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C/C++ on Graviton

Enabling Arm Architecture Specific Features

To build code with the optimal processor features use the following. If you want to support both Graviton and Graviton2 you'll have to limit yourself to the Graviton features.

CPU GCC LLVM
Graviton -march=armv8-a+crc+crypto -march=armv8-a+crc+crypto
Graviton2 -march=armv8.2-a+fp16+rcpc+dotprod+crypto -march=armv8.2-a+fp16+rcpc+dotprod+crypto

Note: GCC-7 does not support +rcpc+dotprod.

Core Specific Tuning

CPU GCC < 9 GCC >=9
Graviton -mtune=cortex-a72 -mtune=cortex-a72
Graviton2 -mtune=cortex-a72 -mtune=neoverse-n1

Compilers

Newer compilers provide better support and optimizations for Graviton processors. We have seen 15% better performance on Graviton2 when using gcc-10 instead of Amazon Linux 2 system's compiler gcc-7. When possible please use the latest compiler version available on your system. The table shows GCC and LLVM compiler versions available in Linux distributions. Starred version marks the default system compiler.

Distribution GCC Clang/LLVM
Amazon Linux 2 7*, 10 7, 11*
Ubuntu 20.04 7, 8, 9*, 10 6, 7, 8, 9, 10, 11
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)

The Graviton2 processor in C6g, M6g, and R6g instances has support for the Armv8.2 instruction set. Armv8.2 specification includes the large-system extensions (LSE) introduced in Armv8.1. LSE provides low-cost atomic operations. LSE improves system throughput for CPU-to-CPU communication, locks, and mutexes. The improvement can be up to an order of magnitude when using LSE instead of load/store exclusives.

POSIX threads library needs LSE atomic instructions. LSE is important for locking and thread synchronization routines. The following systems distribute a libc compiled with LSE instructions:

  • Amazon Linux 2,
  • Ubuntu 18.04 (needs apt install libc6-lse),
  • Ubuntu 20.04.

The compiler needs to generate LSE instructions for applications that use atomic operations. For example, the code of databases like PostgreSQL contain atomic constructs; c++11 code with std::atomic statements translate into atomic operations. GCC's -march=armv8.2-a flag enables all instructions supported by Graviton2, including LSE. To confirm that LSE instructions are created, the output of 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

GCC's -moutline-atomics flag produces a binary that runs on both Graviton and Graviton2. Supporting both platforms with the same binary comes at a small extra cost: one load and one branch. To check that an application has been compiled with -moutline-atomics, nm command line utility displays the name of functions and global variables in an application binary. The boolean variable that GCC uses to check for LSE hardware capability is __aarch64_have_lse_atomics and it should appear in the list of symbols:

$ nm app | grep __aarch64_have_lse_atomics | wc -l
# the output should be 1 if app has been compiled with -moutline-atomics

GCC 10.1+ enables -moutline-atomics by default as does the version of GCC 7 used by Amazon Linux 2.

Porting codes with SSE/AVX intrinsics to NEON

When programs contain code with x64 intrinsics, the following procedure can help to quickly obtain a working program on Arm, assess the performance of the program running on Graviton processors, profile hot paths, and improve the quality of code on the hot paths.

To quickly get a prototype running on Arm, one can use https://github.com/DLTcollab/sse2neon a translator of x64 intrinsics to NEON. sse2neon provides a quick starting point in porting performance critical codes to Arm. It shortens the time needed to get an Arm working program that then can be used to extract profiles and to identify hot paths in the code. A header file sse2neon.h contains several of the functions provided by standard x64 include files like xmmintrin.h, only implemented with NEON instructions to produce the exact semantics of the x64 intrinsic. Once a profile is established, the hot paths can be rewritten directly with NEON intrinsics to avoid the overhead of the generic sse2neon translation.

Signed vs. Unsigned char

The C standard doesn't specify the signedness of char. On x86 char is signed by default while on Arm it is unsigned by default. This can be addressed by using standard int types that explicitly specify the signedness (e.g. uint8_t and int8_t) or compile with -fsigned-char.

Using Graviton2 Arm instructions to speed-up Machine Learning

Graviton2 processors been optimized for performance and power efficient machine learning by enabling Arm dot-product instructions commonly used for Machine Learning (quantized) inference workloads, and enabling Half precision floating point - _float16 to double the number of operations per second, reducing the memory footprint compared to single precision floating point (_float32), while still enjoying large dynamic range.

Using Graviton2 Arm instructions to speed-up common code sequences

The Arm instruction set includes instructions that can be used to speedup common code sequences. The table below lists common operations and links to code sequences:

Operation Description
crc Graviton processors support instructions to accelerate both CRC32 which is used by Ethernet, media and compression and CRC32C (Castagnoli) which is used by filesystems.