Linux 3.15 Can Almost Be Compiled Under LLVM's Clang

A few hours ago I wrote about the most interesting features for the Linux 3.15 kernel from my perspective as it didn't look like anything else interesting would be introduced this late in the merge window before the imminent 3.15-rc1. However, this time I've been happily proven wrong with Clang patches being added to the Linux 3.15 kernel.

Written about too many times to recall on Phoronix has been the four year long effort to build the Linux kernel with LLVM's Clang rather than being bound to using GCC. While the GNU Compiler Collection is very mature, very well optimized for performance, and supports tons of different CPU architectures, many different developers have been interested in "Clang'ing" the Linux kernel. Building the Linux kernel with Clang can lead to better code portability/quality against compilers, Clang is widely accepted as being faster than GCC at compiling code, there's room for possible performance advantages, Clang has a great set of static analysis tools, and there's many other reasons for building the Linux kernel with Clang.

When the initiative got underway years ago -- now formalized as LLVMLinux -- many patches were needed to both LLVM/Clang and the Linux kernel for building a working kernel, albeit even with the "working" kernel there's been several issues and problems with certain modules and other tasks. Over time, all of the LLVM and Clang patches have merged upstream while there's still been many outstanding patches for the Linux kernel to workaround GCC-specific code or handle code not supported by Clang in other ways. With Linux 3.15, the upstream Linux kernel is almost in good shape for being built under this alternate, BSD-licensed compiler.

Linus Torvalds on Saturday afternoon pulled a number of patches to better support compiling the kernel by Behan Webster, one of the developers long involved with this kernel compiler effort. Behan explained, "These are some initial updates to support compiling the kernel with clang. These patches have been through the proper reviews to the best of my ability, and have been soaking in linux-next for a few weeks. These patches by themselves still do not completely allow clang to be used with the kernel code, but lay the foundation for other patches which are still under review. Several other of the LLVMLinux patches have been already added via maintainer trees."

So we're not quite there yet, but almost. Hopefully by the Linux 3.16 kernel we'll be able to compile it against LLVM/Clang rather than only GCC. More information overall on this effort can be found via the LLVMLinux project page hosted by the Linux Foundation.