[Bug other/59648] -O2 -flto compilation of xorg-server-1.15.0 fails

[trippels at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59648

--- Comment #7 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
You cannot use full paths for your test output, because 
creduce will run each iteration in a new directory. So
try changing it from:
 -flto=4 -o /dev/null /home/dave2/$TESTCASE /home/dave2/libxservertest.a >
/home/dave2/test.txt 2>&1
cat /home/dave2/test.txt | grep ...

to:
 -flto=4 -o /dev/null $TESTCASE /home/dave2/libxservertest.a > test.txt 2>&1
cat test.txt | grep ...

To reduce /home/dave2/libxservertest.a first extract its contents:
 ar x /home/dave2/libxservertest.a 
and then produce a list of the object files:
 ls -al *.o > list
Edit "list" and prepend the full path to all object files.
The run delta on "list" as described here:
http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction#Reducing_LTO_bugs
and generate preprocessed source for the files.

Now reduce the preprocessed files one by one. 
After a few iterations you'll end up with something like:

x4 test # cat test.i
long a;
extern int noPanoramiXExtension, PanoramiXNumScreens, CheckVirtualMotion_x;
typedef struct {
  int screens[16];
  int numScreens;
} ScreenInfo;
ScreenInfo screenInfo;
int fn1() {
  if (noPanoramiXExtension) {
    int i;
    i = PanoramiXNumScreens - 1;
    while (i--)
      CheckVirtualMotion_x = (long)screenInfo.screens[i];
  }
  return 0;
}

int main() {
  screenInfo.numScreens = 0;
  a = (long)*fn1;
  return 0;
}

x4 test # cat panoramiX.i
int PanoramiXNumScreens;
int noPanoramiXExtension=1;
int CheckVirtualMotion_x;

x4 test # gcc -O2  -Werror=array-bounds test.i panoramiX.i 
x4 test # gcc -O2 -flto  -Werror=array-bounds test.i panoramiX.i 
test.i: In function ‘fn1’:
test.i:13:54: warning: iteration 16 invokes undefined behavior
[-Waggressive-loop-optimizations]
       CheckVirtualMotion_x = (long)screenInfo.screens[i];
                                                      ^
test.i:12:11: note: containing loop
     while (i--)
           ^
test.i:13:54: error: array subscript is below array bounds
[-Werror=array-bounds]
       CheckVirtualMotion_x = (long)screenInfo.screens[i];
                                                      ^
lto1: some warnings being treated as errors
lto-wrapper: /usr/x86_64-pc-linux-gnu/gcc-bin/4.9.0/gcc returned 1 exit status
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.0/../../../../x86_64-pc-linux-gnu/bin/ld:
fatal error: lto-wrapper failed
collect2: error: ld returned 1 exit status
x4 test #