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From: "su at cs dot ucdavis.edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug lto/61741] New: wrong code by LTO on x86_64-linux-gnu
Date: Tue, 08 Jul 2014 03:49:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: lto
X-Bugzilla-Version: 4.10.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: su at cs dot ucdavis.edu
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61741

            Bug ID: 61741
           Summary: wrong code by LTO on x86_64-linux-gnu
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk miscompiles the following code when using LTO on
x86_64-linux-gnu in both 32-bit and 64-bit modes. 

This also seems to affect 4.6.x -- 4.9.x: 
(1) the generated code hangs for 4.6.x -- 4.8.x; and
(2) the generated code produces wrong result for 4.9.x.

$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-unknown-linux-gnu/4.10.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure --prefix=/usr/local/gcc-trunk
--enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
gcc version 4.10.0 20140707 (experimental) [trunk revision 212319] (GCC) 
$ 
$ gcc-trunk -O0 -c foo.c          
$ gcc-trunk -O0 -c main.c
$ gcc-trunk -Os main.o foo.o 
$ a.out
0
$ 
$ gcc-trunk -flto -O0 -c foo.c
$ gcc-trunk -flto -O0 -c main.c
$ gcc-trunk -flto -Os main.o foo.o
$ a.out
1
$ 
$ cat foo.c
extern int a, b; 

void
foo (void)
{
  char c = 0;
  for (; a; a--)
    for (; c >= 0; c++);
  if (!c)
    b = 1;
}
$ cat main.c
int printf (const char *, ...);

extern void foo (void); 

int a = 1, b;

int
main ()
{
  foo ();
  printf ("%d\n", b);
  return 0;
}
$