util: add new ASSERT_BITFIELD_SIZE() macro (v3)

For checking that bitfields are large enough to hold the largest
expected value.

v2: move into existing util/macros.h header where STATIC_ASSERT() lives.
v3: add MAYBE_UNUSED to variable declaration

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/util/macros.h

@@ -241,6 +241,23 @@ do {                       \
 #define ATTRIBUTE_NOINLINE
 #endif
 
+
+/**
+ * Check that STRUCT::FIELD can hold MAXVAL.  We use a lot of bitfields
+ * in Mesa/gallium.  We have to be sure they're of sufficient size to
+ * hold the largest expected value.
+ * Note that with MSVC, enums are signed and enum bitfields need one extra
+ * high bit (always zero) to ensure the max value is handled correctly.
+ * This macro will detect that with MSVC, but not GCC.
+ */
+#define ASSERT_BITFIELD_SIZE(STRUCT, FIELD, MAXVAL) \
+   do { \
+      MAYBE_UNUSED STRUCT s;                \
+      s.FIELD = (MAXVAL); \
+      assert((int) s.FIELD == (MAXVAL) && "Insufficient bitfield size!"); \
+   } while (0)
+
+
 /** Compute ceiling of integer quotient of A divided by B. */
 #define DIV_ROUND_UP( A, B )  ( (A) % (B) == 0 ? (A)/(B) : (A)/(B)+1 )