The New Features Of LLVM 4.0 & Clang 4.0
LLVM/Clang 4.0 is a big version bump even though it's just been another six month cycle for the project. Beginning with 4.0, they are moving to a new versioning scheme where they will be bumping the major version number each time, so the next feature update will be LLVM/Clang 5.0. New changes to LLVM 4.0 include:
- The GVNHoist pass is enabled by default, which is a "new pass based on Global Value Numbering detects similar computations in branch code and replaces multiple instances of the same computation with a unique expression." GVNHoist can benefit code size and generate better schedules, especially for -Os and -Oz modes.
- Experimental support for LLVM coroutines.
- ARM64/AArch64 for LLVM picked up XRay support, refined support for Qualcomm Falkor and Samsung Exynos processors, and various other updates. The 32-bit ARM back-end also picked up a variety of improvements. The LLD linker support for ARM is also much better off.
- The AVR back-end is new to the mainline LLVM code-base.
- FP16 half-precision support for AMDGPU, among other improvements to the AMDGPU back-end.
- Various LLVM API changes.
Changes pertaining to the Clang 4.0 C/C++ compiler front-end include:
- A number of fixes pertaining to the OpenCL C language support, including OpenCL 2.0 fixes. There is also cl_khr_mipmap_image as one of the newly-supported CL extensions.
- Similar to GCC, the -Og optimization level is now supported for optimizing the debugging experience while otherwise being similar to -O1.
- There's also a new -MJ flag for JSON compilation database output to write a compilation database entry per input.
- Improvements to the -flto=thin (ThinkLTO) mode.
- New checks were added to the Clang static analyzer.
Stay tuned for more LLVM/Clang 4.0 benchmarks on Phoronix shortly. More details on the 4.0 changes via the release_40 branches.
Add A Comment
