[v10,13/13] qemu/bitops.h: Add extract8 and extract16

Commit Message

Yoshinori Sato May 8, 2019, 2:56 p.m. UTC

Signed-off-by: Yoshinori Sato <[e-mail]>
---
 include/qemu/bitops.h | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

Comments

Richard Henderson May 8, 2019, 5:37 p.m. UTC | #1

On 5/8/19 7:56 AM, Yoshinori Sato wrote:
> +static inline uint16_t extract16(uint32_t value, int start, int length)

s/uint32_t/uint16_t/

Aside from the possible value of the more restrictive asserts, I'm not sure
what advantage you see in these routines.  All arithmetic in C is promoted to
type "int", which for all supported hosts is 32-bits.

This suggests an implementation for these functions as

    assert(...);
    return extract32(value, start, length);

because otherwise the (32 - length) subexpression might at first glance appear
to be a cut-and-paste bug.  Whereas it's really required by the larger
subexpression.


r~

Patch

diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
index 3f0926cf40..0dfd81efb2 100644
--- a/include/qemu/bitops.h
+++ b/include/qemu/bitops.h
@@ -301,6 +301,44 @@  static inline uint32_t extract32(uint32_t value, int start, int length)
 }
 
 /**
+ * extract8:
+ * @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
+ *
+ * Extract from the 8 bit input @value the bit field specified by the
+ * @start and @length parameters, and return it. The bit field must
+ * lie entirely within the 8 bit word. It is valid to request that
+ * all 8 bits are returned (ie @length 8 and @start 0).
+ *
+ * Returns: the value of the bit field extracted from the input value.
+ */
+static inline uint8_t extract8(uint8_t value, int start, int length)
+{
+    assert(start >= 0 && length > 0 && length <= 8 - start);
+    return (value >> start) & (~0U >> (32 - length));
+}
+
+/**
+ * extract16:
+ * @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
+ *
+ * Extract from the 16 bit input @value the bit field specified by the
+ * @start and @length parameters, and return it. The bit field must
+ * lie entirely within the 16 bit word. It is valid to request that
+ * all 16 bits are returned (ie @length 16 and @start 0).
+ *
+ * Returns: the value of the bit field extracted from the input value.
+ */
+static inline uint16_t extract16(uint32_t value, int start, int length)
+{
+    assert(start >= 0 && length > 0 && length <= 16 - start);
+    return (value >> start) & (~0U >> (32 - length));
+}
+
+/**
  * extract64:
  * @value: the value to extract the bit field from
  * @start: the lowest bit in the bit field (numbered from 0)