MONDAY, FEBRUARY 24, 2025
libatomic Performance Win
libatomic: Use HWCAPs in AArch64 ifunc tests
Simplifies AArch64 atomic operation selection by using HWCAPs instead of low-level checks.
The libatomic library on AArch64 now uses hardware capability (HWCAP) flags to select the most efficient atomic operations (ifuncs). This replaces older, more complex checks like has_lse128() and has_rcpc3(), simplifying the code and speeding up runtime selection by avoiding expensive system register accesses.
In Details
In libatomic for AArch64, the ifunc selection for atomic operations now relies solely on HWCAPs, removing the need for explicit checks like has_lse128() and has_rcpc3(). This approach, favored by the kernel team, simplifies the host-config.h logic and optimizes the runtime dispatch mechanism by avoiding direct system register polling.
For Context
- libatomic
- A library that provides atomic built-in functions for C, C++, and Fortran, ensuring thread-safe operations on shared data.
- AArch64
- The 64-bit ARM architecture. GCC supports compiling for this popular mobile and server platform.
- ifunc
- Indirect function binding. A mechanism that allows the runtime to select the most appropriate function implementation based on the CPU's capabilities at load time.
- HWCAPs
- Hardware capabilities. Flags exposed by the kernel that indicate features supported by the CPU, used to dynamically select optimized code paths.
- LSE
- Large System Extension. An extension to the ARM architecture that provides enhanced atomic instruction support.
- RCPC3
- Likely refers to a specific ARM instruction set or feature related to atomic operations or memory ordering, possibly Release Consistent Post-cache Coherency.