Re: [PATCH bpf] bpf: Improve bucket_log calculation logic

On Fri, Feb 7, 2020 at 12:13 PM Alexei Starovoitov
[off-list ref] wrote:

In addition to this patch I've tried:
+#define __const_ilog2(n, unique_n) ({                  \
+       typeof(n) unique_n = (n);                       \

Yeah, as you found out, this doesn't work for the case of having
global initializers or things like array sizes.

Which people do do - often nor directly, but through various size macros.

It's annoying, but one of the failures of C is having a nice form of
compile-time constant handling where you can do slightly smarter
arithmetic. The definition of a "const expression" is very very
limited, and hurts us exactly for the array declaration and constant
initializer case.

Oh well.

          Linus