[LLVMdev] [Clang] [lld] [llvm-link] Whole program (original) (raw)

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

Nick Lewycky nicholas at mxc.ca
Sat Jul 18 21:25:07 PDT 2015

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|externallyvisible| 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= and -internalize-public-api-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 linkonceodr 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.orgdocsGoldPlugin.html&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4bhmuhMHA&m=rF94h73bKDdWVhxOWqRXpvw5pSMgvuHQXJ_qw8n2LU&s=PR31BXeMANGrAQP2Tt9Eg5psH82vj8Oq1WmyprGhyn8&e=_ _<https://urldefense.proofpoint.com/v2/url?u=http-3A_llvm.orgdocsGoldPlugin.html&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4bhmuhMHA&m=rF94h73bKDdWVhxOWqRXpvw5pSMgvuHQXJ_qw8n2LU&s=PR31BXeMANGrAQP2Tt9Eg5psH82vj8Oq1WmyprGhyn8&e=>> http://llvm.org/docs/LinkTimeOptimization.html <https://urldefense.proofpoint.com/v2/url?u=http-3A_llvm.orgdocsLinkTimeOptimization.html&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4bhmuhMHA&m=rF94h73bKDdWVhxOWqRXpvw5pSMgvuHQXJ_qw8n2LU&s=PoqmeRXrssdG9xj6FkoSKttwLPWqUVkxFH41dOcg4w&e=_ _<https://urldefense.proofpoint.com/v2/url?u=http-3A_llvm.orgdocsLinkTimeOptimization.html&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4bhmuhMHA&m=rF94h73bKDdWVhxOWqRXpvw5pSMgvuHQXJ_qw8n2LU&s=PoqmeRXrssdG9xj6FkoSKttwLPWqUVkxFH41dOcg4w&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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