Re: [PATCH v3 1/6] ilog2: create truly constant version for sparse

From: Linus Torvalds
Date: Tue Apr 17 2018 - 20:07:12 EST

On Tue, Apr 17, 2018 at 4:35 PM, Martin Wilck <mwilck@xxxxxxxx> wrote:
> Sparse emits errors about ilog2() in array indices because of the use of
> __ilog2_32() and __ilog2_64(),

If sparse warns about it, then presumably gcc with -Wvla warns about it too?

And if thats the case, then __builtin_constant_p() and a ternary
operation isn't good enough, because gcc -Wvla is even more anal than
sparse is.

Which is why we have that __is_constexpr() magic for min/max().

So I suspect that what you'd want is

#define ilog2(n) \
__builtin_choose_expr(__is_constexpr(n), \
const_ilog2(n), \
__builtin_choose_expr(sizeof(n) <= 4, \
__ilog2_u32(n), \

or something. Hmm?