[ovs-dev] [PATCH] lib/flow.h: Improve struct miniflow comment and definition.

[Ben Pfaff]

On Tue, Aug 26, 2014 at 11:32:55PM +0000, Saurabh Shah wrote:
> 
> 
> >Miniflows can nowadays be dynamically allocated to different inline
> >sizes, as done by lib/classifier.c, but this had not been documented
> >at the struct miniflow definition.
> >
> >Also, MINI_N_INLINE had a different value for 32-bit and 64-bit builds
> >due to a historical reason.  Now we use 8 for both.
> >
> >Finally, use change the storage type of 'values_inline' to uint8_t, as
> >uint64_t looks kind of wide for a boolean, even though we intend the
> >bit be carved out from the uint64_t where 'map' resides.
> >
> >Suggested-by: Ben Pfaff <blp at nicira.com>
> >Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>
> >---
> > lib/flow.c |    3 ++-
> > lib/flow.h |   16 +++++++++++++---
> > 2 files changed, 15 insertions(+), 4 deletions(-)
> >
> >diff --git a/lib/flow.c b/lib/flow.c
> >index bef2f27..d322ac2 100644
> >--- a/lib/flow.c
> >+++ b/lib/flow.c
> >@@ -1715,7 +1715,8 @@ miniflow_clone_inline(struct miniflow *dst, const
> >struct miniflow *src,
> > /* Initializes 'dst' with the data in 'src', destroying 'src'.
> >  * The caller must eventually free 'dst' with miniflow_destroy().
> >  * 'dst' must be regularly sized miniflow, but 'src' can have
> >- * larger than default inline values. */
> >+ * storage for more than the default MINI_N_INLINE inline
> >+ * values. */
> > void
> > miniflow_move(struct miniflow *dst, struct miniflow *src)
> > {
> >diff --git a/lib/flow.h b/lib/flow.h
> >index 3b8d24d..2891d60 100644
> >--- a/lib/flow.h
> >+++ b/lib/flow.h
> >@@ -346,7 +346,10 @@ bool flow_equal_except(const struct flow *a, const
> >struct flow *b,
> > ?
> > /* Compressed flow. */
> > 
> >-#define MINI_N_INLINE (sizeof(void *) == 4 ? 7 : 8)
> >+/* Number of 32-bit words present in struct miniflow. */
> >+#define MINI_N_INLINE 8
> >+
> >+/* Maximum number of 32-bit words supported. */
> > BUILD_ASSERT_DECL(FLOW_U32S <= 63);
> > 
> > /* A sparse representation of a "struct flow".
> >@@ -367,6 +370,11 @@ BUILD_ASSERT_DECL(FLOW_U32S <= 63);
> >  * the first element of the values array, the next 1-bit is in the next
> >array
> >  * element, and so on.
> >  *
> >+ * MINI_N_INLINE is the default number of inline words.  When a miniflow
> >is
> >+ * dynamically allocated the actual amount of inline storage may be
> >different.
> >+ * In that case 'inline_values' contains storage at least for the number
> >+ * of words indicated by 'map' (one uint32_t for each 1-bit in the map).
> >+ *
> >  * Elements in values array are allowed to be zero.  This is useful for
> >"struct
> >  * minimatch", for which ensuring that the miniflow and minimask members
> >have
> >  * same 'map' allows optimization.  This allowance applies only to a
> >miniflow
> >@@ -375,12 +383,14 @@ BUILD_ASSERT_DECL(FLOW_U32S <= 63);
> >  */
> > struct miniflow {
> >     uint64_t map:63;
> >-    uint64_t values_inline:1;
> >+    uint8_t values_inline:1;
> >     union {
> >         uint32_t *offline_values;
> >-        uint32_t inline_values[MINI_N_INLINE];
> >+        uint32_t inline_values[MINI_N_INLINE]; /* Minimum inline size. */
> >     };
> > };
> >+BUILD_ASSERT_DECL(sizeof(struct miniflow)
> >+                  == sizeof(uint64_t) + MINI_N_INLINE * sizeof(uint32_t))
> 
> This fails on windows:
> 
> c:\mingw\msys\1.0\home\administrator\work\ro\ovs\lib\hash.h(148) : warning
> C4028
> : formal parameter 2 different from declaration
> c:\mingw\msys\1.0\home\administrator\work\ro\ovs\lib\flow.h(393) : error
> C2034:
> 'build_assert_failed' : type of bit field too small for number of bits

Does it fail because of the uint64_t -> uint8_t change or because of
the change to MINI_N_INLINE?  (If it's the former let's just drop that
change.)