RE: Why is (2 < 2) true? Is it a gcc bug?

[Dorau, Lukasz]

On Friday, January 17, 2014 10:44 PM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> On Fri, Jan 17, 2014 at 1:02 PM, Markus Trippelsdorf
> <markus@xxxxxxxxxxxxxxx> wrote:
> > On 2014.01.17 at 11:58 -0800, Alexei Starovoitov wrote:
> >> On Fri, Jan 17, 2014 at 9:58 AM, Alexei Starovoitov
> >> <alexei.starovoitov@xxxxxxxxx> wrote:
> >> > 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 ?
> >> >
> >>
> >> 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)
> >
> > Your testcase is invalid:
> >
> > markus@x4 tmp % clang -fsanitize=undefined -Wall -Wextra -O2 bug.c
> > markus@x4 tmp % ./a.out
> > (0 < 2) == 1
> > (1 < 2) == 1
> > bug.c:16:20: runtime error: index 2 out of bounds for type 'struct isci_host *[2]'
> >
> > As Jakub Jelinek said on IRC, changing the loop to e.g.:
> >
> >   for (i = 0;
> >        i < 2 && (isci_host = v.hosts[i]);
> >        i++) {
> >
> > fixes the issue.
> 
> testcase was reduced from drivers/scsi/isci/host.h written back in
> 2011 (commit b329aff107)
> #define for_each_isci_host(id, ihost, pdev) \
>         for (id = 0, ihost = to_pci_info(pdev)->hosts[id]; \
>              id < ARRAY_SIZE(to_pci_info(pdev)->hosts) && ihost; \
>              ihost = to_pci_info(pdev)->hosts[++id])
> 
> yes, it does access 3rd element of 2 element array and will read junk.
> 
> C standard says the behavior is undefined and comes handy in compiler defense,
> but in this case compiler has obviously all the information to make
> right decision
> instead of misoptimizing the code.
> So yes, the loop is erroneous, non-portable, etc, but gcc can be smarter.
> --

Thank you for explanation!

Alexei,

Will you file a gcc bug for this case?

Thanks,
Lukasz

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