LLVM Weekly - #2, Jan 13th 2014

Welcome to the second issue of LLVM Weekly, a weekly newsletter (published every Monday) covering developments in LLVM, Clang, and related projects. Thank you to everyone who has given positive feedback or suggestions, I'm planning in future editions to integrate some sort of statistics of git activity and contributions as well as activity on Bugzilla. LLVM Weekly is brought to you by Alex Bradbury. Subscribe to future issues at https://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.

News and articles from around the web

Stephen Diehl has published a Haskell adaptation of the 'Kaleidoscope' tutorials on how to implement aJIT compiled language using LLVM. This was received very positively and prompted some discussion on HN.

Registration for EuroLLVM is now open. There's still time to submit your talk, poster or workshop proposals too.

Rust 0.9 has been released. Rust is a systems programming language which targetting LLVM with a particular emphasis on type safety, memory safety and concurrency. One of the core Rust developers at Mozilla, Niko Matsakis, also gave a talk at linux.conf.au introducing Rust.

Coverity 7 now includes Clang compiler support.

On the mailing lists

• Apple are advertising for interns with their LLVM compiler frontend and runtime teams this summer. Note internships are restricted to students.

• Amara Emerson shares a proposal on handling the removal of the -mfpu option for AArch64. This proposal aims to match gcc, where instead of -mfpu to specify FPU/NEON, the same information will be passed using feature modifiers on -march. It was pointed out that GCC also has -mcpu to complicate things. It's hoped collaboration with the GCC devs will lead to a simplification that both can use.

• Tobias Grosser has set up some new machines tracking performance at -O3 using LNT, which is a framework used for performance tracking of LLVM over time. Tobias is asking for someone to step in and improve the LNT infrastructure to take advantage of this new data. LNT is written in Python, using Flask and SQLAlchemy so this would be an ideal project for someone interested in contributing to LLVM but perhaps doesn't match the typical profile of the LLVM contributor.

• Timothy Baldridge asks whether LLVM can inline external library functions in a JITted method. Replies point out that for the specific example given (memcpy), LLVM is already able to inline. Kevin Modzelewski describes a technique he's used.

• Rafael Espíndola announces a new milestone, all in-tree backends now use MCInst for printing assembly. Out of tree targets will need to lower MachineInstr to MCInst to use the new printer and the recent conversion of the Sparc backend to do this is probably as good a guide as any.

• As part of the effort to use C++11 inside the LLVM codebase, Chandler Carruth has asked anyone running build-bot to look at upgrading to the new baseline compiler versions. These are MSVC 2012, GCC 4.7, or Clang 3.1.

• Jasper Neumann has been working on switch lowering using hashing as an alternative to using a jump table. He later further elaborates on the technique and points to an interesting looking paper, Multiway Branch Code Generation by Roger Anthony Sayle.

• Kevin Modzelewski is encountering disappointing bitcode parsing performance, in particular when debug info is present. The issue is not identified or solved yet, I'm hoping to be able to point to an explanation of the problem in next week's issue!

• Chandler Carruth suggests that the interprocedual analysis passes be merged in to the Analysis library. Right now, various interprocedural analyses live in lib/Analysis/IPA and are built as the LLVMipa library. Chris Lattner agrees so we can expect the merge to happen in the not too distant future.

• Chandler Carruth has suggested that the LLVM coding standards be updated so that namespaces are never indented, as opposed to the current confusing mixture. Despite the fact that debating whitespace is a favourite pastime of most programmers, the thread has been fairly uncontroversial thus far.

• Milian Wolff wonders about using the clang C API to parse default values.

• Renato Golin is working towards adding various pragmas to Clang and is seeking feedback on how to implement it. A few people on the mailing list suggest it might be worth trying to instead implement these using C++11 generalized attributes and perhaps enabling support for the syntax in C99.

• The __has_attribute feature macro is commonly used to detect support for a particular attribute in the compiler. Aaron Ballman suggests extending it so you can test for the precise form of attribute syntax you are after.

LLVM commits

• The Sparc ASM parser has seen further development, learning to parse branch instructions, conditional moves, and floating point instructions and more. It also gained an initial disassembler implementation. r198738, r199033, r198591

• In SimplifyCFG will now understand that when building a lookup table for a switch with no holes that it doesn't need a default result. According to the commit message, this saves 164KB off the clang binary in a bootstrap build. r199025

• The new pass manager saw further developments. r198998 and many others (prefixed with [PM] in the commit logs)

• The SampleProfile pass saw further development. Profile samples are propagated through the CFG, heuristically converting instruction samples to branch weights. Work was done to extend and simplify the format of sample profile input files. r198972, r198973

• The LoopVectorizer can use versioning for strided memory access. r198950

• In DataLayout, 'w' is now used rather than 'c' for win32 name mangling. r198938

• The LLVM developer policy was updated to more clearly spell out how to submit patches to the project. r198901

• WriteAsOperand has been killed in favour of the the printAsOperand method. r198836

• The x86 backend learned some more AVX-512 intrinsics. r198745

Clang commits

• Support for the enable_if attribute was added. This can be placed on function declarations to control which overload is selected based on the values of the function's arguments. See the commitdiff (particularly the docs change) for more info. r198996

• More work on MS VC++ ABI compatibility was committed. r198907, r198975, r198977, r198900 and more.

• __has_attribute now understands target-specific attributes. r198897

• The clang plugin infrastructure is now covered by at least some tests. r198747

• The documentation on clang internals was updated with more up to date information on adding new attributes. r198705

• An initial instrumentation-based profile guided optimisation implementation landed. r198640