LLVM Weekly - #4, Jan 27th 2014

Welcome to the fourth issue of LLVM Weekly, a weekly newsletter (published every Monday) covering developments in LLVM, Clang, and related projects. This marks the end of the first month of operation, here's to many more! 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. I've been keeping the @llvmweekly Twitter account updated throughout the week, so follow that if you want more frequent news updates.

News and articles from around the web

The biggest compiler-related news this week has been the discussions on the GCC mailing list. Things kicked off with Eric S. Raymond's post suggesting that technical progress in GCC is being held back by concerns about reusing parts of GCC in a way that bypasses the copyleft license. Ian Lance Taylor responded to point out that GCC now has a plugin system, albeit with an unstable interface, which mostly put a stop to that line of discussion. However a later post to the mailing list from Richard Stallman has proved very controversial by claiming that "The existence of LLVM is a terrible setback for our community precisely because it is not copylefted and can be used as the basis for nonfree compilers". There's plenty of discussion of these comments around the web at LWN, Hacker News, Reddit, Slashdot etc. Although many of us may have a preference for non-copyleft ('permissive') free software licenses, RMS has consistently and over a long period of time argued that copyleft licenses ultimately do a better job of spreading free software and preserving its freedom. As such, it's not clear to me why this mailing list post has come as a surprise to many. I'm personally surprised he didn't bring up the fact that the BSD-style license used by LLVM contains no explicit patent grant (though LLVM does have a patent policy to help protect its users).

Rapidly moving away from controversial topics, an exciting milestone for the LLVM project was hit this week. The 200000th commit has been applied. Takumi Nakamura was lucky enough to be the one to author that commit.

The Khronos group has released the SPIR 1.2 specification. SPIR is a standardised intermediate representation meant for use with OpenCL, and is based on LLVM 3.2 IR. With the release, the Khronos Group have open sourced a modified Clang 3.2 which can generate SPIR from OpenCL C programs as well as a module verifier.

Joaquín M López Muñoz has published a benchmark comparing hash table performance on Clang. He compares GCC's libstdc++-v3 to the LLVM project's libc++.

The Cambridge (UK) LLVM socials are starting up again, with the next one on the 29th Jan at 7.30pm. Sadly I can't make it, hopefully the next one!

On the mailing lists

• Chandler Carruth asks whether LoopPass and LoopPassManager can be removed. He lists the confusion and issues they cause, most damning to me is that they reoutinely modify IR from outside of the loop thus breaking the implied contract of a 'loop pass'. The discussion ended up with the proposal that LoopSimplify, LCSSA, and LoopVectorizer become function passes. As you can see from the LLVM commit section below, Chandler quickly submitted patches to do this which have been applied.

• Vadim asks for input on his idea of using LLVM to generate code with position-independent stacks. This would be usual for implementing something similar to Python greenlets. Mark Seaborn points to some similar work he has done which restricts memory access to a range of address space.

• Tom Stellard has completed a suggested policy and process for LLVM minor releases.

• Shea Levy, a maintainer for nixpkgs writes to the list with the issues he has encountered packaging LLVM and Clang for Nix.

• Raul Silvera asks for opinions on removing the ReadOnly attribute from math intrinsics on the grounds that changes to the floating point environment (such as rounding mode) aren't modelled anyway. As Raul points out in a later followup "We currently appear to be at the worst of both worlds as we do not have support for this environment but are always being constrained by it."

• Sebastien Riou asks how to force a MachineFunctionPass to be the last one. Peter Cooper suggests that at the moment you can add the pass to the end of X86PassConfig::addPreEmitPass or add the pass after the call to preEmitBass and before the ASM printer is added.

• Jasper Neumann has spotted various cases where LLVM leaves unnecessary register moves in the generated code. Andrew Trick suggests copy-eliminating peepholes could be added after register allocation, but that in general there is reluctance to introduce extra complexity to optimise special cases involving multiple physical register copies due to the small impact they have on performance.

• David Chisnall enquires whether LLVM relies on the assumption that the stack is in address space 0. Matt Arsenault notes that OpenCL local variables currently work by created globals rather than alloca'ing a nondefault address space. He suggests the limitation stems from the fact a number of places in the compiler assume the address space is 0 and never check by retrieving the address space of the relevant pointer.

• Christian Schafmeister has posted a preannouncement of his soon-to-be-released C++ refactoring tool, written in Lisp. It makes use of clang's ASTMatcher/Refactoring library. I'm looking forward to the promised open source release.

• David Fang has posted a summary of the status of libc++ on powerpc-darwin.

LLVM commits

• LoopSimplify is no longer a LoopPass, instead it is both a utility function and a FunctionPass. The motivation was to be able to compute function analysis passes after running LoopSimplify, but the change has a bunch of other advantages described in detail in the commit message. r199884. Additionally, the LCSSA (loop-closed SSA) pass was made a utility with a function pass and the LoopVectorizer became a FunctionPass. r200067, r200074.

• The Constant Hoisting Pass was born. r200022.

• InstCombine learned how to deal with vectors for most fmul/fvid/add/sub/mul/div combines. r199598, r199602.

• Type-based alias analysis has, for the time being, been disabled when using alias analysis in CodeGen due to two shortcomings described in the commit message. r200093.

• LTO gained new methods which allows the user to parse metadata nodes, extract linker options, and extract dependent libraries from a bitcode module. r199759.

• The Sparc backend now supports the inline assembly constraint 'I'. r199781.

• The x86 backend allows segment and address-size overrides for movs/lods/outs, fixing bug 9385. r199803 and more.

• llvm-ar no longer opens or fstats file twice. r199815.

• When compiling a function with the minsize attribute, the ARM backend will now use literal pools even for normal i32 immediates. r199891.

• There was a fair bit of activity on the R600 backend. I haven't had the time to properly summarise that activity or pick out the most important commits, so I recommend those interested take a look through the commit logs.

• JIT is now supported for Sparc64. r199977.

• llvm-readobj gained support for the PE32+ format (used for Windows 64-bit executables). r200117.

Clang commits

• Registry::getCompletions was implemented. This returns a list of valid completions for a given context. r199950.

• Clang gained basic support for the attribute returns_nonnull. r199626, r199790.

• getResultType on function and method declarations was renamed to getReturnType which is a semantically more accurate name. r200082. Similarly, getResultLoc was renamed to getReturnLoc. r200105.

• All applicable accessors in FunctionProtoType have been renamed from *argument* to *parameter*. r199686.

• Clang was taught to look in its installation libdir for libraries such as libc++ when the installation is within the system root. r199769.

• A module.map file is now required to load a module. r199852.

Other project commits

• lldb learned the 'step-avoid-libraries' setting, which allows a user to list libraries to avoid. r199943.

• In compiler-rt, support was added for intercepting and sanitizing arguments passed to printf functions in AddressSanitizer and ThreadSanitizer. r199729.

• A fix was committed to ThreadSanitizer to prevent deadlocking after a fork. r199993.

• Dragonegg can now be built with CMake. r199994.

• Compiler-rt gained support in its udiv/umod implementations for ARMv4 which lacks bx and clz. Code changes also resulting in a 30%+ performance improvement on the Raspberry Pi (armv7, ARM1176) and 5-10% on a Cortex A9. r200001.

• In AddressSanitizer on Android, all AddressSanitizer output is duplicated to the system log. r199887.

• lld gained support for emitting a PE32+ file header. r200128.

• lldb now supports Haswell on x86-64. r199854.