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

[arnd@arndb.de (Arnd Bergmann)]

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