Reverse lower 16 bits, instead of swapping lower 2 bytes

Author: zhengyu123

ddprof-lib/src/main/cpp/reverse_bits.h

@@ -0,0 +1,23 @@
+//
+// Borrow the implementation from openjdk
+// https://github.com/openjdk/jdk/blob/master/src/hotspot/share/utilities/reverse_bits.hpp
+//
+
+#ifndef REVERSE_BITS_H
+#define REVERSE_BITS_H
+#include "arch_dd.h"
+#include <stdint.h>
+
+static constexpr u32 rep_5555 = static_cast<u32>(UINT64_C(0x5555555555555555));
+static constexpr u32 rep_3333 = static_cast<u32>(UINT64_C(0x3333333333333333));
+static constexpr u32 rep_0F0F = static_cast<u32>(UINT64_C(0x0F0F0F0F0F0F0F0F));
+
+inline u16 reverse16(u16 v) {
+  u32 x = static_cast<u32>(v);
+  x = ((x & rep_5555) << 1) | ((x >> 1) & rep_5555);
+  x = ((x & rep_3333) << 2) | ((x >> 2) & rep_3333);
+  x = ((x & rep_0F0F) << 4) | ((x >> 4) & rep_0F0F);
+  return __builtin_bswap16(static_cast<u16>(x));
+}
+
+#endif //REVERSE_BITS_H

ddprof-lib/src/main/cpp/threadFilter.cpp

@@ -17,6 +17,7 @@
 #include "threadFilter.h"
 #include "counters.h"
 #include "os.h"
+#include "reverse_bits.h"
 #include <stdlib.h>
 #include <string.h>
 
@@ -89,7 +90,7 @@ int ThreadFilter::mapThreadId(int thread_id) {
   // We want to map the thread_id inside the same bitmap
   static_assert(BITMAP_SIZE >= (u16)0xffff, "Potential verflow");
   u16 lower16 = (u16)(thread_id & 0xffff);
-  lower16 = ((lower16 & 0x00ff) << 8) | ((lower16 & 0xff00) >> 8);
+  lower16 = reverse16(lower16);
   int tid = (thread_id & ~0xffff) | lower16;
   return tid; 
 }