[Bug c/36862] New: Optimization O2 causes infinite loop

[Bug c/36862] New: Optimization O2 causes infinite loop

[ben at zeus dot com]

Sorry, not sure if this is a real bug or whether the code is broken and relying
on undefined behaviour when an integer overflows. The test case is very simple:

#include <stdio.h>

int main( int argc, char *argv[] )
{
   int bit;
   for( bit = 1; bit; bit *= 2 ) {
      printf( "%d\n", bit );
   }
   return 0;
}

Compiling this with just the argument '-O2' on GCC 4.3.1 will cause this loop
to run forever, printing out an endless stream of zeros. Lower levels of
optimization don't hit this problem. Changing the type of 'bit' to unsigned
also prevents an infinite loop. Earlier GCCs (GCC 4.2.3 and earlier) did not
have this behaviour.


-- 
           Summary: Optimization O2 causes infinite loop
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ben at zeus dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36862

[Bug c/36862] Optimization O2 causes infinite loop

[ben at zeus dot com]

------- Comment #1 from ben at zeus dot com  2008-07-17 09:53 -------
Sorry, forgot to include any compiler details:

[hera ben]$ /usr/local/gcc-4.3/bin/gcc -O2 -v -save-temps loop.c                

Using built-in specs.

Target: i686-pc-linux-gnu

Configured with: ../gcc-4.3.1/configure --prefix=/usr/local/gcc-4.3.1
--with-gmp=/usr/local/gmp-4.2.2 --with-mpfr=/usr/local/mpfr-2.3.1 :
(reconfigured) ../gcc-4.3.1/configure --prefix=/usr/local/gcc-4.3.1
--with-gmp=/usr/local/gmp-4.2.2 --with-mpfr=/usr/local/mpfr-2.3.1

Thread model: posix

gcc version 4.3.1 (GCC) 

COLLECT_GCC_OPTIONS='-O2' '-v' '-save-temps' '-mtune=generic'

 /usr/local/gcc-4.3.1/libexec/gcc/i686-pc-linux-gnu/4.3.1/cc1 -E -quiet -v
loop.c -mtune=generic -O2 -fpch-preprocess -o loop.i

ignoring nonexistent directory
"/usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../../i686-pc-linux-gnu/include"

#include "..." search starts here:

#include <...> search starts here:

 /usr/local/include

 /usr/local/gcc-4.3.1/include

 /usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/include

 /usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/include-fixed

 /usr/include

End of search list.

COLLECT_GCC_OPTIONS='-O2' '-v' '-save-temps' '-mtune=generic'

 /usr/local/gcc-4.3.1/libexec/gcc/i686-pc-linux-gnu/4.3.1/cc1 -fpreprocessed
loop.i -quiet -dumpbase loop.c -mtune=generic -auxbase loop -O2 -version -o
loop.s

GNU C (GCC) version 4.3.1 (i686-pc-linux-gnu)

        compiled by GNU C version 4.3.1, GMP version 4.2.2, MPFR version 2.3.1.

GGC heuristics: --param ggc-min-expand=64 --param ggc-min-heapsize=64563

Compiler executable checksum: 93e4b24a169a3647e9429593228b6881

COLLECT_GCC_OPTIONS='-O2' '-v' '-save-temps' '-mtune=generic'

 as -V -Qy -o loop.o loop.s

GNU assembler version 2.15 (i386-linux) using BFD version 2.15

COMPILER_PATH=/usr/local/gcc-4.3.1/libexec/gcc/i686-pc-linux-gnu/4.3.1/:/usr/local/gcc-4.3.1/libexec/gcc/i686-pc-linux-gnu/4.3.1/:/usr/local/gcc-4.3.1/libexec/gcc/i686-pc-linux-gnu/:/usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/:/usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/

LIBRARY_PATH=/usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/:/usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/../../../:/lib/:/usr/lib/

COLLECT_GCC_OPTIONS='-O2' '-v' '-save-temps' '-mtune=generic'

 /usr/local/gcc-4.3.1/libexec/gcc/i686-pc-linux-gnu/4.3.1/collect2
--eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o
/usr/lib/crti.o /usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/crtbegin.o
-L/usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1
-L/usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/../../.. loop.o -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/local/gcc-4.3.1/lib/gcc/i686-pc-linux-gnu/4.3.1/crtend.o /usr/lib/crtn.o


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36862

[Bug c/36862] Optimization O2 causes infinite loop

[paolo dot carlini at oracle dot com]

------- Comment #2 from paolo dot carlini at oracle dot com  2008-07-17 10:06 -------
(In reply to comment #0)
>    int bit;
>    for( bit = 1; bit; bit *= 2 )

This is pretty straightforward, isn't it? At some point bit - a *signed*
integral type - overflows and this leads to undefined bahavior.


-- 

paolo dot carlini at oracle dot com changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36862

[Bug c/36862] Optimization O2 causes infinite loop

[brian at dessent dot net]

------- Comment #3 from brian at dessent dot net  2008-07-17 10:21 -------
Subject: Re:   New: Optimization O2 causes infinite loop

Yes, this code is broken as indeed overflow of signed integers is
undefined, so transforming this to an infinite loop is legitimate.  Here
are some links to read:

http://gcc.gnu.org/gcc-4.2/changes.html
http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fwrapv-1854
http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fstrict_002doverflow-698
http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wstrict_002doverflow-316
http://www.gnu.org/software/autoconf/manual/html_node/Integer-Overflow.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36862