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Beware of GNU-ARM compiler for Cortex-M0/M0+/M1

Miro Samek
The popular GNU-ARM toolset has had long-known issues for the Cortex-M0/M0+/M1 (ARMv6-M architecture). Specifically, people have reported very inefficient code generated, see "Cortex M0/M0+/M1/M23 BAD Optimisation in GCC" https://embdev.net/topic/426508 . 

But while so far people reported only inefficient code, I would like to make people aware of *incorrect* code generated by GNU-ARM for Cortex-M0/M0+. 

The issue was detected with interrupt disabling and has been documented in a bug report for the QP framework, see https://sourceforge.net/p/qpc/bugs/184/. The experiments performed with the latest available GUN-ARM (GNU Tools for ARM Embedded Processors 6-2017-q2-update, 6.3.1 20170620 release) clearly show incorrect code generated at optimization level -O, while the same code compiled at -O2 level seemed to be correct. 

Please be careful with GNU-ARM for ARMv6-M architecture and preferably avoid using it for these CPUs as long as the issue remains unresolved. 

Miro Samek 
state-machine.com 
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