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Date:	Fri, 18 Jul 2014 13:20:13 -0700
From:	Andy Lutomirski <>
To:	"H. Peter Anvin" <>
Cc:	Stephen Rothwell <>,
	Thomas Gleixner <>,
	Ingo Molnar <>,
	Peter Zijlstra <>,
	"" <>,
	"" <>,
	Linus Torvalds <>
Subject: Re: linux-next: build warning after merge of the tip tree

On Fri, Jul 18, 2014 at 1:15 PM, H. Peter Anvin <> wrote:
> On 07/18/2014 01:08 PM, Andy Lutomirski wrote:
>> 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.
> No, the issue is that gcc is telling you that the code will do the wrong
> thing in this case.  Yes, only for one iteration, but still.
> The reason this is a concern is that: (x > x + n) and its variants is
> often used to mean (x > INT_MAX - n) without the type knowledge, but
> that is actually invalid standard C because signed types are not
> guaranteed to wrap.

Right, but the constant in this case is *much* less than INT_MAX.
Anyway, this is moot.

I do wonder whether the kind of people who build hardened kernels
should enable -fwrapv, though.


>         -hpa

Andy Lutomirski
AMA Capital Management, LLC
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