[Qemu-devel] [PATCH 1/2] bitops: Provide sextract32() and sextract64()

[Peter Maydell]

A common operation in instruction decoding is to take a field
from an instruction that represents a signed integer in some
arbitrary number of bits, and sign extend it into a C signed
integer type for manipulation. Provide new functions sextract32()
and sextract64() which perform this operation; they are like
the existing extract32() and extract64() except that the field
is sign-extended into the returned result.

Signed-off-by: Peter Maydell <address@hidden>
---
 include/qemu/bitops.h |   50 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
index affcc96..06e2e6f 100644
--- a/include/qemu/bitops.h
+++ b/include/qemu/bitops.h
@@ -222,6 +222,56 @@ static inline uint64_t extract64(uint64_t value, int 
start, int length)
 }
 
 /**
+ * sextract32:
+ * @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 32 bit input @value the bit field specified by the
+ * @start and @length parameters, and return it, sign extended to
+ * an int32_t (ie with the most significant bit of the field propagated
+ * to all the upper bits of the return value). The bit field must lie
+ * entirely within the 32 bit word. It is valid to request that
+ * all 32 bits are returned (ie @length 32 and @start 0).
+ *
+ * Returns: the sign extended value of the bit field extracted from the
+ * input value.
+ */
+static inline int32_t sextract32(uint32_t value, int start, int length)
+{
+    assert(start >= 0 && length > 0 && length <= 32 - start);
+    /* Note that this implementation relies on right shift of signed
+     * integers being an arithmetic shift.
+     */
+    return ((int32_t)(value << (32 - length - start))) >> (32 - length);
+}
+
+/**
+ * sextract64:
+ * @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 64 bit input @value the bit field specified by the
+ * @start and @length parameters, and return it, sign extended to
+ * an int64_t (ie with the most significant bit of the field propagated
+ * to all the upper bits of the return value). The bit field must lie
+ * entirely within the 64 bit word. It is valid to request that
+ * all 64 bits are returned (ie @length 64 and @start 0).
+ *
+ * Returns: the sign extended value of the bit field extracted from the
+ * input value.
+ */
+static inline uint64_t sextract64(uint64_t value, int start, int length)
+{
+    assert(start >= 0 && length > 0 && length <= 64 - start);
+    /* Note that this implementation relies on right shift of signed
+     * integers being an arithmetic shift.
+     */
+    return ((int64_t)(value << (64 - length - start))) >> (64 - length);
+}
+
+/**
  * deposit32:
  * @value: initial value to insert bit field into
  * @start: the lowest bit in the bit field (numbered from 0)
-- 
1.7.9.5