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<p class="MsoNormal"><span style="color:#1F497D">Hello,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Let me ask a LTO simple question again. For the llvm LTO example in the link:
<a href="http://llvm.org/docs/LinkTimeOptimization.html">http://llvm.org/docs/LinkTimeOptimization.html</a>, I use below build commands to generate three different optimization level binary: -O0, -O1, -O2. By nm listing the foo1~4 symbols , I can see these
different optimizations really works. <o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">How can I know what different optimizations are used by the clang LTO among -O0, -O1 and -O2?
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="color:black"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">Is the compiler domain optimization (e.g. clang/llvm) or the linker (e.g. ld) domain optimization make these difference?
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">3.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:black">How can I explicitly enable or disable these specific optimizations besides using -O0, -O1, -O2?</span><span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$clang -emit-llvm -c main.c -o main.bc<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$clang -emit-llvm -c a.c -o a.bc<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$llvm-ar cr main.lib main.bc<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$llvm-ar cr a.lib a.bc<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$clang -O0 -flto main.lib a.lib -o main0<o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span style="color:#1F497D">$clang -O1 -flto main.lib a.lib -o main1<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$clang -O2 -flto main.lib a.lib -o main2<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$nm main0<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">…<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">00000000004005a0 t foo1<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">0000000000400580 t foo2<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">00000000004005e0 t foo3<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">0000000000400530 t foo4<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">0000000000400500 t frame_dummy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">…<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$</span> <span style="color:#1F497D">
nm main1<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">…<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">0000000000400550 t foo1<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">0000000000400580 t foo3<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">0000000000400530 t foo4<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">0000000000400500 t frame_dummy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">…<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$</span> <span style="color:#1F497D">
nm main2<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">…<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">00000000004004d0 t frame_dummy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">…<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">From blew verbose output, tt seems only linker( e.g. ld) is invovled to do the optimization?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">$ clang -O2 -flto main.lib a.lib -o main2 -v<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">clang version 3.8.0 (tags/RELEASE_380/final)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Target: x86_64-unknown-linux-gnu<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Thread model: posix<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">InstalledDir: /usr/local/bin<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.3<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.3.1<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Found candidate GCC installation: /usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Selected GCC installation: /usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Candidate multilib: .;@m64<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Candidate multilib: 32;@m32<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Selected multilib: .;@m64<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">"/usr/bin/ld" -z relro --hash-style=gnu --build-id --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o main2 /usr/lib/x86_64-linux-gnu/crt1.o /usr/lib/x86_64-linux-gnu/crti.o /usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0/crtbegin.o
-L/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0 -L/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0/../../../../lib64 -L/usr/local/bin/../lib64 -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0/../../..
-L/usr/local/bin/../lib -L/lib -L/usr/lib -plugin /usr/local/bin/../lib/LLVMgold.so -plugin-opt=mcpu=x86-64 -plugin-opt=O2 main.lib a.lib -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0/crtend.o
/usr/lib/x86_64-linux-gnu/crtn.o<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">Steven Shi</span></b><b><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D"><o:p></o:p></span></b></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:#1F497D">Intel\SSG\STO\UEFI Firmware</span></b><b><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#1F497D"><o:p></o:p></span></b></p>
<p class="MsoNormal"><b><span style="font-family:"Arial",sans-serif;color:#1F497D"><o:p> </o:p></span></b></p>
<p class="MsoNormal" style="text-align:justify"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#1F497D">Tel: +86 021-61166522<o:p></o:p></span></p>
<p class="MsoNormal" style="text-align:justify"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#1F497D">iNet: 821-6522<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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