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Re: Issue with LTO/-fwhole-program
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?
Manuel.