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Fortran on Arm64

Availability and Standards Compliance

There are many Fortran compilers available for Arm64 including:

The NVIDIA HPC Compiler is a direct decedent of the popular PGI Fortran compiler, so it accepts the same compiler flags. GCC and LLVM operate more-or-less the same on Arm64 as on other architectures except for the -mcpu flag, as described below. The Arm Compiler for Linux is based on LLVM and includes additional optimizations for Arm Neoverse cores that make it a highly performant compiler for CPU-only applications. However, it is missing some Fortran 2008 and OpenMP 4.5+ features that may be desirable for GPU-accelerated applications.

Enabling Arm Architecture Specific Features

For gfortran on Arm64, -mcpu= acts as both specifying the appropriate architecture and tuning and it's generally better to use that vs -march or -mtune if you're building for a specific CPU. You can find additional details in this presentation from Arm Inc. to Stony Brook University.

CPU Flag gfortran version LLVM verison
Any Arm64 -mcpu=native gfortran-9+ flang/LLVM 10+
Ampere Altra -mcpu=neoverse-n1 gfortran-9+ flang/LLVM 10+

Compilers

Whenever possible, please use the latest compiler version available on your system. Newer compilers provide better support and optimizations for Arm64 processors. Many codes will demonstrate at least 15% better performance when using GCC 10 vs. GCC 7. The table below shows GCC and LLVM compiler versions available in Linux distributions. Starred version marks the default system compiler.

Distribution GCC Clang/LLVM
Ubuntu 22.04 9, 10, 11*, 12 11, 12, 13, 14*
Ubuntu 20.04 7, 8, 9*, 10 6, 7, 8, 9, 10, 11, 12
Ubuntu 18.04 4.8, 5, 6, 7*, 8 3.9, 4, 5, 6, 7, 8, 9, 10
Debian10 7, 8* 6, 7, 8
Red Hat EL8 8*, 9, 10 10
SUSE Linux ES15 7*, 9, 10 7

Large-System Extensions (LSE)

All server-class Arm64 processors support low-cost atomic operations which can improve system throughput for CPU-to-CPU communication, locks, and mutexes. On recent Arm64 CPUs, the improvement can be up to an order of magnitude when using LSE atomics instead of load/store exclusives. See Locks, Synchronization, and Atomics for details.

Enabling LSE

gfortran's -mcpu=native flag enables all instructions supported by the host CPU, including LSE. If you're cross compiling, use the appropriate -mcpu option for your target CPU, e.g. -mcpu=neoverse-n1 for the Ampere Altra CPU. You can check which Arm features gfortran will enable with the -mcpu=native flag by using this command:

gcc -dM -E -mcpu=native - < /dev/null | grep ARM_FEATURE

For example, on the Ampere Altra CPU with GCC 9.4, we see "__ARM_FEATURE_ATOMICS 1 " indicating that LSE atomics are enabled:

gcc -dM -E -mcpu=native - < /dev/null | grep ARM_FEATURE
#define __ARM_FEATURE_ATOMICS 1
#define __ARM_FEATURE_UNALIGNED 1
#define __ARM_FEATURE_AES 1
#define __ARM_FEATURE_IDIV 1
#define __ARM_FEATURE_QRDMX 1
#define __ARM_FEATURE_DOTPROD 1
#define __ARM_FEATURE_CRYPTO 1
#define __ARM_FEATURE_FP16_SCALAR_ARITHMETIC 1
#define __ARM_FEATURE_FP16_VECTOR_ARITHMETIC 1
#define __ARM_FEATURE_FMA 1
#define __ARM_FEATURE_CLZ 1
#define __ARM_FEATURE_SHA2 1
#define __ARM_FEATURE_CRC32 1
#define __ARM_FEATURE_NUMERIC_MAXMIN 1

Checking for LSE in a binary

To confirm that LSE instructions are used, the output of the objdump command line utility should contain LSE instructions:

$ objdump -d app | grep -i 'cas\|casp\|swp\|ldadd\|stadd\|ldclr\|stclr\|ldeor\|steor\|ldset\|stset\|ldsmax\|stsmax\|ldsmin\|stsmin\|ldumax\|stumax\|ldumin\|stumin' | wc -l

To check whether the application binary contains load and store exclusives:

$ objdump -d app | grep -i 'ldxr\|ldaxr\|stxr\|stlxr' | wc -l