[LLVMdev] [Clang] [lld] [llvm-link] Whole program / dead-code optimization

[ed at modk.it]

After digging a bit more it seems we can achieve the same as gcc's
-fwhole-program by simply marking the mult function as "static" which is
all -fwhole-program does anyway.  From
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html:

 *-fwhole-program**Assume that the current compilation unit represents the
whole program being compiled. All public functions and variables with the
exception of main and those merged by attributeexternally_visible become
static functions and in effect are optimized more aggressively by
interprocedural optimizers.*

So we can accomplish that for now with a simple pass on the source.  But
that had me thinking, how do we accomplish the same for unused C++ classes
or member functions within classes.  I figured we could accomplish that by
changing the linkage type within the llvm IR.  But it turns out these
already get linkonce_odr linkage.  http://llvm.org/docs/LangRef.html states
"Unreferenced linkonce globals are allowed to be discarded"

class num{

  private:

    int number;

  public:

    num(int n):number(n){}

    int mult(int other){

        return number*other;

    }

};


int main(void){

    return 0;
}

If I compile the above to LLVM IR there is actually no trace of the num
class which kind of baffles me because what if I was compiling a library
and this class was needed in the library consumer?

Either way with this knowledge I think we can get the results we're looking
for in the short term and will follow up if we find or come up with
anything that could be generally useful to others.

Thanks,
Ed

On Sat, Jul 18, 2015 at 9:46 AM, ed at modk.it <ed at modk.it> wrote:

> Thanks Nick.  I've been pursuing Gao's technique but can't seem to get opt
> to remove obviously dead code from even the following trivial example:
>
> int mult(int a, int b){
>
>     return a*b;
>
> }
>
>
> int main(void){
>
>     return 0;
>
> }
>
>
> While mult is never called it still is not removed.  I just can't seem to
> get opt to understand it's seeing the whole program so it can remove this
> globally accessible function.  What am I missing?  Seems related to the
> missing -fwhole-program flag in clang.  Perhaps this is not even possible?
> If I can't get any answers here I may repost that specific question since I
> didn't list [opt] in the original question subject.
>
>
> Thanks,
>
> Ed
>
> On Fri, Jul 17, 2015 at 1:15 AM, Nick Lewycky <nicholas at mxc.ca> wrote:
>
>> ed at modk.it wrote:
>>
>>>
>>>     Is there a reason why LLVM's link-time optimization won't work for
>>> you?
>>>
>>>     http://llvm.org/docs/GoldPlugin.html
>>>     <
>>> https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_docs_GoldPlugin.html&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=rF94h73bKDdWVhxOWqRXpvw5pSMgvuHQXJ__qw8n2LU&s=PR31BXeMANGrAQP2Tt9Eg5psH82vj8Oq1WmyprGhyn8&e=
>>> >
>>>     http://llvm.org/docs/LinkTimeOptimization.html
>>>     <
>>> https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_docs_LinkTimeOptimization.html&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=rF94h73bKDdWVhxOWqRXpvw5pSMgvuHQXJ__qw8n2LU&s=PoqmeRXrssdG9xj6Fko_SKttwLPWqUVkxFH41dOcg4w&e=
>>> >
>>>
>>>
>>> Well the primary motivation to move to LLVM is licensing which is why we
>>> also ditched binutils since we can't package gcc for iOS due to the
>>> GPL.  So in the end the gold plugin wouldn't work for licensing reasons
>>> even if we can get it to work technically but thanks for the links I'm
>>> still trying to wrap my head around the problem and any info helps.
>>>
>>
>> The right future is a world where lld performs llvm lto for you.
>>
>> Until then, the technique in Gao's PDF is what I would recommend.
>>
>> Nick
>>
>
>
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