<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 4, 2015, at 4:51 PM, David Callahan <<a href="mailto:dcallahan@fb.com" class="">dcallahan@fb.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<div class="">Thanks</div>
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<div class="">I had two small learnings. </div>
<div class="">1. Don’t name the output of llvm-link as a “.o” (the subsequent clang ignores it) </div>
<div class="">2. You need to have optimization flags like –O3 on the second clang, the on on the first is dropped.</div>
<div class="">But this did as I had hoped:</div>
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<div class="">bin/clang -flto -O3 -c {a,b,c}.o</div>
<div class="">bin/llvm-link -o abc.bc {a,b,c}.o</div>
<div class="">bin/clang -O3 -o abc.o -c abc.bc</div>
<div class="">bin/clang -o test main.c abc.o</div></div></div></div></blockquote><div><br class=""></div><div>Note that you may want to disable the optimization on the first run of clang:</div><div><br class=""></div><div>bin/clang -flto -O3 -c {a,b,c}.o -mllvm -disable-llvm-optzns</div><div><br class=""></div>— </div><div>Mehdi</div><div><br class=""><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; font-size: 14px; font-family: Calibri, sans-serif;" class=""><div class="">
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<span style="font-weight:bold" class="">From: </span><<a href="mailto:mehdi.amini@apple.com" class="">mehdi.amini@apple.com</a>> on behalf of Mehdi Amini <<a href="mailto:mehdi.amini@apple.com" class="">mehdi.amini@apple.com</a>><br class="">
<span style="font-weight:bold" class="">Date: </span>Friday, December 4, 2015 at 4:27 PM<br class="">
<span style="font-weight:bold" class="">To: </span>David Callahan <<a href="mailto:dcallahan@fb.com" class="">dcallahan@fb.com</a>><br class="">
<span style="font-weight:bold" class="">Cc: </span>LLVM Dev Mailing list <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>><br class="">
<span style="font-weight:bold" class="">Subject: </span>Re: [llvm-dev] LTO on libraries<br class="">
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<div class="">On Dec 4, 2015, at 4:12 PM, David Callahan <<a href="mailto:dcallahan@fb.com" class="">dcallahan@fb.com</a>> wrote:</div>
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<div class="">Thanks for the response.</div>
<div class="">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?</div>
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<div class="">Yes.</div>
<div class="">You can use clang to generate a .o (as a real binary), that you can then wrap in an archive library.</div>
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<div class="">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.</div>
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<div class="">You’re probably be interested in ThinLTO that is currently being implemented: scalability and incrementally are scheduled.</div>
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<div class="">I need to trigger the optimizer on the intermediate rather than defer to final link for these goals.</div>
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<div class="">Just run clang on the output of llvm-link :)</div>
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<div class="">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.</div>
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<div class="">—david</div>
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<span style="font-weight:bold" class="">From: </span><<a href="mailto:mehdi.amini@apple.com" class="">mehdi.amini@apple.com</a>> on behalf of Mehdi Amini <<a href="mailto:mehdi.amini@apple.com" class="">mehdi.amini@apple.com</a>><br class="">
<span style="font-weight:bold" class="">Date: </span>Friday, December 4, 2015 at 4:02 PM<br class="">
<span style="font-weight:bold" class="">To: </span>David Callahan <<a href="mailto:dcallahan@fb.com" class="">dcallahan@fb.com</a>><br class="">
<span style="font-weight:bold" class="">Cc: </span>LLVM Dev Mailing list <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>><br class="">
<span style="font-weight:bold" class="">Subject: </span>Re: [llvm-dev] LTO on libraries<br class="">
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<div class="">On Dec 4, 2015, at 3:54 PM, David Callahan via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div>
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<div class="">Hello,</div>
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<div class="">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.</div>
<div class="">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.</div>
<div class="">Using llvm-link will linke the bitcode files but does not trigger compilation.</div>
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<div class="">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.</div>
<div class="">You may also want to look at llvm-lto, but it can be annoying because it requires (AFAIK) an export list.</div>
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<div class="">What is your use-case?</div>
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<div class="">— </div>
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<div class="">Thanks</div>
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