License: Creative Commons Attribution 3.0 Unported license (CC-BY 3.0)
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DOI: 10.4230/LIPIcs.ECOOP.2015.124
URN: urn:nbn:de:0030-drops-52202
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Bhandari, Abhilash ; Nandivada, V. Krishna

Loop Tiling in the Presence of Exceptions

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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.

BibTeX - Entry

  author =	{Abhilash Bhandari and V. Krishna Nandivada},
  title =	{{Loop Tiling in the Presence of Exceptions}},
  booktitle =	{29th European Conference on Object-Oriented Programming (ECOOP 2015)},
  pages =	{124--148},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-939897-86-6},
  ISSN =	{1868-8969},
  year =	{2015},
  volume =	{37},
  editor =	{John Tang Boyland},
  publisher =	{Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{},
  URN =		{urn:nbn:de:0030-drops-52202},
  doi =		{10.4230/LIPIcs.ECOOP.2015.124},
  annote =	{Keywords: Compiler optimizations, semantics preservation, exceptions, loop-tiling}

Keywords: Compiler optimizations, semantics preservation, exceptions, loop-tiling
Collection: 29th European Conference on Object-Oriented Programming (ECOOP 2015)
Issue Date: 2015
Date of publication: 29.06.2015

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