't a dead spot in the buffer, provided the ring size < INT_MAX.
+ * -- David Howells 2019-09-23.

Hi David,

Is "ring size < INT_MAX" constraint correct?

No. As long as one always uses a[idx % size] to access the array, the
only requirement is that size is representable in an unsigned int. Then
because one also wants to do the % using simple bitmasking, that further
restricts one to sizes that are a power of 2, so the end result is that
the max size is 2^31 (aka INT_MAX+1).

I've never had to implement this free running indices scheme, but
the way I've always visualized it is that the top bit of the index is
used as a lap (as in a race) indicator, leaving 31 bits to work with
(in case of unsigned ints).  Should that be

  ring size <= 2^31

or more precisely

  ring size is a power of two <= 2^31

Exactly. But it's kind of moot since the ring size would never be
allowed to grow anywhere near that.

Rasmus