bitops.h: Implement half-shuffle and half-unshuffle ops

A half-shuffle operation takes a word with zeros in the high half:
 0000 0000 0000 0000 ABCD EFGH IJKL MNOP
and spreads the bits out so they are in every other bit of the word:
 0A0B 0C0D 0E0F 0G0H 0I0J 0K0L 0M0N 0O0P
A half-unshuffle performs the reverse operation.

Provide functions in bitops.h which implement these operations
for 32-bit and 64-bit inputs, and add tests for them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Tested-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1465915112-29272-3-git-send-email-peter.maydell@linaro.org

1 files changed, 108 insertions, 0 deletions

diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
index 755fdd1293..15418a86df 100644
--- a/include/qemu/bitops.h
+++ b/include/qemu/bitops.h
@@ -428,4 +428,112 @@ static inline uint64_t deposit64(uint64_t value, int start, int length,
     return (value & ~mask) | ((fieldval << start) & mask);
 }
 
+/**
+ * half_shuffle32:
+ * @value: 32-bit value (of which only the bottom 16 bits are of interest)
+ *
+ * Given an input value:
+ *  xxxx xxxx xxxx xxxx ABCD EFGH IJKL MNOP
+ * return the value where the bottom 16 bits are spread out into
+ * the odd bits in the word, and the even bits are zeroed:
+ *  0A0B 0C0D 0E0F 0G0H 0I0J 0K0L 0M0N 0O0P
+ *
+ * Any bits set in the top half of the input are ignored.
+ *
+ * Returns: the shuffled bits.
+ */
+static inline uint32_t half_shuffle32(uint32_t x)
+{
+    /* This algorithm is from _Hacker's Delight_ section 7-2 "Shuffling Bits".
+     * It ignores any bits set in the top half of the input.
+     */
+    x = ((x & 0xFF00) << 8) | (x & 0x00FF);
+    x = ((x << 4) | x) & 0x0F0F0F0F;
+    x = ((x << 2) | x) & 0x33333333;
+    x = ((x << 1) | x) & 0x55555555;
+    return x;
+}
+
+/**
+ * half_shuffle64:
+ * @value: 64-bit value (of which only the bottom 32 bits are of interest)
+ *
+ * Given an input value:
+ *  xxxx xxxx xxxx .... xxxx xxxx ABCD EFGH IJKL MNOP QRST UVWX YZab cdef
+ * return the value where the bottom 32 bits are spread out into
+ * the odd bits in the word, and the even bits are zeroed:
+ *  0A0B 0C0D 0E0F 0G0H 0I0J 0K0L 0M0N .... 0U0V 0W0X 0Y0Z 0a0b 0c0d 0e0f
+ *
+ * Any bits set in the top half of the input are ignored.
+ *
+ * Returns: the shuffled bits.
+ */
+static inline uint64_t half_shuffle64(uint64_t x)
+{
+    /* This algorithm is from _Hacker's Delight_ section 7-2 "Shuffling Bits".
+     * It ignores any bits set in the top half of the input.
+     */
+    x = ((x & 0xFFFF0000ULL) << 16) | (x & 0xFFFF);
+    x = ((x << 8) | x) & 0x00FF00FF00FF00FFULL;
+    x = ((x << 4) | x) & 0x0F0F0F0F0F0F0F0FULL;
+    x = ((x << 2) | x) & 0x3333333333333333ULL;
+    x = ((x << 1) | x) & 0x5555555555555555ULL;
+    return x;
+}
+
+/**
+ * half_unshuffle32:
+ * @value: 32-bit value (of which only the odd bits are of interest)
+ *
+ * Given an input value:
+ *  xAxB xCxD xExF xGxH xIxJ xKxL xMxN xOxP
+ * return the value where all the odd bits are compressed down
+ * into the low half of the word, and the high half is zeroed:
+ *  0000 0000 0000 0000 ABCD EFGH IJKL MNOP
+ *
+ * Any even bits set in the input are ignored.
+ *
+ * Returns: the unshuffled bits.
+ */
+static inline uint32_t half_unshuffle32(uint32_t x)
+{
+    /* This algorithm is from _Hacker's Delight_ section 7-2 "Shuffling Bits".
+     * where it is called an inverse half shuffle.
+     */
+    x &= 0x55555555;
+    x = ((x >> 1) | x) & 0x33333333;
+    x = ((x >> 2) | x) & 0x0F0F0F0F;
+    x = ((x >> 4) | x) & 0x00FF00FF;
+    x = ((x >> 8) | x) & 0x0000FFFF;
+    return x;
+}
+
+/**
+ * half_unshuffle64:
+ * @value: 64-bit value (of which only the odd bits are of interest)
+ *
+ * Given an input value:
+ *  xAxB xCxD xExF xGxH xIxJ xKxL xMxN .... xUxV xWxX xYxZ xaxb xcxd xexf
+ * return the value where all the odd bits are compressed down
+ * into the low half of the word, and the high half is zeroed:
+ *  0000 0000 0000 .... 0000 0000 ABCD EFGH IJKL MNOP QRST UVWX YZab cdef
+ *
+ * Any even bits set in the input are ignored.
+ *
+ * Returns: the unshuffled bits.
+ */
+static inline uint64_t half_unshuffle64(uint64_t x)
+{
+    /* This algorithm is from _Hacker's Delight_ section 7-2 "Shuffling Bits".
+     * where it is called an inverse half shuffle.
+     */
+    x &= 0x5555555555555555ULL;
+    x = ((x >> 1) | x) & 0x3333333333333333ULL;
+    x = ((x >> 2) | x) & 0x0F0F0F0F0F0F0F0FULL;
+    x = ((x >> 4) | x) & 0x00FF00FF00FF00FFULL;
+    x = ((x >> 8) | x) & 0x0000FFFF0000FFFFULL;
+    x = ((x >> 16) | x) & 0x00000000FFFFFFFFULL;
+    return x;
+}
+
 #endif