Code Size Optimisation: Epic Summary (23Q2)

[tom-cosgrove-arm]

Summary of Requirements

• Timescale
  • no external deadline, timeboxing to Q2 2023
• Description:
  • Reduce flash usage (i.e. code size + initialised memory) for Mbed TLS libmbedcrypto (PSA)
• Deliverables:
  • Code and initialised data size measurements
  • Documentation where it changes
• Exclusions / out-of-scope:
  • libmbedx509 and libmbedtls.a
  • RAM (heap and stack) usage
• Dependencies
  • None
• Relevant contacts
  • Shebu Varghese Kuriakose
  • TF-M (Antonio De Angelis)

Epic Summary

• Is epic fully broken down?
  • Yes
• Epic estimate
  • pS + qM = ? weeks
• Parallelism?
  • Most tasks can be worked on concurrently, but some issues form dependency chains
  • It would be best to have size measurement in the CI before the first code reduction PR is merged.
• Potential for conflicts?
  • Yes, as we will be working across the code base (except X.509 and TLS code)
• Description/Use Case
  • See requirements.
• DoD (default for each issue)
  • Code changes reviewed and merged, with no functional changes (unless previously agreed)
  • Expected code + initialised data reduction met
  • CI passing
• Experts list
  • @tom-cosgrove-arm, Antonio De Angelis, @mpg (for baremetal and driver-only pieces)
• Design documentation
  • n/a
• Dependencies
  • None
• Resources needed
  • None
• Infra/Test automation/CI needs
  • Persistence of code size results, to be able to track improvements over time
• Requirement link https://jira.arm.com/browse/SECLIB-632
• Review use of a separator to split must-have from nice-to-have
  • Yes
• Risks
  • no particular risks
• Testing notes
  • None
• Holiday
  • TC: 2 weeks in May
• Other resourcing notes
  • AD available until the end of April 2023

Note: we refer to "code size" throughout, but are including the size of initialised data, i.e. the total flash memory required

Useful Information

We primarily care about Armv8-M or Armv8.1-M (Thumb and Thumb-2 instructions, not Aarch64/A64). TF-M takes measurements for an521 platform, i.e. Cortex-M33, so that's what we will use.

We care about the TF-M SMALL and MEDIUM configurations.

Compiler options should be -Os, as used by TF-M. (Ask them about -Oz)

Target compiler is Armclang v6.19 with (TF-M) build type MinSizeRel.

For changes that are size/performance trade-offs, this needs to be documented as part of any pull request, and the performance reduction included in the review.

The TF-M v1.70 release notes with sizes of Mbed TLS libmbedcrypto are at https://tf-m-user-guide.trustedfirmware.org/releases/1.7.0.html

Kanban to track progress: https://github.com/orgs/Mbed-TLS/projects/7

Building TF-M

$ git clone https://git.trustedfirmware.org/TF-M/trusted-firmware-m.git

$ cd trusted-firmware-m

$ mkdir build && cd build

$ /usr/local/ArmCompilerforEmbedded6.19/bin/suite_exec $SHELL

$ cmake .. -DTFM_PLATFORM=arm/mps2/an521 -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake -DTFM_PROFILE=profile_medium -DCMAKE_BUILD_TYPE=MinSizeRel -DTEST_S=ON -DTEST_NS=ON

$ cmake --build .

[tom-cosgrove-arm]

Compiler options should be -Os, as used by TF-M. (Ask them about -Oz)

TF-M uses MinSizeRel for which the default optimisation level is -Os.

On ArmClang they have modified MinSizeRel to use -Oz, but with gcc still use -Os (gcc11 doesn't support -Oz - gcc12 will, but he highest version of gcc supported by TF-M is 11.3.1)

@dave-rodgman @tom-daubney-arm @yanrayw ^