[PATCH] D15564: [ELF] - Optimize .eh_frame section: remove CIE if all FDEs referencing it were removed.

Sat Jan 2 05:28:59 PST 2016

grimar added a comment.

In http://reviews.llvm.org/D15564#318375, @davide wrote:

> To be more clear, I think what you posted is a little bit strange. It's at least counter intuitive that doing more work makes linking faster, so I'd rather think that's just noise.
>  Also, how did you get those numbers? Which OS, preconditions (cache, disk) etc.. ? How many trials did you run? Your comment says twice, do you mean you run each experiment two times, or you run once with patch and once without (in any case I'll suggest to increase the number of trials)? What's variance, average, stddev ?

Actually I dont understand why so many attention is about this patch since the only thing its do is delays extending this->Header.sh_size until moment of any FDE appear. It does not do more work if we dont count one added comparsion as "work" of course. That actually less than any common patch usually do. 
So I also think results is it just a noise and I see no anything strange here. Thats why I wrote "what is more looks like a truth for me: it just has no any visible negative affect on speed".

As I said in comment I used "perf stat --repeat 50", so I ran it 2x50 times for code without patch and 2x50 times for code with one. 
First run, w/o patch, 50 repeats had 0.310183332 average link time.
Second run, w/o patch, 50 repeats had 0.306395272 average link time.
Third run, with patch, 50 repeats had 0.298682542 average link time.
Fourth run, with  patch, 50 repeats had 0.300793454 average link time.

I am using 64 bit ubuntu 15.04 VM installed on SSD drive, CPU is i7-4970K at 4.00

