Add prepareSkipTo() method to EliasFanoReader

Summary:
`prepareSkipTo(x`) allows the client of EliasFanoReader to "hint" that
`skipTo(x)` will be called in the near future. The primary benefit of doing so
is that memory which is needed for `skipTo(x)` can be prefetched to minimize
cache misses incurred when calling `skipTo(x)`.

Reviewed By: ot, philippv

Differential Revision: D5508995

fbshipit-source-id: 4876b566256849f76193db3dc0404768aeeeb30d

folly/experimental/EliasFanoCoding.h

@@ -46,6 +46,8 @@ namespace folly { namespace compression {
 
 static_assert(kIsLittleEndian, "EliasFanoCoding.h requires little endianness");
 
+constexpr size_t kCacheLineSize = 64;
+
 template <class Pointer>
 struct EliasFanoCompressedListBase {
   EliasFanoCompressedListBase() = default;
@@ -450,6 +452,38 @@ class UpperBitsReader : ForwardPointers<Encoder::forwardQuantum>,
     return value_;
   }
 
+  /**
+   * Prepare to skip to `value`. This is a constant-time operation that will
+   * prefetch memory required for a `skipTo(value)` call.
+   *
+   * @return position of reader
+   */
+  SizeType prepareSkipTo(ValueType v) const {
+    auto position = position_;
+
+    if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
+      auto outer = outer_;
+      const size_t steps = v / Encoder::skipQuantum;
+      const size_t dest = folly::loadUnaligned<SkipValueType>(
+          this->skipPointers_ + (steps - 1) * sizeof(SkipValueType));
+
+      position = dest - 1;
+      outer = (dest + Encoder::skipQuantum * steps) / 8;
+
+      // Prefetch up to the beginning of where we linear search. After that,
+      // hardware prefetching will outperform our own. In addition, this
+      // simplifies calculating what to prefetch as we don't have to calculate
+      // the entire destination address. Two cache lines are prefetched because
+      // this results in fewer cycles used (based on practical results) than
+      // one. However, three cache lines does not have any additional effect.
+      const auto addr = start_ + outer;
+      __builtin_prefetch(addr);
+      __builtin_prefetch(addr + kCacheLineSize);
+    }
+
+    return position;
+  }
+
   ValueType jump(size_t n) {
     if (Encoder::forwardQuantum == 0 || n <= Encoder::forwardQuantum) {
       reset();
@@ -611,6 +645,29 @@ class EliasFanoReader {
     return true;
   }
 
+  /**
+   * Prepare to skip to `value` by prefetching appropriate memory in both the
+   * upper and lower bits.
+   */
+  void prepareSkipTo(ValueType value) const {
+    // Also works when value_ == kInvalidValue.
+    if (value != kInvalidValue) {
+      DCHECK_GE(value + 1, value_ + 1);
+    }
+
+    if ((!kUnchecked && value > lastValue_) || (value == value_)) {
+      return;
+    }
+
+    // Do minimal computation required to prefetch address used in
+    // `readLowerPart()`.
+    ValueType upperValue = (value >> numLowerBits_);
+    const auto upperPosition = upper_.prepareSkipTo(upperValue);
+    const auto addr = lower_ + (upperPosition * numLowerBits_ / 8);
+    __builtin_prefetch(addr);
+    __builtin_prefetch(addr + kCacheLineSize);
+  }
+
   bool jump(SizeType n) {
     if (LIKELY(n < size_)) {  // Also checks that n != -1.
       value_ = readLowerPart(n) | (upper_.jump(n + 1) << numLowerBits_);