[cfe-dev] "You don't pay for what you don't use" principle violation?

[David Blaikie]

On Sun, May 4, 2014 at 3:56 PM, Fernando Pelliccioni
<fpelliccioni at gmail.com> wrote:
>
>
>
> On Sun, May 4, 2014 at 7:27 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Sun, May 4, 2014 at 3:08 PM, Fernando Pelliccioni
>> <fpelliccioni at gmail.com> wrote:
>> > Hi all,
>> >
>> > Is there any reason why the following code produces the attached
>> > assembly
>> > code?
>> >
>> >     struct X { int v; };
>> >
>> >     int func( X** x, int n )
>> >     {
>> >         int sum = 0;
>> >
>> >         while ( n > 0 )
>> >         {
>> >             sum += (*x)->v;
>> >             ++x;
>> >             --n;
>> >         }
>> >         return sum;
>> >     }
>>
>> This is an externally visible function - it can't be omitted from the
>> object file as it may be used by some other object file that will be
>> linked in. (it could be a global ctor, so the absence of code in
>> 'main' doesn't indicate that it will never be used)
>>
>> LTO can remove functions like this.
>>
>> >     int main() {}
>> >
>> > The "func" function is not used, I think it should be omitted by the
>> > compiler.
>> >
>> > If I change the function to a template function
>> >
>> >     template <typename T>
>> >     int func( T** x, int n )
>> >     {
>> >         int sum = 0;
>> >
>> >         while ( n > 0 )
>> >         {
>> >             sum += (*x)->v;
>> >             ++x;
>> >             --n;
>> >         }
>> >         return sum;
>> >     }
>> >
>> > Then, no code is generated.
>>
>> Since there's no call, there's no template instantiation, and no code.
>> The C++ language requires callers in other translation units to have
>> an instantiation of the template somewhere, it just doesn't have to be
>> here.
>>
>> >
>> >
>> > Compiled using:
>> >     clang++ -S -O3 -std=c++11 -S code.cpp
>> >
>> > clang++ --version
>> >     clang version 3.5 (trunk 200620)
>> >     Target: x86_64-apple-darwin13.1.0
>> >     Thread model: posix
>> >
>> >
>> > Best,
>> > Fernando.
>> >
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> >
>
>
>
> Thanks guys, sorry for the lack of precision in my words. You are right, the
> template function example is useless.
> Actually I was thinking in both, compile and linking time optimizations.
> I rephrase my question. Why the function is not omitted by the linker? (Is
> the -S flag the culprit?)

Could be - I'm not particularly familiar with linker flags, though
that's a bit out of scope for this list anyway.

- David