quoted

I remember that this construction may bring horrible code on some architectures
with some version(s) of the compiler (*).

Wow, even the trunk Clang and GCC seem to generate better code for
your version of this line: https://godbolt.org/z/36Kqxhe6j

Oh wait.
First, my Godbolt reproducer is incorrect, it is using sizeof(unsigned
long) instead of 8 * sizeof(unsigned long) for BITS_PER_LONG.

quoted

To fix that I found an easy refactoring:

                map[index] &= ~(GENMASK(nbits, 0) << offset));

Second, the line above should probably be:
  map[index] &= ~(GENMASK(nbits - 1, 0) << offset));

, right?

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