I am trying to understand the build process of llvm + clang from source.
These are the commands I executed, on a Linux Mint 19 Tara machine with a i5-6600T CPU @ 2.70GHz and 24GB of RAM:
git clone https://github.com/llvm/llvm-project.git llvm.git mkdir llvm.bld cd llvm.bld cmake ../llvm.git/llvm -G "Unix Makefiles" -DLLVM_ENABLE_PROJECTS=clang -DCMAKE_BUILD_TYPE=Release time make -j 4
The first time, this takes a few hours to compile; that is to be expected. However, when I run
make -j 4 again without touching any source files, it still takes more than 3 minutes. Why is that? Here are the actual timings I got on one run:
real 2m3.950s user 3m15.365s sys 0m14.494s
To get better insight, I tried
time make VERBOSE=1 so that it shows what commands it is executing. It seems to be calling cmake recursively, e.g.:
/usr/bin/cmake -H/home/johan/work/llvm.git/llvm -B/home/johan/work/llvm.bld --check-build-system CMakeFiles/Makefile.cmake 0
It is also calling
make recursively, possibly to generate dependencies, e.g.:
make -f lib/Demangle/CMakeFiles/LLVMDemangle.dir/build.make lib/Demangle/CMakeFiles/LLVMDemangle.dir/depend
Finally, it also seems to be regenerating some
*.inc files, e.g.:
cd /home/johan/work/llvm.bld/tools/clang/include/clang/StaticAnalyzer/Checkers && ../../../../../../bin/clang-tblgen -gen-clang-sa-checkers -I /home/johan/work/llvm.git/clang/include/clang/StaticAnalyzer/Checkers -I /home/johan/work/llvm.git/clang/include -I /home/johan/work/llvm.bld/tools/clang/include -I /home/johan/work/llvm.bld/include -I /home/johan/work/llvm.git/llvm/include /home/johan/work/llvm.git/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td --write-if-changed -o /home/johan/work/llvm.bld/tools/clang/include/clang/StaticAnalyzer/Checkers/Checkers.inc
Why is it doing so much work when nothing changed?
What about the lessons in “Recursive Make Considered Harmful”? (See e.g. Recursive Make Considered Harmful)
When I am only interested in a specific executable, say
clang-11, is there a faster way to rebuild it after a change in a single file, or do I always have to wait the full 2 minutes plus actual compile and line time?