LLVM Project News and Details from the Trenches

Monday, October 26, 2015

LLVM Weekly - #95, Oct 26th 2015

Welcome to the ninety-fifth issue of LLVM Weekly, a weekly newsletter (published every Monday) covering developments in LLVM, Clang, and related projects. LLVM Weekly is brought to you by Alex Bradbury. Subscribe to future issues at http://llvmweekly.org and pass it on to anyone else you think may be interested. Please send any tips or feedback to asb@asbradbury.org, or @llvmweekly or @asbradbury on Twitter.

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News and articles from around the web

The C++ Standardization Committee just finished up their most recent meeting, and STL (Stephan T. Lavavej) has provided a useful summary. Herb Sutter has also posted a trip report.

The HHVM team have posted an update on the status of LLVM code generation in HHVM. They managed to get LLVM to equal the performance of their custom backend, but are not going to deploy the LLVM backend to production for now. They're no longer actively working on the LLVM backend, but hope to ensure it doesn't regress.

Hal Finkel is proposing an LLVM social in Austin on the evening of November 15th. There should be a high density of LLVM users due to the LLVM in HPC workshop.

On the mailing lists

LLVM commits

  • The TargetLowerBase::LibCall LegalizeAction has been introduced. This allows backends to control whether they prefer expansion or conversion to a libcall. r250826.

  • The Hexagon backend continues to accumulate sophisticated target-specific optimisations. HexagonBitSimplify continues a number of transformations to perform simplifications, redundant code elimination etc. r250868.

  • The new AliasAnalysis infrastructure gained an optional 'external' AA wrapper pass, to allow users to merge in external AA results. The unit test included in the patch gives a good example of how to use this. r250894.

  • CodeGenPrepare can now transform select instructions into branches and sink expensive operands. r250743.

  • Loop rotation can now use profile data in making decisions during MachineBlockPlacement. r250754.

  • ValueTracking now has a isKnownNonEqual predicate. r251012.

Clang commits

  • Basic (currently parsing and basic semantic analysis) support for the anticipated C++1z coroutine feature was added. r250980, r250985, r250993.

  • -fvisibility=internal is now aliased to -fvisibility=hidden, as LLVM doesn't currently support internal visibility. r250954.

  • Clang's static analyzer learnt to associate hashes with found issues. This hash aims to be resilient to code changes, so should be useful for suppressing false positives. r251011.

Other project commits

  • lld gained support for lazy relocations on x86-64. r250808.

  • The new LLD ELF linker now supports the --gc-sections parameter. This increases the time to link Clang by 8% bus reduces the size of the output binary by 4%. r251043.

  • LLDB gained a REPL. r250753, r250773.

  • DWARF parsing in LLDB can now be multi-threaded, which can drastically incrase the speed of loading debug info. r251106.