[llvm-dev] Who wants faster LLVM/Clang builds?

[NAKAMURA Takumi via llvm-dev]

By the way, have you tried -fmodules ?

Takumi

On Wed, Dec 6, 2017 at 1:41 PM Mikhail Zolotukhin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> Recently I've done some experiments on the LLVM/Clang code and discovered
> that many of our source files often include unnecessary header files. I
> wrote a simple tool that eliminates redundant includes and estimates
> benefits of doing it, and the results were quite nice: for some files we
> were able to save 90% of compile time! I think we want to apply some of the
> cleanups I found, but I'm not sure how to better do it: the total patches
> are 8k lines of code for LLVM and 3k lines of code for clang (I'll attach
> them for reference). My suggestion would be that people take a look at the
> list of changed files and pick the changes for the piece of code they are
> working on if the changes look sane (the changes do need some checking
> before committing). Does it sound like a good idea? I'd appreciate any
> feedback on what can we do here.
>
> The list of files for which removing redundant headers improved compile
> time (the numbers are compile time in seconds for a Debug build):
>
> *LLVM top 10*
> *Filename Old New Delta*
> lib/CodeGen/GlobalISel/GlobalISel.cpp 0.26 0.02 -91.9%
> lib/MC/MCLabel.cpp 0.19 0.02 -88.2%
> tools/llvm-readobj/ObjDumper.cpp 0.43 0.10 -76.5%
> lib/MC/MCWinEH.cpp 0.51 0.13 -74.3%
> lib/Transforms/Vectorize/Vectorize.cpp 0.72 0.29 -59.7%
> tools/llvm-diff/DiffLog.cpp 0.58 0.26 -54.6%
> lib/Target/ARM/MCTargetDesc/ARMMachORelocationInfo.cpp 0.46 0.26 -44.1%
> lib/DebugInfo/DWARF/DWARFExpression.cpp 0.68 0.38 -43.3%
> lib/LTO/LTOModule.cpp 2.25 1.33 -41.1%
> lib/Target/TargetMachine.cpp 1.76 1.10 -37.8%
>
> Full list:
>
>
> *Clang top 10*
> *Filename Old New Delta*
> tools/libclang/CXString.cpp 1.70 0.25 -85.2%
> lib/Tooling/CommonOptionsParser.cpp 1.69 0.55 -67.3%
> lib/AST/StmtViz.cpp 1.02 0.44 -57.4%
> tools/driver/cc1_main.cpp 2.26 0.97 -57.1%
> unittests/CodeGen/BufferSourceTest.cpp 3.08 1.83 -40.6%
> lib/CodeGen/CGLoopInfo.cpp 1.91 1.34 -29.9%
> unittests/Tooling/RefactoringActionRulesTest.cpp 2.46 1.79 -27.0%
> unittests/CodeGen/CodeGenExternalTest.cpp 3.43 2.52 -26.5%
> tools/libclang/CXStoredDiagnostic.cpp 1.67 1.26 -24.8%
> tools/clang-func-mapping/ClangFnMapGen.cpp 2.48 1.89 -23.8%
>
> Full list:
>
> The corresponding patches (careful, they are big):
>
> *Methodology*
> My tool took the compile_commands.json from LLVM build and  iterated over
> files trying to remove redundant headers. To find which header files could
> be removed it scanned the file for "#include" lines and tried to remove
> them one by one (checking if the file still compiles after the removal).
> When there were no more include lines to remove, we verified the change
> with ninja+ninja check. After it we compared preprocessed file size before
> and after the change hoping to see that it dropped and then checked the
> compile time impact.
> NB: As a side effect of this approach we removed all include-lines from
> inactive "ifdef" sections, which means that the patches **will** break
> other configurations if applied as-is.
>
> Thanks,
> Michael
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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