LLVM Weekly - #112, Feb 22nd 2016

Welcome to the one hundred and twelfth 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 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

Filip Pizlo has written a fantastic article introducing the new B3 JIT compiler for WebKit's JavaScriptCore. This intends to replace LLVM as the optimising backend to their fourth-tier JIT. The article describes in detail their reasons for moving away from LLVM (mainly compile-time) and the design trade-offs made, such as in reducing memory allocations and minimising pointer-chasing in the IR. This reminds me of the trade-offs Mike Pall made in the LuaJIT 2.0 IR. Philip Reames also shared some initial thoughts on B3. I know some people have expressed disappointment about WebKit moving away from LLVM, but if you'll allow me to insert just a little bit of editorial I'd argue B3 is a very positive development for LLVM and the wider compiler community. B3 explores a different set of design trade-offs to those chosen for LLVM, and these sort of changes are probably easiest to explore in a fresh codebase.Thanks to this write-up (and hopefully future B3/AIR documentation), we can learn from the B3 developers' experiences and consider if some of their choices will make sense for LLVM. It's also good to remember that LLVM isn't the only feasible route for code generation and optimisation, and we shouldn't treat LLVM's design choices as the one-true way to do things. Impressively, B3 was developed to its current state in only 6 months of developer-time.

Version 0.17.0 of LDC, the LLVM-based compiler for the D programming language has been released. You can view a detailed changelog here.

GCC6 will feature a whole bunch of new warnings, and this blog post details many of them.

The schedule for EuroLLVM 2016 has now been posted. This will be held March 17th-18th in Barcelona.

On the mailing lists

• Bob Wilson proposes that format-security warnings in Clang default to error. Nico Weber posted a handy summary of the thread.

• Sanjoy Das has posted an RFC on adding guard intrinsics to LLVM. These would be used in a similar way to the Check opcode in WebKit's new B3 compiler.

• Alina Sbirlea proposes adding bitcode tests to the LLVM test-suite. Hal Finkel suggests going further and just pulling Halide in to the LLVM test-suite as a front-end example that should provide greater test coverage.

• Andrew Trick shared some thoughts on LLVM in light of the WebKit B3 announcement. "Even when LLVM's compile time problems are largely solved, and I believe they can be, there will always be systemic compile time and memory overhead from design decisions that achieve generality, flexibility, and layering. These are software engineering tradeoffs."

LLVM commits

• The PPCLoopDataPrefetch pass has been moved to Transforms/Scalar/LoopDataPrefetch in preparation for it becoming a target-agnostic pass. r261265.

• The cmpxchg LLVM instruction now allows pointer type operands. r261281.

• The X86 backend gained support for a new stack symbol ordering optimisation. This is primarily intended to reduce code size, and produces small but measurable improvements across some SPEC CPU 2000 benchmarks. r260917.

• The LLVM C API has been extended to allow it to be used to manipulate the datalayout. r260936.

• Some major work on the LazyCallGraph has been checked in. r261040.

• The AMDGPU backend gained a basic disassembler. r261185.

• The PostOrderFuctionAttrs pass has been ported to the new pass manager. As described in the commit message, this actually represents a major milestone. r261203.

• The Hexagon backend gained support for thread-local storage. r261218.

Clang commits

• A nullPointerConstant AST matcher was added. r261008.

• Clang gained a -Wcomma warning, which will warn for most uses of the builtin comma operator. r261278

Other project commits

• LLD has sprouted a release notes document. r260960.

• The LLVM test-suite's CMake build system saw a number of fixes for SPEC. r261470.