folly test: fix typo in RandomDataHolder::RandomDataHolder

Summary:
Running the folly/io/test:compression_test with a UBSAN-enabled binary would fail with this:

  folly/io/test/CompressionTest.cpp:96:36: runtime error: shift exponent 16777216 is too large for 64-bit type 'size_t' (aka 'unsigned long')

That exposed the unwanted left-shift.
That made it so the actual "start" offsets were in the range [0..numThreads-1], rather than being
e.g., with numThreads == 8, [0, 16777216, 33554432, ...].  Oops.

Reviewed By: luciang

Differential Revision: D3277606

fbshipit-source-id: 4bc48f84b0065de070b62a810f08b96e36ea7136

diff --git a/folly/io/test/CompressionTest.cpp b/folly/io/test/CompressionTest.cpp
index 020d74876ae3d7464902abee12638af37efcf3d1..8fe347f1d9df9431273d7261dd820dd855ac6b70 100644 (file)
--- a/folly/io/test/CompressionTest.cpp
+++ b/folly/io/test/CompressionTest.cpp
@@ -92,7 +92,7 @@ RandomDataHolder::RandomDataHolder(size_t sizeLog2)
     threads.emplace_back(
         [this, seed, t, numThreadsLog2, sizeLog2] () {
           std::mt19937 rng(seed + t);
-          size_t countLog2 = size_t(1) << (sizeLog2 - numThreadsLog2);
+          size_t countLog2 = sizeLog2 - numThreadsLog2;
           size_t start = size_t(t) << countLog2;
           for (size_t i = 0; i < countLog2; ++i) {
             this->data_[start + i] = rng();