LLVM and the Automatic Vectorization of Loops Invoking Math Routines: -fsimdmath
Authors: Francesco Petrogalli (ARM Ltd)
Abstract: The vectorization of loops invoking math function is an important optimization that is available in most commercial compilers. This paper describes a new command line option, -fsimdmath, available in Arm Compiler for HPC, that enables auto-vectorization of math functions in C and C++ code, and that will also be applicable to Fortran code in a future version.
The design of -fsimdath is based on open standards and public architectural specifications. The library that provides the vector implementation of the math routines, libsimdmath.so, is shipped with the compiler and based on the SLEEF library libsleefgnuabi.so. SLEEF is a project that aims to provide a vector implementation of all C99 math functions, for a wide variety of vector extensions and architectures, across multiple platforms.
This feature is very important for HPC programmers, because the vector units of new CPUs are getting wider. Whether you are targeting Intel architectures with the AVX512 vector extension, or Arm architectures with the Scalable Vector Extension, good quality auto-vectorization is of increasing importance.
Although -fsimdmath has been implemented in a commercial compiler, it has been designed with portability and compatibility in mind, so that its use is not limited only to the vector extensions of the Arm architectures, but can be easily introduced as a major optimization for all the vector extensions that LLVM supports.
If accepted upstream, this new feature will enlarge the set of loops that LLVM will be able to auto-vectorize.
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