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

[John Criswell]

On 7/16/15 1:39 PM, ed at modk.it wrote:
> Hi All,
>
> After the initial learning curve, we're excited to have put together a 
> completely gcc/binutils-free toolchain based on LLVM.  Now that we 
> have things working, we desperately need to optimize the resulting 
> binaries.  Our bin files are up to 10x their fully optimized gcc 
> equivalent (1.5k vs 16k).  This is for a bare metal ARM based system 
> so this is significant.
>
> We're using lld for linking and the following dead code elimination 
> techniques seem to be dead ends:
>
> 1) whole program optimization on our most egregious space waster 
> (-fwhole-program not supported by clang)
> 2) link time optimization (looks like this is only supported by lld 
> for the COFF path not the ELF path)
> 3) using a linker plugin like gold (-fuse-linker-plugin doesn't seem 
> to be supported by clang)
>
> We have control over the whole codebase and could essentially compile 
> all of our C/C++ code as single file if there was a way to tell clang 
> that it is seeing the whole program.
>
> Any thoughts on how we could achieve this?  This slidedeck suggests 
> using llvm-link to accomplish this: 
> http://llvm.org/devmtg/2013-11/slides/Gao-LTO.pdf 
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_devmtg_2013-2D11_slides_Gao-2DLTO.pdf&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=TLoIRs6M_cYUZSKAcynkcq55H57t5tpplD2x6T-b3mM&s=kZfwoBElwGePaEljcvFWBQ2n9bCYoAS2bojmIpl1-QE&e=>. 
> Is this the most promising way forward?

Is there a reason why LLVM's link-time optimization won't work for you?

http://llvm.org/docs/GoldPlugin.html
http://llvm.org/docs/LinkTimeOptimization.html

Regards,

John Criswell

>
> Thanks,
> Ed
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev


-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150716/cf6694ff/attachment.html>