Merge pull request #1374 from mathias-lang-sociomantic/bitswap-ulong

Add core.bitopt.bitswap(ulong)
dlang · Sep 6, 2015 · 3e3426f · 3e3426f
2 parents 46fdff3 + 10821c6
commit 3e3426f
Showing 1 changed file with 72 additions and 0 deletions.
diff --git a/src/core/bitop.d b/src/core/bitop.d
@@ -414,3 +414,75 @@ unittest
     foreach(i; 0 .. 32)
         assert(bitswap(1 << i) == 1 << 32 - i - 1);
 }
+
+/**
+ * Reverses the order of bits in a 64-bit integer.
+ */
+ulong bitswap ( ulong x ) pure @trusted
+{
+    version (D_InlineAsm_X86_64)
+    {
+        asm pure nothrow @nogc { naked; }
+
+        version (Win64)
+            asm pure nothrow @nogc { mov RAX, RCX; }
+        else
+            asm pure nothrow @nogc { mov RAX, RDI; }
+
+        asm pure nothrow @nogc
+        {
+            // Author: Tiago Gasiba.
+            mov RDX, RAX;
+            shr RAX, 1;
+            mov RCX, 0x5555_5555_5555_5555L;
+            and RDX, RCX;
+            and RAX, RCX;
+            shl RDX, 1;
+            or  RAX, RDX;
+
+            mov RDX, RAX;
+            shr RAX, 2;
+            mov RCX, 0x3333_3333_3333_3333L;
+            and RDX, RCX;
+            and RAX, RCX;
+            shl RDX, 2;
+            or  RAX, RDX;
+
+            mov RDX, RAX;
+            shr RAX, 4;
+            mov RCX, 0x0f0f_0f0f_0f0f_0f0fL;
+            and RDX, RCX;
+            and RAX, RCX;
+            shl RDX, 4;
+            or  RAX, RDX;
+            bswap RAX;
+            ret;
+        }
+    }
+    else
+    {
+        // swap odd and even bits
+        x = ((x >> 1) & 0x5555_5555_5555_5555L) | ((x & 0x5555_5555_5555_5555L) << 1);
+        // swap consecutive pairs
+        x = ((x >> 2) & 0x3333_3333_3333_3333L) | ((x & 0x3333_3333_3333_3333L) << 2);
+        // swap nibbles
+        x = ((x >> 4) & 0x0f0f_0f0f_0f0f_0f0fL) | ((x & 0x0f0f_0f0f_0f0f_0f0fL) << 4);
+        // swap bytes
+        x = ((x >> 8) & 0x00FF_00FF_00FF_00FFL) | ((x & 0x00FF_00FF_00FF_00FFL) << 8);
+        // swap shorts
+        x = ((x >> 16) & 0x0000_FFFF_0000_FFFFL) | ((x & 0x0000_FFFF_0000_FFFFL) << 16);
+        // swap ints
+        x = ( x >> 32 ) | ( x << 32);
+        return x;
+    }
+}
+
+unittest
+{
+    assert (bitswap( 0b1000000000000000000000010000000000000000100000000000000000000001)
+        == 0b1000000000000000000000010000000000000000100000000000000000000001);
+    assert (bitswap( 0b1110000000000000000000010000000000000000100000000000000000000001)
+        == 0b1000000000000000000000010000000000000000100000000000000000000111);
+    foreach (i; 0 .. 64)
+        assert(bitswap(1UL << i) == 1UL << 64 - i - 1);
+}