The State Of 64-Bit ARM (AArch64) On LLVM/Clang

ARM's AArch64 back-end for LLVM to handle the 64-bit ARMv8 architecture is working, but there's still more work ahead of the hardware's general availability in about one year's time.

There's AArch64 in GCC's code-base with version 4.8 following months of development and its approval by the steering committee. Initial AArch64 architecture support was also merged into the Linux 3.7 kernel. However, on the LLVM/Clang side there hasn't been much public-facing news.

Earlier this month, however, at the LLVM Developers' Meeting in San Jose, ARM Holdings provided an update on their AArch64 back-end. ARM engineers have been working on a 64-bit ARMv8 LLVM back-end targeting ELF output on Linux with an integrated assembler by default, using all of the latest LLVM APIs, strong-testing, and compiling standard-compliant C/C++ code. However, of expressed lower priority were optimizations and not commonly used features of the C and C++ languages. ARM will ultimately focus upon AArch64 compiler optimizations, but for now they just want working executables.

In the San Jose meeting, ARM shared that their C++98 and C99 coverage is now well supported and that SPEC2000 and SPEC2006 tests are being built and running successfully. Self-built Clang and LLVM binaries also pass the regression test suite. There's also NEON work for ARM64 that's still going on, a few failures within the LLVM test-suite, and MC Hammer passing on just scalar instructions.

For the AArch64 back-end so far it required about one and a half months to work on the basic layout, four months for implementing instructions systematically, and about another four months of integration work.

More information on the AArch64 LLVM back-end status can be found from the PDF slides.