Issue with LTO/-fwhole-program

[Richard Guenther]

On Fri, Jun 11, 2010 at 2:57 PM, Manuel López-Ibáñez
<lopezibanez@gmail.com> wrote:
> On 11 June 2010 14:40, Richard Guenther <richard.guenther@gmail.com> wrote:
>> On Fri, Jun 11, 2010 at 2:36 PM, Manuel López-Ibáñez
>> <lopezibanez@gmail.com> wrote:
>>> On 11 June 2010 14:23, Richard Guenther <richard.guenther@gmail.com> wrote:
>>>> On Fri, Jun 11, 2010 at 2:22 PM, Manuel López-Ibáñez
>>>> <lopezibanez@gmail.com> wrote:
>>>>> On 11 June 2010 14:07, Richard Guenther <richard.guenther@gmail.com> wrote:
>>>>>> On Fri, Jun 11, 2010 at 2:04 PM, Bingfeng Mei <bmei@broadcom.com> wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> I am still puzzled by the effect of LTO/-fwhole-program.
>>>>>>> For the following simple tests:
>>>>>>>
>>>>>>> a.c:
>>>>>>>
>>>>>>> #include <stdio.h>
>>>>>>> int v;
>>>>>>>
>>>>>>> extern void bar();
>>>>>>> int main()
>>>>>>> {
>>>>>>>  v = 5;
>>>>>>>  bar();
>>>>>>>
>>>>>>>  printf("v = %d\n", v);
>>>>>>>  return 0;
>>>>>>> }
>>>>>>>
>>>>>>> b.c:
>>>>>>> int v;
>>>>>>> void bar()
>>>>>>> {
>>>>>>>  v = 4;
>>>>>>> }
>>>>>>>
>>>>>>> If I just compile plainly, the output is:
>>>>>>> v = 4
>>>>>>>
>>>>>>> If I compile  as:
>>>>>>> ~/work/install-x86/bin/gcc  a.c -O2 -c -save-temps -flto
>>>>>>> ~/work/install-x86/bin/gcc  b.c -O2 -c -save-temps
>>>>>>> ~/work/install-x86/bin/gcc  a.o b.o -O2 -fuse-linker-plugin -o f -flto -fwhole-program
>>>>>>>
>>>>>>> The output is:
>>>>>>> v = 5
>>>>>>>
>>>>>>> We get two copies of v here. One is converted to static by whole-program optimizer,
>>>>>>> and the other is global. I know I can add externally_visible in a.c to solve
>>>>>>> the issue.  But since compiler is not able to give any warning here, it could make
>>>>>>> program very tricky to debug.
>>>>>>>
>>>>>>> What is the value to convert variables to static ones? I know unreferenced ones can
>>>>>>> be optimized out, but this can be achieved by -fdata-sections & -gc-collection as
>>>>>>> well, I believe.
>>>>>>
>>>>>> You make inter-procedural/file data-flow operations possible.
>>>>>
>>>>> But this is a bug, isn't it?
>>>>
>>>> No, you lied to the compiler by specifying -fwhole-file.
>>>
>>> I don't understand. The final link was the whole program.
>>
>> GCC does not see the whole program if you didn't build all units
>> you are linking with -flto.
>
> Ah, so the problem is the missing -flto in the second compilation
> step? I think this is a bug in the compiler for not reporting this
> somehow. Is there are PR open for this?

Well, we can't.  We specifically support mixed LTO/non LTO objects
(think of shared libraries for example).  With the linker-plugin and gold
we can do better, but with just GNU ld and collect2 we can't.

Richard.

> Manuel.
>