Add port-optimised task selection for ARMv8-M

[jefftenney]

Add port-optimised task selection for ARMv8-M

Description

Add port-optimised task selection for ARMv8-M, patterned after the existing implementation on ARMv7-M. Include a compile-time error for baseline implementations of ARMv8-M like CM23 (without the "main" extension described in the Architecture Reference Manual) attempting to use configPORT_OPTIMISED_TASK_SELECTION. Those CPUs do not support the clz instruction used in the implementation of port-optimised task selection.

During the migration of this feature from ARMv7-M to ARMv8-M, the helper function ucPortCountLeadingZeros() became ulPortCountLeadingZeros() (returning a uint32_t, not a uint8_t) to stop forcing the compiler to generate an unnecessary uxtb instruction to clear out the upper 24 bits of the register. The clz instruction always writes a full 32-bit value to the destination register.

Test Steps

Copy the port files from this PR into a local clone of the full FreeRTOS distribution. Then:

• Build CM33 demo app (NXP LPC55S69) in IAR with and without configPORT_OPTIMISED_TASK_SELECTION. Verify build is successful. Review generated file task.s for use/non-use of clz in vTaskSwitchContext().
• Build CM33 demo app (NXP LPC55S69) in GCC with and without configPORT_OPTIMISED_TASK_SELECTION. Verify build is successful.
• Build CM23 demo app (Nuvoton M2351) with configPORT_OPTIMISED_TASK_SELECTION set to 1 and verify error is generated during compilation.

Copy the port and kernel files from this PR into a local clone of https://github.com/jefftenney/LPTIM-Tick-U5/ (for STM32U585, CM33). Then:

• Build and run with configPORT_OPTIMISED_TASK_SELECTION set to 1. Use debugger to verify use of clz in disassembly for vTaskSwitchContext().
• Verify correct operation of application.

Checklist:

[X] I have tested my changes. No regression in existing tests.
[X] I have modified and/or added unit-tests to cover the code changes in this Pull Request. (N/A)

Related Issues

• https://forums.freertos.org/t/port-optimized-task-selection-on-cortex-m33/12521
• Closes [Feature Request] Port optimized task selection on Cortex M33 #319

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.08 🎉

Comparison is base (71662b5) 94.35% compared to head (2d50d8c) 94.43%.

❗ Current head 2d50d8c differs from pull request most recent head d53ec5f. Consider uploading reports for the commit d53ec5f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #703      +/-   ##
==========================================
+ Coverage   94.35%   94.43%   +0.08%     
==========================================
  Files           6        6              
  Lines        2428     2428              
  Branches      596      596              
==========================================
+ Hits         2291     2293       +2     
+ Misses         87       85       -2     
  Partials       50       50              

Flag	Coverage Δ
unittests	94.43% <ø> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

see 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[jefftenney]

Also tested this on https://github.com/FreeRTOS/iot-reference-stm32u5 just now and it works.

[jefftenney]

@aggarg Great catch on the default setting. Thanks!