libkern.h: avoid errors with roundup_pow_of_two() in __aligned()

[bz]

Summary

iwlwifi uses: __aligned(roundup_pow_of_two(sizeof(struct ...)))
which results in 'statement expression not allowed at file scope'
errors.

Make order_base_2() not use a local variable to avoid this problem.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days

[dougm]

My concern with this is that if the argument n to the macro has side effects, they will be effected twice. I don't find examples of that in the kernel, though I do find somewhere that n is a function call, so that a function would be invoked twice.

Let me suggest an alternative.

#define order_base_2(n) ilog2(2*(n)-1)

I believe that addresses your problem. What it costs is overflow when n has its high order bit set. I don't think that's a significant cost.

[bz]

In D46800#1067105, @dougm wrote:

My concern with this is that if the argument n to the macro has side effects, they will be effected twice. I don't find examples of that in the kernel, though I do find somewhere that n is a function call, so that a function would be invoked twice.

Let me suggest an alternative.

#define order_base_2(n) ilog2(2*(n)-1)

I believe that addresses your problem. What it costs is overflow when n has its high order bit set. I don't think that's a significant cost.

Seems to work for me. Do you want to commandeer the change for this?

[dougm]

In D46800#1067418, @bz wrote:

In D46800#1067105, @dougm wrote:

My concern with this is that if the argument n to the macro has side effects, they will be effected twice. I don't find examples of that in the kernel, though I do find somewhere that n is a function call, so that a function would be invoked twice.

Let me suggest an alternative.

#define order_base_2(n) ilog2(2*(n)-1)

I believe that addresses your problem. What it costs is overflow when n has its high order bit set. I don't think that's a significant cost.

Seems to work for me. Do you want to commandeer the change for this?

I will post it for review in the next two hours.