bitreverse

Summary:
Adds a bitreverse function to Bits.h

AFAIK there is no great way to do this on x86, takes ~4-5ns.
arm has a single instruction we can drop in in the future.

Reviewed By: yfeldblum

Differential Revision: D6459283

fbshipit-source-id: 129db196b2fac95386f601ae57843aa87523b915

folly/Bits.h

@@ -372,4 +372,13 @@ inline void storeUnaligned(void* p, T value) {
   }
 }
 
+template <typename T>
+T bitReverse(T n) {
+  auto m = static_cast<typename std::make_unsigned<T>::type>(n);
+  m = ((m & 0xAAAAAAAAAAAAAAAA) >> 1) | ((m & 0x5555555555555555) << 1);
+  m = ((m & 0xCCCCCCCCCCCCCCCC) >> 2) | ((m & 0x3333333333333333) << 2);
+  m = ((m & 0xF0F0F0F0F0F0F0F0) >> 4) | ((m & 0x0F0F0F0F0F0F0F0F) << 4);
+  return static_cast<T>(Endian::swap(m));
+}
+
 } // namespace folly

folly/test/BitsBenchmark.cpp

@@ -38,6 +38,15 @@ BENCHMARK(isPowTwo, iters) {
   }
 }
 
+BENCHMARK_DRAW_LINE();
+BENCHMARK(reverse, iters) {
+  uint64_t b = 0;
+  for (unsigned long i = 0; i < iters; ++i) {
+    b = folly::bitReverse(i + b);
+    folly::doNotOptimizeAway(b);
+  }
+}
+
 int main(int argc, char** argv) {
   gflags::ParseCommandLineFlags(&argc, &argv, true);
   folly::runBenchmarks();
@@ -48,7 +57,13 @@ int main(int argc, char** argv) {
 Benchmarks run on dual Xeon X5650's @ 2.67GHz w/hyperthreading enabled
   (12 physical cores, 12 MB cache, 72 GB RAM)
 
-Benchmark                               Iters   Total t    t/iter iter/sec
-------------------------------------------------------------------------------
-*       nextPowTwoClz                 1000000  1.659 ms  1.659 ns  574.8 M
+============================================================================
+folly/test/BitsBenchmark.cpp                    relative  time/iter  iters/s
+============================================================================
+nextPowTwoClz                                                0.00fs  Infinity
+----------------------------------------------------------------------------
+isPowTwo                                                   731.61ps    1.37G
+----------------------------------------------------------------------------
+reverse                                                      4.84ns  206.58M
+============================================================================
 */

folly/test/BitsTest.cpp

@@ -18,6 +18,7 @@
 
 #include <folly/Bits.h>
 
+#include <folly/Random.h>
 #include <random>
 
 #include <folly/portability/GTest.h>
@@ -185,3 +186,25 @@ TEST(Bits, Endian_swap_real) {
   EXPECT_NE(d, Endian::swap(d));
   EXPECT_EQ(d, Endian::swap(Endian::swap(d)));
 }
+
+uint64_t reverse_simple(uint64_t v) {
+  uint64_t r = 0;
+
+  for (int i = 0; i < 64; i++) {
+    r <<= 1;
+    r |= v & 1;
+    v >>= 1;
+  }
+  return r;
+}
+
+TEST(Bits, BitReverse) {
+  EXPECT_EQ(folly::bitReverse<uint8_t>(0), 0);
+  EXPECT_EQ(folly::bitReverse<uint8_t>(1), 128);
+  for (int i = 0; i < 100; i++) {
+    uint64_t v = folly::Random::rand64();
+    EXPECT_EQ(folly::bitReverse(v), reverse_simple(v));
+    uint32_t b = folly::Random::rand32();
+    EXPECT_EQ(folly::bitReverse(b), reverse_simple(b) >> 32);
+  }
+}