Messages in this thread Patch in this message | | | From | Arnd Bergmann <> | Subject | Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness | Date | Wed, 19 Oct 2016 17:11:40 +0200 |
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On Wednesday, October 19, 2016 4:01:58 PM CEST Ard Biesheuvel wrote: > On 19 October 2016 at 15:59, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: > > On 19 October 2016 at 14:35, Will Deacon <will.deacon@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@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; + } } }
Arnd
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