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

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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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Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Will Deacon <will.deacon@arm.com> - 2016-10-17 20:40 +0200
  Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-10-17 21:50 +0200
    Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-10-19 17:10 +0200
      Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Arnd Bergmann <arnd@arndb.de> - 2016-10-19 17:20 +0200
        Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-10-19 17:30 +0200
          Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Arnd Bergmann <arnd@arndb.de> - 2016-10-19 17:50 +0200
        Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Gregory CLEMENT <gregory.clement@free-electrons.com> - 2016-10-19 17:40 +0200
    Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-10-19 17:10 +0200
    Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Will Deacon <will.deacon@arm.com> - 2016-10-19 17:50 +0200
      Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-10-19 18:00 +0200
  Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Markus Trippelsdorf <markus@trippelsdorf.de> - 2016-10-19 17:40 +0200
    Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Linus Torvalds <torvalds@linux-foundation.org> - 2016-10-19 18:00 +0200
      Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Markus Trippelsdorf <markus@trippelsdorf.de> - 2016-10-19 18:00 +0200
        Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-10-19 18:10 +0200
        Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Linus Torvalds <torvalds@linux-foundation.org> - 2016-10-19 18:10 +0200
          Re: Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness Will Deacon <will.deacon@arm.com> - 2016-10-19 18:30 +0200

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