Warnings about conflicting options

[emblocks]

Hi,

Someone uses in a project the two following switches which is fine in 4.8:

-mcpu=cortex-m7 and -march=armv7e-m

the cortex-m7 is also a armv7e-m as far as I know.

5.3.1 is giving me the warnings for every file:

"warning: switch -mcpu=cortex-m7 conflicts with -march=armv7e-m switch"

why?

[Andre Vieira (andre-simoesdiasvieira)]

Hi emblocks,

This is because we define an extra feature for the cortex-m7. This is the FL_NO_VOLATILE feature which indicates that volatile memory should not be put in an IT block.

In our check to detect whether an -march option is compatible with a -mcpu option we check that they define the same set of features. This is why you see that warning. We advise against the use of both -mcpu and -march switches.

Best Regards,
Andre

[emblocks (gnugcc)]

Andre, thanks for your quick reply.

Where can we find more about that FL_NO_VOLATILE feature? Google shows nothing.

Regards
Gerard

[emblocks (gnugcc)]

BTW what is the preferred method? -mcpu I guess>

[Thomas Preud'homme (thomas-preudhomme)]

Hi,

-mcpu is the same as -march + -mtune. For instance -mcpu=cortex-m0 is the same as -march=armv6-m -mtune=cortex-m0.

Best regards.

[Thomas Preud'homme (thomas-preudhomme)]

About FL_NO_VOLATILE, it's a GCC internal flag to represent the CPU capability. As Andre said, it drives GCC into not having any access to volatile memory inside an IT block. Other armv7e-m processor will not necessarily have this consideration.

Best regards