linux-kernel - Re: linux-next: build warning after merge of the tip tree

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Message-ID: <CALCETrXP+Rz8wi_N2G7WLwnGuFMC8zeo5o+jgGmvpZkndrM+2Q@mail.gmail.com>
Date:	Fri, 18 Jul 2014 13:08:51 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: linux-next: build warning after merge of the tip tree

On Fri, Jul 18, 2014 at 1:05 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 07/18/2014 12:57 PM, Andy Lutomirski wrote:
>>
>> This particular warning is IMO in a particularly dumb category: GCC
>> optimizes some code and then warns about a construct that wasn't there
>> in the original code.  In this case, I think it unrolled a loop and
>> discovered that one iteration contained a test that was always true.
>> Big deal.
>>
>> (OTOH, the code in question was buggy, but not all for the reason that
>> GCC thought it was.)
>>
>
>                 if (syms[sym_vvar_start] > syms[i] + 4096)
>                         fail("%s underruns begin_vvar\n",
>                              required_syms[i].name);
>
> if i == sym_vvar_start then this is at least a valid warning.  It could
> easily be quieted by chaning syms[] to an unsigned array.

Hah -- fooled you, too :)

i isn't an index in to the syms array at all.  This code is completely
wrong.  See the patch I sent in reply to Stephen's original email.

But, to your earlier point, presumably this could warn:

for (int i = 0; i < 10; i++)
  if (array[i] > array[5] + 1)
    fail();

I think that's absurd.  There's nothing wrong with that code.  A given
test should have to be always true or always false on *all* loop
iterations to be flagged, I think.

--Andy

>
>         -hpa
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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