Command line was (I took it from ninja build script):
perf stat --repeat 50 /home/umb/LLVM/build/bin/clang++ -std=c++11 -fuse-ld=lld -fvisibility-inlines-hidden -Wall -W -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wnon-virtual-dtor -Wno-comment -std=c++11 -fcolor-diagnostics -fno-common -Woverloaded-virtual -O3 -DNDEBUG  -Wl,-allow-shlib-undefined   -Wl,--export-dynamic  -Wl,-O3, -Wl,--gc-sections tools/clang/tools/driver/CMakeFiles/clang.dir/driver.cpp.o tools/clang/tools/driver/CMakeFiles/clang.dir/cc1_main.cpp.o tools/clang/tools/driver/CMakeFiles/clang.dir/cc1as_main.cpp.o  -o bin/clang-3.8  lib/libLLVMAArch64CodeGen.a lib/libLLVMAArch64AsmPrinter.a lib/libLLVMAArch64AsmParser.a lib/libLLVMAArch64Desc.a lib/libLLVMAArch64Info.a lib/libLLVMAArch64Disassembler.a lib/libLLVMAMDGPUCodeGen.a lib/libLLVMAMDGPUAsmPrinter.a lib/libLLVMAMDGPUAsmParser.a lib/libLLVMAMDGPUDesc.a lib/libLLVMAMDGPUInfo.a lib/libLLVMARMCodeGen.a lib/libLLVMARMAsmPrinter.a lib/libLLVMARMAsmParser.a lib/libLLVMARMDesc.a lib/libLLVMARMInfo.a lib/libLLVMARMDisassembler.a lib/libLLVMBPFCodeGen.a lib/libLLVMBPFAsmPrinter.a lib/libLLVMBPFDesc.a lib/libLLVMBPFInfo.a lib/libLLVMCppBackendCodeGen.a lib/libLLVMCppBackendInfo.a lib/libLLVMHexagonCodeGen.a lib/libLLVMHexagonAsmParser.a lib/libLLVMHexagonDesc.a lib/libLLVMHexagonInfo.a lib/libLLVMHexagonDisassembler.a lib/libLLVMMipsCodeGen.a lib/libLLVMMipsAsmPrinter.a lib/libLLVMMipsAsmParser.a lib/libLLVMMipsDesc.a lib/libLLVMMipsInfo.a lib/libLLVMMipsDisassembler.a lib/libLLVMMSP430CodeGen.a lib/libLLVMMSP430AsmPrinter.a lib/libLLVMMSP430Desc.a lib/libLLVMMSP430Info.a lib/libLLVMNVPTXCodeGen.a lib/libLLVMNVPTXAsmPrinter.a lib/libLLVMNVPTXDesc.a lib/libLLVMNVPTXInfo.a lib/libLLVMPowerPCCodeGen.a lib/libLLVMPowerPCAsmPrinter.a lib/libLLVMPowerPCAsmParser.a lib/libLLVMPowerPCDesc.a lib/libLLVMPowerPCInfo.a lib/libLLVMPowerPCDisassembler.a lib/libLLVMSparcCodeGen.a lib/libLLVMSparcAsmPrinter.a lib/libLLVMSparcAsmParser.a lib/libLLVMSparcDesc.a lib/libLLVMSparcInfo.a lib/libLLVMSparcDisassembler.a lib/libLLVMSystemZCodeGen.a lib/libLLVMSystemZAsmPrinter.a lib/libLLVMSystemZAsmParser.a lib/libLLVMSystemZDesc.a lib/libLLVMSystemZInfo.a lib/libLLVMSystemZDisassembler.a lib/libLLVMX86CodeGen.a lib/libLLVMX86AsmPrinter.a lib/libLLVMX86AsmParser.a lib/libLLVMX86Desc.a lib/libLLVMX86Info.a lib/libLLVMX86Disassembler.a lib/libLLVMXCoreCodeGen.a lib/libLLVMXCoreAsmPrinter.a lib/libLLVMXCoreDesc.a lib/libLLVMXCoreInfo.a lib/libLLVMXCoreDisassembler.a lib/libLLVMAnalysis.a lib/libLLVMCodeGen.a lib/libLLVMCore.a lib/libLLVMipo.a lib/libLLVMInstCombine.a lib/libLLVMInstrumentation.a lib/libLLVMMC.a lib/libLLVMMCParser.a lib/libLLVMObjCARCOpts.a lib/libLLVMOption.a lib/libLLVMScalarOpts.a lib/libLLVMSupport.a lib/libLLVMTransformUtils.a lib/libLLVMVectorize.a lib/libclangBasic.a lib/libclangCodeGen.a lib/libclangDriver.a lib/libclangFrontend.a lib/libclangFrontendTool.a lib/libLLVMAArch64Desc.a lib/libLLVMAArch64AsmPrinter.a lib/libLLVMAArch64Info.a lib/libLLVMAArch64Utils.a lib/libLLVMAMDGPUAsmPrinter.a lib/libLLVMAMDGPUUtils.a lib/libLLVMARMDesc.a lib/libLLVMARMAsmPrinter.a lib/libLLVMARMInfo.a lib/libLLVMBPFAsmPrinter.a lib/libLLVMHexagonDesc.a lib/libLLVMHexagonInfo.a lib/libLLVMMipsAsmPrinter.a lib/libLLVMMipsInfo.a lib/libLLVMMSP430AsmPrinter.a lib/libLLVMNVPTXAsmPrinter.a lib/libLLVMPowerPCAsmPrinter.a lib/libLLVMPowerPCInfo.a lib/libLLVMSparcAsmPrinter.a lib/libLLVMSparcInfo.a lib/libLLVMSystemZDesc.a lib/libLLVMSystemZAsmPrinter.a lib/libLLVMSystemZInfo.a lib/libLLVMX86CodeGen.a lib/libLLVMX86Desc.a lib/libLLVMX86AsmPrinter.a lib/libLLVMX86Utils.a lib/libLLVMX86Info.a lib/libLLVMXCoreAsmPrinter.a lib/libLLVMAsmPrinter.a lib/libLLVMSelectionDAG.a lib/libLLVMCodeGen.a lib/libLLVMXCoreInfo.a lib/libLLVMMCDisassembler.a lib/libclangCodeGen.a lib/libLLVMipo.a lib/libLLVMVectorize.a lib/libLLVMInstrumentation.a lib/libLLVMObjCARCOpts.a lib/libLLVMScalarOpts.a lib/libLLVMInstCombine.a lib/libLLVMTarget.a lib/libLLVMBitWriter.a lib/libLLVMIRReader.a lib/libLLVMAsmParser.a lib/libLLVMLinker.a lib/libLLVMTransformUtils.a lib/libLLVMAnalysis.a lib/libLLVMProfileData.a lib/libLLVMObject.a lib/libclangRewriteFrontend.a lib/libclangARCMigrate.a lib/libclangStaticAnalyzerFrontend.a lib/libclangFrontend.a lib/libclangDriver.a lib/libLLVMOption.a lib/libclangParse.a lib/libLLVMMCParser.a lib/libclangSerialization.a lib/libLLVMBitReader.a lib/libclangSema.a lib/libclangEdit.a lib/libclangStaticAnalyzerCheckers.a lib/libclangStaticAnalyzerCore.a lib/libclangAnalysis.a lib/libclangAST.a lib/libclangRewrite.a lib/libclangLex.a lib/libclangBasic.a lib/libLLVMCore.a lib/libLLVMMC.a lib/libLLVMSupport.a -lrt -ldl -ltinfo -lpthread -lz -lm -Wl,-rpath,"\$ORIGIN/../lib"

> I'm not necessarily asking you to revert this until we get more detailed analysis, but I'll appreciate if you can gather informations and make them public.

>  In the history of LLD (the old one) some changes were made "in the name of performance" without really understanding the effects. I'm not exactly looking forward to see this again.

> 

> Thanks!

Repository:
  rL LLVM

http://reviews.llvm.org/D15564