[llvm-dev] LTO on libraries

[David Callahan via llvm-dev]

Thanks

I had two small learnings.
1. Don’t name the output of llvm-link as a “.o” (the subsequent clang ignores it)
2. You need to have optimization flags like –O3 on the second clang, the on on the first is dropped.
But this did as I had hoped:

bin/clang -flto -O3 -c {a,b,c}.o
bin/llvm-link -o abc.bc {a,b,c}.o
bin/clang -O3 -o abc.o -c abc.bc
bin/clang -o test main.c abc.o

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 4:27 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 4:12 PM, David Callahan <dcallahan at fb.com<mailto:dcallahan at fb.com>> wrote:

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?

Yes.
You can use clang to generate a .o (as a real binary), that you can then wrap in an archive library.



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.

You’re probably be interested in ThinLTO that is currently being implemented: scalability and incrementally are scheduled.

I need to trigger the optimizer on the intermediate rather than defer to final link for these goals.

Just run clang on the output of llvm-link :)

Note: you’re not having the same linker semantic with an archive containing one .o compared to an archive with multiple .o. So you need to be careful.

—
Mehdi



—david


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:

Hello,

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?

—
Mehdi




Thanks
david
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