bufmgr: Make it easier to change number of buffer state bits

In an upcoming commit I'd like to change the number of bits for the usage
count (the current max is 5, fitting in three bits, but we reserve four
bits). Until now that required adjusting a bunch of magic constants, now the
constants are defined based on the number of bits reserved.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/lxzj26ga6ippdeunz6kuncectr5gfuugmm2ry22qu6hcx6oid6@lzx3sjsqhmt6
Discussion: https://postgr.es/m/riivolmg6uzfvpzfn6wjo3ghwt42rcec43ok6mv4oenfg654y7@x7dbposbskwd

Author: anarazel

src/include/storage/buf_internals.h

@@ -39,12 +39,19 @@
  *
  * The definition of buffer state components is below.
  */
+#define BUF_REFCOUNT_BITS 18
+#define BUF_USAGECOUNT_BITS 4
+#define BUF_FLAG_BITS 10
+
+StaticAssertDecl(BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS + BUF_FLAG_BITS == 32,
+				 "parts of buffer state space need to equal 32");
+
 #define BUF_REFCOUNT_ONE 1
-#define BUF_REFCOUNT_MASK ((1U << 18) - 1)
-#define BUF_USAGECOUNT_MASK 0x003C0000U
-#define BUF_USAGECOUNT_ONE (1U << 18)
-#define BUF_USAGECOUNT_SHIFT 18
-#define BUF_FLAG_MASK 0xFFC00000U
+#define BUF_REFCOUNT_MASK ((1U << BUF_REFCOUNT_BITS) - 1)
+#define BUF_USAGECOUNT_MASK (((1U << BUF_USAGECOUNT_BITS) - 1) << (BUF_REFCOUNT_BITS))
+#define BUF_USAGECOUNT_ONE (1U << BUF_REFCOUNT_BITS)
+#define BUF_USAGECOUNT_SHIFT BUF_REFCOUNT_BITS
+#define BUF_FLAG_MASK (((1U << BUF_FLAG_BITS) - 1) << (BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS))
 
 /* Get refcount and usagecount from buffer state */
 #define BUF_STATE_GET_REFCOUNT(state) ((state) & BUF_REFCOUNT_MASK)
@@ -77,6 +84,9 @@
  */
 #define BM_MAX_USAGE_COUNT	5
 
+StaticAssertDecl(BM_MAX_USAGE_COUNT < (1 << BUF_USAGECOUNT_BITS),
+				 "BM_MAX_USAGE_COUNT doesn't fit in BUF_USAGECOUNT_BITS bits");
+
 /*
  * Buffer tag identifies which disk block the buffer contains.
  *