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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 17 Apr 2018 17:07:04 -0700
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Subject: Re: [PATCH v3 1/6] ilog2: create truly constant version for sparse
To:     Martin Wilck <mwilck@suse.com>
Cc:     "Martin K. Petersen" <martin.petersen@oracle.com>,
        Linux SCSI List <linux-scsi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hannes Reinecke <hare@suse.de>,
        James Bottomley <jejb@linux.vnet.ibm.com>,
        Xose Vazquez Perez <xose.vazquez@gmail.com>,
        Bart Van Assche <Bart.VanAssche@sandisk.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jonathan Corbet <corbet@lwn.net>,
        Randy Dunlap <rdunlap@infradead.org>
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On Tue, Apr 17, 2018 at 4:35 PM, Martin Wilck <mwilck@suse.com> 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), \
                        __ilog2_u64(n)))

or something. Hmm?

           Linus