Ionut Balosin - Runtime vs. compile time (JIT vs AOT) optimizations in Java and C++

The talk reveals how Just In Time Compiler (e.g. JIT C2) from HotSpot/OpenJDK internally manages runtime optimizations for hot methods in comparison to ahead of time approach triggered by LLVM clang on similar C++ source code, emphasizing all of the internals and strategies used by each Compiler to achieve better performance.

For each optimization there is similar Java and C++ source code and corresponding generated assembly code in order to prove what really happens under the hood. Each test is covered by a dedicated language benchmark and conclusions. Main topics of the agenda:

Different sequential sums (e.g. N elements array, N integers, two arrays, etc.)
Loop unrolling, loop peeling
Fields object layout
Null checks
Uncommon traps
Lock coarsening
Lock elision
Virtual calls
Scalar replacement
Concurrency implications (e.g. memory access optimization which are prevented in case inlining do not happen). …
The tools used during our research study are: JITWatch, Java Measurement Harness, C++ Google Benchmark and perf. All test scenarios are launched against latest official Java release (e.g. 9.0.1) and a recent LLVM clang version (e.g. 5.0.0).