[Bug c/37067] New: gcc creating wrong code with -O2

[Bug c/37067] New: gcc creating wrong code with -O2

[stefan dot bruens at rwth-aachen dot de]

The following code is producing false results with -O2 optimization (-O1 and O3
are correct):
-------
#include <stdio.h>
#define BLOCK_SIZE 100000

int main()
{
  int i;
  float block_f[BLOCK_SIZE];
  int d_phase = 0;
  int d_phase_inc = 0x100000;

  for (i = 0; i < BLOCK_SIZE; i++){
      block_f[i] = (float)(d_phase);
      d_phase += d_phase_inc;
    }

  printf ("%x %x\n", (int)block_f[1695], (int)block_f[1696]);
  return 0;
}
-----
With -O2, the loop is run only 0x6a0 times, instead of 0x186a0 times.

gcc is from openSUSE 11.0 (4.3.1 20080507 (prerelease) [gcc-4_3-branch revision
135036] (SUSE Linux)), but fc9 is most probably affected as well.

Failing code is a very simplified test case from gnuradio
(http://www.ruby-forum.com/topic/156807)


-- 
           Summary: gcc creating wrong code with -O2
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: stefan dot bruens at rwth-aachen dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067

[Bug c/37067] gcc creating wrong code with -O2

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2008-08-09 18:01 -------
This works for me as far as I can see (you didn't specify the expected
output, and certainly if the loop doesn't run as often as you want you
may print uninitialized memory).  Anyway, if it doesn't work then it is
because d_phase overflows and so invokes undefined behavior.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067

[Bug c/37067] gcc creating wrong code with -O2

[stefan dot bruens at rwth-aachen dot de]

------- Comment #2 from stefan dot bruens at rwth-aachen dot de  2008-08-09 20:37 -------
The expected output is quite clear - 69f00000 6a000000 - but output is 69f00000
0.
The loop should run 100000 times, as i does not overflow.
d_phase does overflow after 2048 steps, but this should not influence the loop.

with -O2, gcc seems to do the following:
- d_phase is the same as i*0x100000
- the block should run 0x186a0 times
- (0x186a0 * 0x100000) & 0xffffffff = 0x6a000000
- break when d_phase reaches 0x6a000000 (1696 increment steps)

with -O3 and -O1, d_phase and i get their own variables
there should be at least a warning


-- 

stefan dot bruens at rwth-aachen dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067

[Bug c/37067] gcc creating wrong code with -O2

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2008-08-09 20:42 -------
>The loop should run 100000 times, as i does not overflow.

i does not overflow but d_phase_inc does ...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067

[Bug c/37067] gcc creating wrong code with -O2

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2008-08-09 20:43 -------
As I said - it works for me (i686, gcc (Debian 4.3.1-8) 4.3.1).  Still the
overflow invokes undefined behavior and thus _can_ affect the loop.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067

[Bug c/37067] gcc creating wrong code with -O2

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2008-08-09 20:46 -------
>The loop should run 100000 times, as i does not overflow.

i does not overflow but d_phase does ...

2046 7ff00000
2047 80000000
That is an overflow.

So is:
4094 fff00000
4095 0


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067

[Bug c/37067] gcc creating wrong code with -O2

[stefan dot bruens at rwth-aachen dot de]

------- Comment #6 from stefan dot bruens at rwth-aachen dot de  2008-08-09 20:56 -------
So is there any possibility to flag a variable to be expected to overflow and
"wrap around"? I think there is a lot of code in the wild assuming the loop to
run 100000 times and d_phase to overflow all the time, becoming negative.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067

[Bug c/37067] gcc creating wrong code with -O2

[brian at dessent dot net]

------- Comment #7 from brian at dessent dot net  2008-08-09 21:00 -------
Subject: Re:  gcc creating wrong code with -O2

Just make it unsigned since unsigned overflow is always defined.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067

[Bug c/37067] gcc creating wrong code with -O2

[rguenth at gcc dot gnu dot org]

------- Comment #8 from rguenth at gcc dot gnu dot org  2008-08-09 21:07 -------
You can use -fwrapv.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37067