Loop Tiling in the Presence of Exceptions

Exceptions in OO languages provide a convenient mechanism to deal with anomalous situations. However, many of the loop optimization techniques cannot be applied in the presence of conditional throw statements in the body of the loop, owing to possible cross iteration control dependences. Compilers either ignore such throw statements and apply traditional loop optimizations (semantic non-preserving), or conservatively avoid invoking any of these optimizations altogether (inefficient). We define a loop optimization to be xception-safe, if the optimization can be applied even on (possibly) exception throwing loops, in a semantics preserving manner. In this paper, we present a generalized scheme to do exception-safe loop optimizations and present a scheme of optimized exception-safe loop tiling (oESLT), as a specialization thereof. oESLT tiles the input loops, assuming that exceptions will never be thrown. To ensure the semantics preservation (in case an exception is thrown), oESLT generates code to rollback the updates done in the advanced iterations (iterations that the unoptimized code would not have executed, but executed speculatively by the oESLT generated code) and safely-execute the delayed iterations (ones that the unoptimized code would have executed, but not executed by the code generated by oESLT). For the rollback phase to work efficiently, oESLT identifies a minimal number of elements to backup and generates the necessary code. We implement oESLT, along with a naive scheme (nESLT, where we backup every element and do a full rollback and safe-execution in case an exception is thrown), in the Graphite framework of GCC 4.8. To help in this process, we define a new program region called ESCoPs (Extended Static Control Parts) that helps identify loops with multiple exit points and interface with the underlying polyhedral representation. We use the popular PolyBench suite to present a comparative evaluation of nESLT and oESLT against the unoptimized versions

On-stack replacement, distilled

On-stack replacement (OSR) is essential technology for adaptive optimization, allowing changes to code actively executing in a managed runtime. The engineering aspects of OSR are well-known among VM architects, with several implementations available to date. However, OSR is yet to be explored as a general means to transfer execution between related program versions, which can pave the road to unprecedented applications that stretch beyond VMs. We aim at filling this gap with a constructive and provably correct OSR framework, allowing a class of general-purpose transformation functions to yield a special-purpose replacement. We describe and evaluate an implementation of our technique in LLVM. As a novel application of OSR, we present a feasibility study on debugging of optimized code, showing how our techniques can be used to fix variables holding incorrect values at breakpoints due to optimizations

Progress of Delhi Light Source at IUAC, New Delhi

The first phase of the pre-bunched Free Electron Laser (FEL) based on the RF electron gun, has been initiated at Inter University Accelerator Centre (IUAC), New Delhi. The photoinjector-based electron gun made from OFHC copper was fabricated and tested with low power RF. The beam optics calculation by using ASTRA and GPT codes are performed and radiation produced from the pre-bunched electron bunches are being calculated. The high-power RF system was ordered and will be commissioned at IUAC by the beginning of 2018. The design of the laser system is being finalised and assembly/testing of the complete laser system will be started soon in collaboration with KEK, Japan. The initial design of the photocathode deposition system has been completed and its procurement/development process is also started. The first version of the undulator magnet design is completed and its further im-provements are underway. The initial design of the DLS beam line have been worked out and various beam diagnostics components are being finalised. Production of the electron beam and THz radiation is expected by 2018 and 2019, respectively