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