Optimization of a real world application BLAST is used to demonstrate the limitations of static and profile-guided optimizations and to highlight the potential of runtime optimization systems. We analyze the performance profile of this application to determine performance bottlenecks and evaluate the effect of aggressive compiler optimizations on BLAST. We find that applying common optimizations (e.g. O3) can degrade performance. Profile guided optimizations do not show much improvement across the board, as current implementations do not address critical performance bottlenecks in BLAST. In some cases, these optimizations lower performance significantly due to unexpected secondary effects of aggressive optimizations. We also apply runtime optimization to BLAST using the ADORE framework. ADORE is able to detect performance bottlenecks and deploy optimizations resulting in performance gains up to 58% on some queries using data cache prefetching.

Optimizing programs at run-time provides opportunities to apply aggressive optimizations to programs based on information that was not available at compile time. At run time, programs can be adapted to better exploit architectural features, optimize the use of dynamic libraries, and simplify code based on run-time constants. Our profiling system provides a framework for collecting information required for performing run-time optimization. We sample the performance hardware registers available on an Itanium processor, and select a set of code that is likely to lead to important performance-events. We gather distribution information about the performance-events we wish to monitor, and test our traces by estimating the ability for dynamic patching of a program to execute run-time generated traces. Our results show that we are able to capture 58% of execution time across various SPEC2000 integer benchmarks using our profile and patching techniques on a relatively small number of frequently executed execution paths. Our profiling and detection system overhead increases execution time by only 2-4%.

Traditional software controlled data cache prefetching is often ineffective due to the lack of runtime cache miss and miss address information. To overcome this limitation, we implement runtime data cache prefetching in the dynamic optimization system ADORE (ADaptive Object code Reoptimization). Its performance has been compared with static software prefetching on the SPEC2000 benchmark suite. Runtime cache prefetching shows better performance. On an Itanium 2 based Linux workstation, it can increase performance by more than 20% over static prefetching on some benchmarks. For benchmarks that do not benefit from prefetching, the runtime optimization system adds only 1%-2% overhead. We have also collected cache miss profiles to guide static data cache prefetching in the ORC compiler. With that information the compiler can effectively avoid generating prefetches for loops that hit well in the data cache.