Optimize ZlibCodec::doUncompress()

Summary:
`ZlibCodec::doUncompress()` always allocates at least 4 MiB when the uncompressed size is unkown.
We can get a 10-20% speed boost for small inputs (as well as significant memory savings) if we dynamically choose the buffer size.

Benchmarks before the change:
  ============================================================================
  experimental/terrelln/CompressionBenchmark.cpp  relative  time/iter  iters/s
  ============================================================================
  uncompress(ZLIB_100)                                         2.96us  338.19K
  uncompress(ZLIB_1000)                                        7.22us  138.44K
  uncompress(ZLIB_10000)                                      36.86us   27.13K
  uncompress(ZLIB_100000)                                    299.70us    3.34K
  uncompress(ZLIB_1000000)                                     2.74ms   365.32
  uncompress(ZLIB_10000000)                                   26.91ms    37.16
  uncompress(ZLIB_100000000)                                 273.92ms     3.65
  ============================================================================
After:
  ============================================================================
  experimental/terrelln/CompressionBenchmark.cpp  relative  time/iter  iters/s
  ============================================================================
  uncompress(ZLIB_100)                                         2.34us  426.68K
  uncompress(ZLIB_1000)                                        6.24us  160.38K
  uncompress(ZLIB_10000)                                      35.52us   28.16K
  uncompress(ZLIB_100000)                                    296.30us    3.37K
  uncompress(ZLIB_1000000)                                     2.74ms   365.25
  uncompress(ZLIB_10000000)                                   27.16ms    36.82
  uncompress(ZLIB_100000000)                                 273.72ms     3.65
  ============================================================================

Reviewed By: yfeldblum

Differential Revision: D4783871

fbshipit-source-id: 0fcce191ea520fd2117551db679c86f5ff734893

folly/io/Compression.cpp

@@ -846,6 +846,13 @@ std::unique_ptr<IOBuf> ZlibCodec::doCompress(const IOBuf* data) {
   return out;
 }
 
+static uint64_t computeBufferLength(uint64_t const compressedLength) {
+  constexpr uint64_t kMaxBufferLength = uint64_t(4) << 20; // 4 MiB
+  constexpr uint64_t kBlockSize = uint64_t(32) << 10; // 32 KiB
+  const uint64_t goodBufferSize = 4 * std::max(kBlockSize, compressedLength);
+  return std::min(goodBufferSize, kMaxBufferLength);
+}
+
 std::unique_ptr<IOBuf> ZlibCodec::doUncompress(const IOBuf* data,
                                                uint64_t uncompressedLength) {
   z_stream stream;
@@ -878,8 +885,9 @@ std::unique_ptr<IOBuf> ZlibCodec::doUncompress(const IOBuf* data,
   };
 
   // Max 64MiB in one go
-  constexpr uint32_t maxSingleStepLength = uint32_t(64) << 20;    // 64MiB
-  constexpr uint32_t defaultBufferLength = uint32_t(4) << 20;     // 4MiB
+  constexpr uint64_t maxSingleStepLength = uint64_t(64) << 20; // 64MiB
+  const uint64_t defaultBufferLength =
+      computeBufferLength(data->computeChainDataLength());
 
   auto out = addOutputBuffer(
       &stream,