[Qemu-devel] [PATCH v2] bitops.h: Add field32() and field64() functions to extract bitfields

[Peter Maydell]

Add field32() and field64() functions which extract a particular
bit field from a word and return it. Based on an idea by Jia Liu.

Suggested-by: Jia Liu <address@hidden>
Signed-off-by: Peter Maydell <address@hidden>
---
v1->v2: added missing brackets to field32() to bring it in to line
        with field64()
(Still using 'int' rather than 'unsigned' for bit numbers as per
rationale in previous discussion.)

 bitops.h |   28 ++++++++++++++++++++++++++++
 1 files changed, 28 insertions(+), 0 deletions(-)

diff --git a/bitops.h b/bitops.h
index 07d1a06..ffbb387 100644
--- a/bitops.h
+++ b/bitops.h
@@ -269,4 +269,32 @@ static inline unsigned long hweight_long(unsigned long w)
     return count;
 }
 
+/**
+ * field64 - return a specified bit field from a uint64_t value
+ * @value: The value to extract the bit field from
+ * @start: The lowest bit in the bit field (numbered from 0)
+ * @length: The length of the bit field
+ *
+ * Returns the value of the bit field extracted from the input value.
+ */
+static inline uint64_t field64(uint64_t value, int start, int length)
+{
+    assert(start >= 0 && start <= 63 && length > 0 && start + length <= 64);
+    return (value >> start) & (~0ULL >> (64 - length));
+}
+
+/**
+ * field32 - return a specified bit field from a uint32_t value
+ * @value: The value to extract the bit field from
+ * @start: The lowest bit in the bit field (numbered from 0)
+ * @length: The length of the bit field
+ *
+ * Returns the value of the bit field extracted from the input value.
+ */
+static inline uint32_t field32(uint32_t value, int start, int length)
+{
+    assert(start >= 0 && start <= 31 && length > 0 && start + length <= 32);
+    return (value >> start) & (~0U >> (32 - length));
+}
+
 #endif
-- 
1.7.1