On Fri, Jan 17, 2014 at 5:37 AM, Dorau, Lukasz <lukasz.dorau@xxxxxxxxx> 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.
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