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

[Nick Lewycky]

ed at modk.it wrote:
> 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
>     attribute|externally_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.

The pass is called "-internalize". You can control its operation a bit 
with the -internalize-public-api-file=<filename> and 
-internalize-public-api-list=<list> flags.

   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"

The answer is still internalize. Don't include their names in the list 
of public APIs and they'll be switched to 'internal' linkage, then 
deleted by the LTO passes.

>
> classnum{
>
> private:
>
> intnumber;
>
> public:
>
>      num(intn):number(n){}
> https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.htm
>
> intmult(intother){
>
> returnnumber*other;
>
>      }
>
> };
>
>
> intmain(void){
>
> return0;
>
> }
>
> 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?

There is no representation of a class in llvm or .o files. Instead, 
there's a plain struct which represents the typed memory (ie., storage 
for the non-static data members only), plus a pile of functions which 
take a pointer to that memory as their first argument (the member 
functions taking their 'this' argument).

Note that 'static' functions also just change the linkage to internal. 
So does putting code in an anonymous namespace. The main thing you get 
out of linker integration into LLVM LTO is that the linker can look at 
the pile of non-llvm code and determine which symbols are required by 
the rest of the system and compute that public API list for internalize.

Nick

> 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 <mailto:ed at modk.it>
> <ed at modk.it <mailto: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:
>
>     intmult(inta, intb){
>
>     returna*b;
>
>     }
>
>
>     intmain(void){
>
>     return0;
>
>     }
>
>
>     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
>     <mailto:nicholas at mxc.ca>> wrote:
>
>         ed at modk.it <mailto: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=
>             <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=
>             <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
>
>
>