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Arm M-Profile Assembly Tricks

Published at
Mar 13, 2025
Main Article
https://github.com/pornin/arm-asm-notes

ARM Assembly Tricks

This repository contains the description of various tricks and other optimization notes that can be used to make assembly implementations smaller and/or faster on small ARM embedded CPUs, targeting in particular the ARM Cortex-M0+ and the ARM Cortex-M4.

These notes are envisioned in the context of cryptographic implementations, so there is some emphasis on mathematical aspects and on constant-time processing (making computations in such a way that no private information can be inferred from timing measurements, including computation time but also indirect timing-based effects such as cache misses or stalls due to memory access contention).

Conventions

In all descriptions:

  • Each code snippet is provided as a macro in GAS (GNU assembly) syntax, which used register names provided as macro parameters.

  • In ARMv6-M code (for ARM Cortex-M0+), the registers should be chosen among the "low half" (r0 to r7). On ARMv7-M (for ARM Cortex-M4), the "high" registers (r8 to r12, and r14) can also be used (though r9 could be reserved by the platform).

  • Unless specified explicitly, all registers provided as parameters shall be distinct.

  • The following terms are used to qualify the registers used as input:

    • preserved: the input register value is not modified

    • consumed: the register contents are set to an unspecified value

    • scratch: the register contents on input are ignored; an unspecifed value is written into the register

  • All ARMv6-M code also works on ARMv7-M, hence code that can run on an ARM Cortex-M0+ is said to be "compatible ARMv6-M". Variants specific to ARMv7-M are listed if they provide an advantage (usually in terms of speed) over the ARMv6-M code.

  • The notation rx:ry means that two 32-bit registers are considered as a single 64-bit value, with rx being the low half, and ry the high half. The notation extends to more than two words, e.g. ra:rb:rc:rd for a 128-bit integer (ra is least significant, rd is most significant).