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

[Michael Zolotukhin via cfe-dev]

> On Dec 5, 2017, at 9:38 PM, Chris Lattner <clattner at nondot.org> wrote:
> 
> I, for one, want faster builds.
Good, we have at least two people on board then :)
> 
> Beyond that though, this seems like obvious goodness to reduce coupling in the codebase.  I’ve only skimmed the patch, but this seems like a clearly amazingly great ideas.  Did you use the IWYU tool or something else?
I tried using it, but while it gave me some interesting hints, I stopped using it when the build broke after the proposed changes. Probably, I could’ve figured out what went wrong and made it work, but I also noticed that the proposed by IWYU changes are much more intrusive - i.e. it tries to forward declare symbols, analyze include chains and leave only the last include etc (and it only would work if one applies the changes to all affected files at once). While these all are good ideas, I'd expect some objections against mechanical application of such clean-ups. Plus the patch would be much less obvious.

So, instead, I implemented a light-weight version of it that just tries to remove #include lines, making the footprint of this cleanup local. As a result, here I get a patch consisting of many independent changes (it can be applied per file and everything should work fine). More details of how that was done is in “Methodology” section in the end of the original e-mail.

Thanks,
Michael

> 
> -Chris
> 
> 
>> On Dec 5, 2017, at 3:40 PM, Mikhail Zolotukhin via llvm-dev <llvm-dev at lists.llvm.org <mailto: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:
>> <llvm.txt>
>> 
>> 
>> 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:
>> <clang.txt>
>> 
>> The corresponding patches (careful, they are big):
>> <llvm_redundant_headers.patch>
>> <clang_redundant_headers.patch>
>> 
>> 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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