Platform

Arm (Thumb assembly).

What

Depending on the GCC compiler options used, you can receive an error:

error: r7 cannot be used in asm here

Reason

The assembly code in bn_mul.h is optimized for the Arm platform and uses some registers, including r7 to efficiently perform an operation. GCC also uses r7 as the frame pointer under the Arm Thumb assembly.

Solution

Add -fomit-frame-pointer to your GCC compiler options.

If you have already added for example -O or -O2 you do not need to add -fomit-frame-pointer as the optimization options already include it on most systems by default.

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