llvm dev - Dec 2015 - LTO on libraries

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.

Thanks
david
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> On Dec 4, 2015, at 3:54 PM, David Callahan via llvm-dev <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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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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.

—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
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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