linux-kernel - Re: Why is (2 < 2) true? Is it a gcc bug?

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Date:	Fri, 17 Jan 2014 11:58:28 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	"Dorau, Lukasz" <lukasz.dorau@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	sebastian.riemer@...fitbricks.com, richard.weinberger@...il.com
Subject: Re: Why is (2 < 2) true? Is it a gcc bug?

On Fri, Jan 17, 2014 at 9:58 AM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Fri, Jan 17, 2014 at 5:37 AM, Dorau, Lukasz <lukasz.dorau@...el.com> wrote:
>> Hi
>>
>> My story is very simply...
>> I applied the following patch:
>>
>> diff --git a/drivers/scsi/isci/init.c b/drivers/scsi/isci/init.c
>> --- a/drivers/scsi/isci/init.c
>> +++ b/drivers/scsi/isci/init.c
>> @@ -698,8 +698,11 @@ static int isci_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>>         if (err)
>>                 goto err_host_alloc;
>>
>> -       for_each_isci_host(i, isci_host, pdev)
>> +       for_each_isci_host(i, isci_host, pdev) {
>> +               pr_err("(%d < %d) == %d\n",\
>> +                      i, SCI_MAX_CONTROLLERS, (i < SCI_MAX_CONTROLLERS));
>>                 scsi_scan_host(to_shost(isci_host));
>> +       }
>>
>>         return 0;
>>
>> --
>> 1.8.3.1
>>
>> Then I issued the command 'modprobe isci' on platform with two SCU controllers (Patsburg D or T chipset)
>> and received the following, very strange, output:
>>
>> (0 < 2) == 1
>> (1 < 2) == 1
>> (2 < 2) == 1
>>
>> Can anyone explain why (2 < 2) is true? Is it a gcc bug?
>
> gcc sees that i < array_size is the same as i < 2 as part of loop condition, so
> it optimizes (i < sci_max_controllers) into constant 1.
> and emits printk like:
>   printk ("\13(%d < %d) == %d\n", i_382, 2, 1);
>
>> (The kernel was compiled using gcc version 4.8.2.)
>
> it actually looks to be gcc 4.8 bug.
> Can you try gcc 4.7 ?
>
> gcc 4.7 compiles your loop into the following:
> <bb 74>:
>   # i_382 = PHI <0(73), i_73(74)>
>   # isci_host_148 = PHI <isci_host_63(73), isci_host_74(74)>
>   printk ("\13(%d < %d) == %d\n", i_382, 2, 1);
>   D.43295_70 = MEM[(struct isci_host *)isci_host_148 + 18632B];
>   # DEBUG D#6 => isci_host_148
>   # DEBUG ihost s=> ihost
>   scsi_scan_host (D.43295_70);
>   # DEBUG pdev => pdev_17(D)
>   # DEBUG pdev => pdev_17(D)
>   D.43629_353 = dev_get_drvdata (D.42809_20);
>   i_73 = i_382 + 1;
>   # DEBUG i => i_73
>   isci_host_74 = MEM[(struct isci_pci_info *)D.43629_353].hosts[i_73];
>   # DEBUG isci_host => isci_host_74
>   # DEBUG isci_host => isci_host_74
>   # DEBUG i => i_73
>   i.9_79 = (unsigned int) i_73;
>   D.42849_65 = i.9_79 <= 1;
>   D.42850_66 = isci_host_74 != 0B;
>   D.42851_67 = D.42850_66 & D.42849_65;
>   if (D.42851_67 != 0)
>     goto <bb 74>;
>   else
>     goto <bb 77>;
>
> which looks correct to me.
>
> while gcc 4.8.2 into:
>   <bb 92>:
>   # i_73 = PHI <i_82(93), 0(91)>
>   # isci_host_274 = PHI <isci_host_83(93), isci_host_71(91)>
>   # DEBUG isci_host => isci_host_274
>   # DEBUG i => i_73
>   printk ("\13(%d < %d) == %d\n", i_73, 2, 1);
>   _79 = MEM[(struct isci_host *)isci_host_274 + 18632B];
>   # DEBUG D#6 => isci_host_274
>   # DEBUG ihost => D#6
>   scsi_scan_host (_79);
>   # DEBUG pdev => pdev_26(D)
>   # DEBUG pdev => pdev_26(D)
>   _97 = dev_get_drvdata (_29);
>   i_82 = i_73 + 1;
>   # DEBUG i => i_82
>   isci_host_83 = MEM[(struct isci_pci_info *)_97].hosts[i_82];
>   # DEBUG isci_host => isci_host_83
>   # DEBUG isci_host => isci_host_83
>   # DEBUG i => i_82
>   if (isci_host_83 != 0B)
>     goto <bb 93>;
>   else
>     goto <bb 90>;
>
>   <bb 93>:
>   goto <bb 92>;
>
> in case of gcc4.8 the i<=1 comparison got optimized out and only
> isci_host !=0 is left,
> which looks incorrect.

It is interesting GCC 4.8 bug,
since it seems to expose issues in two compiler passes.

here is test case:

struct isci_host;
struct isci_orom;

struct isci_pci_info {
  struct isci_host *hosts[2];
  struct isci_orom *orom;
} v = {{(struct isci_host *)1,(struct isci_host *)1}, 0};

int printf(const char *fmt, ...);

int isci_pci_probe()
{
  int i;
  struct isci_host *isci_host;

  for (i = 0, isci_host = v.hosts[i];
       i < 2 && isci_host;
       isci_host = v.hosts[++i]) {
    printf("(%d < %d) == %d\n", i, 2, (i < 2));
  }

  return 0;
}

int main()
{
  isci_pci_probe();
}

$ gcc bug.c
$./a.out
0 < 2) == 1
(1 < 2) == 1
$ gcc bug.c -O2
$ ./a.out
(0 < 2) == 1
(1 < 2) == 1
Segmentation fault (core dumped)

workaround:

disable Value Range Propagation pass:
-fdisable-tree-vrp1 -fdisable-tree-vrp2

and complete unroll pass:
-fdisable-tree-cunrolli
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