Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness

[Ard Biesheuvel]

On 19 October 2016 at 16:11, Arnd Bergmann <arnd at arndb.de> wrote:
> On Wednesday, October 19, 2016 4:01:58 PM CEST Ard Biesheuvel wrote:
>> On 19 October 2016 at 15:59, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
>> > On 19 October 2016 at 14:35, Will Deacon <will.deacon at arm.com> wrote:
>> >> On Mon, Oct 17, 2016 at 08:43:19PM +0100, Ard Biesheuvel wrote:
>> >>> On 17 October 2016 at 19:38, Will Deacon <will.deacon at arm.com> wrote:
>> >
>> > Yes, and that would be perfectly legal from a correctness point of
>> > view, and would likely help performance as well. By using
>> > __builtin_constant_p(), you are choosing to perform a build time
>> > evaluation of an expression that would ordinarily be evaluated only at
>> > runtime. This implies that you have to address undefined behavior at
>> > build time rather than at runtime as well.
>> >
>> >>> If order_base_2() is not defined for input 0, it should BUG() in that
>> >>> case, and the associated __builtin_unreachable() should prevent the
>> >>> special version from being emitted. If order_base_2() is defined for input
>> >>> 0, it should not invoke ilog2() with that argument, and the problem should
>> >>> go away as well.
>> >>
>> >> I don't necessarily think it should BUG() if it's not defined for input
>> >> 0; things like __ffs don't do that and we'd be introducing conditional
>> >> checks for cases that should not happen. The comment above order_base_2
>> >> does suggest that ob2(0) should return 0, but it can actually end up
>> >> invoking ilog2(-1), which is obviously wrong.
>> >>
>> >> I could update the comment, but that doesn't fix the build issue.
>> >>
>> >
>> > Fixing roundup_pow_of_two() [which is arguably incorrect]
>>
>> I just spotted the comment that says it is undefined. But that means
>> it could legally return 1 for input 0, i suppose
>
> I think having the link error in roundup_pow_of_two() is safer than
> returning 1.
>
> Why not turn it into a runtime warning in this driver?
>
> diff --git a/drivers/clk/mvebu/armada-37xx-periph.c b/drivers/clk/mvebu/armada-37xx-periph.c
> index cecb0fdfaef6..711d1d9842cc 100644
> --- a/drivers/clk/mvebu/armada-37xx-periph.c
> +++ b/drivers/clk/mvebu/armada-37xx-periph.c
> @@ -349,8 +349,10 @@ static int armada_3700_add_composite_clk(const struct clk_periph_data *data,
>                         rate->reg = reg + (u64)rate->reg;
>                         for (clkt = rate->table; clkt->div; clkt++)
>                                 table_size++;
> -                       rate->width = order_base_2(table_size);
> -                       rate->lock = lock;
> +                       if (!WARN_ON(table_size == 0)) {
> +                               rate->width = order_base_2(table_size);
> +                               rate->lock = lock;
> +                       }
>                 }
>         }
>

I guess Will is not looking for a way to fix the driver, but for a way
to eliminate this issue entirely going forward.

In general, I think the issue where constant folding results in
ilog2() or other similar functions being called with invalid build
time constant parameter values is simply something we have to deal
with.

In this case, it is in fact order_base_2() that deviates from its
documented behavior (as Will points out), and fixing /that/ should
make this particular issue go away afaict.