The New Compiler Features Of LLVM 10.0 / Clang 10.0
Written by Michael Larabel in LLVM on 8 March 2020 at 10:26 AM EDT. 4 Comments
LLVM --
After running behind schedule from the planned release last month and an extra release candidate being warranted, LLVM 10.0 should be releasing this coming weeks along with its sub-projects -- most notably, the Clang 10.0 C/C++ compiler. Here is a look at the big ticket items of LLVM/Clang 10.0.

The release of LLVM/Clang 10.0 is expected in the coming days while GCC 10 will be releasing in the next few weeks. As for the changes with this half-year update to this innovative compiler infrastructure, LLVM 10.0 highlights include:

- For Intel AVX-512 CPUs, -mprefer-vector-width=256 is now the default behavior for limiting the use of 512-bit registers due to the AVX-512 downclocking that can occur. This matches the behavior of GCC now while those wanting the previous behavior can pass -mprefer-vector-width=512 if wanting to increase the use of 512-bit registers but with possible performance implications from the AVX-512 frequency impact.

- AMD Znver2 (Zen 2) improvements.

- An option to help with the JCC microcode erratum impact.

- Support for Arm's Cortex-A65, A65AE, Neoverse N1, and Neoverse E1 cores.

- Octeon+ MIPS CPUs are now supported and improved support for existing Octeon processors.

- IBM z15 target support.

- Besides new Arm CPU targets, the AArch64 back-end for LLVM 10 also has more optimized ARMv8.1-M code generation, auto-vectorization for the ARMv8.1-M MVE vector extension, and other improvements.

- IBM POWER has seen a number of improvements too including better register pressure estimates, improved cost model for the vectorizer, vectorization of math routines using the IBM MASSV library, and other enhancements.

- LLVM's WebAssembly target has much better SIMD support, thread-local storage (TLS) now works, and other support improvements.

- Many improvements to RISC-V's LLVM support.

- LLDB can now handle debugging Windows ARM/ARM64 binaries and also has better support for being built by MinGW.

- MLIR landed as the promising new IR being picked up by an increasing number of projects.

- Numerous AMDGPU LLVM back-end improvements.

The Clang 10.0 C/C++ front-end meanwhile has going for it:

- Expanded C++20 support, including C++ Concepts support and other features, but the support isn't yet finished in full.

- A variety of diagnostics improvements with continuing to ramp up the usefulness of their warnings and ensuring they are accurate.

- Skylake-AVX512/Icelake/Cascadelake/Cooperlake targets will now default to not using 512-bit ZMM registers in vectorized code unless 512-bit intrinsics are used in the source code, due to the AVX-512 frequency hit that can lead to lower performance. Similar to the LLVM change, -mprefer-vector-width=512 can be used to restore the previous behavior.

- When building for WebAssembly, wasm-opt will be run if found on the system for helping to reduce the generated code size.

- Various other changes to better match GCC's behavior for different commands and outputs.

- Various minor improvements to Clang's OpenCL C/C++ kernel language support.

- Expanded OpenMP 5.0 support including for range-based loops, collapsing of imperfectly nested loops, unified shared memory for NVIDIA NVPTX, and other additions.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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