[llvm-dev] LTO on libraries

David Callahan via llvm-dev llvm-dev at lists.llvm.org
Fri Dec 4 16:12:37 PST 2015

Thanks for the response.
To clarify in your suggestion, llvm-link will combine the modules but not run the optimization pass, that is still delayed until the final binary is built, correct?

My use case is apply LTO to roughly program subsets; sacrificing effectiveness to avoid scaling problems and to allow the artifacts to be reused like archives and cached like .o’s.
I need to trigger the optimizer on the intermediate rather than defer to final link for these goals.


From: <mehdi.amini at apple.com<mailto:mehdi.amini at apple.com>> on behalf of Mehdi Amini <mehdi.amini at apple.com<mailto:mehdi.amini at apple.com>>
Date: Friday, December 4, 2015 at 4:02 PM
To: David Callahan <dcallahan at fb.com<mailto:dcallahan at fb.com>>
Cc: LLVM Dev Mailing list <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] LTO on libraries

On Dec 4, 2015, at 3:54 PM, David Callahan via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:


Is there a combination of the current tools would would allow me to apply LTO to a set of object files where are less than an entire application? Similar to “ld –r”  for doing a partial link.
I assume it would start with “clang –flto –c {a,b,c}.c” but the a subsequent link step fails  do to unresolved references of course.
Using llvm-link will linke the bitcode files but does not trigger compilation.

You can llvm-link and then run clang.  Note that the result of LTO in this case is not as powerful as when it is linker driver, since you can’t internalize.
You may also want to look at llvm-lto, but it can be annoying because it requires (AFAIK) an export list.

What is your use-case?


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