Compile time (no link time) comparison between ELF and REPO targets. about llvm-prepo

[MaggieYingYi]

Commands for each step:

1. Build llvm-repo/clang-repo toolchain with Release configuration.

% cd  ~/git/github/llvm-prepo
% tools/pstore/utils/make_build.py -DCAMKE_BUILD_TYPE=Release -o Release

2. Modify the Makefile of ~/git/github/llvm-prepo/tools/pstore to compile the project only, no link.
   Build a static library, which is basically just a container for object files.

Modifications:
a) set (CMAKE_CXX_FLAGS_DEBUG  "-O0")
b) set (CMAKE_CXX_FLAGS_RELEASE "-O3 -DNDEBUG")
c) change  add_pstore_executable to add_pstore_library
d) change add_pstore_unit_test to add_pstore_test_library

3. Build pstore library using built llvm-repo/clang-repo compiler targeted on ELF, get the compile
   time.

change CMakefile targeted on ELF:   set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -target x86_64-pc-linux-gnu-elf")

% cd  ~/git/github/llvm-prepo/tools/pstore
% utils/make_build.py -DCAMKE_BUILD_TYPE=Debug(/Release) -DCMAKE_C_COMPILER=~/git/github/llvm-prepo/Release/bin/clang -DCMAKE_C++_COMPILER=~/git/github/llvm-prepo/Release/bin/clang++ -o ELF_Debug(/ELF_Release)
% cd ELF_Debug(/ELF_Release)
% ninja clean
% /usr/bin/time -o time_report_for_elf.log ninja

4. Build pstore library using built llvm-repo/clang-repo compiler targeted on Repo, get the compile
   time.

Change CMakefile targeted on Repo:  set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -target x86_64-pc-linux-gnu-repo")

% cd  ~/git/github/llvm-prepo/tools/pstore
% utils/make_build.py -DCAMKE_BUILD_TYPE=Debug(/Release) -DCMAKE_C_COMPILER=~/git/github/llvm-prepo/Release/bin/clang -DCMAKE_C++_COMPILER=~/git/github/llvm-prepo/Release/bin/clang++ -o Repo_Debug(/Repo_Release)
% cd Repo_Debug(/Repo_Release)
% rm -f clang.db
% ninja clean
% /usr/bin/time -o time_report_for_repo_first_time.log ninja

5. Build pstore library again and get the compile time.

% cd ~/git/github/llvm-prepo/tools/pstore/Repo_Debug(/Repo_Release)
% ninja clean
% /usr/bin/time -o time_report_for_repo_seond_time.log ninja

[MaggieYingYi]

The detailed compile time has been given in https://github.com/SNSystems/llvm-prepo/wiki/Performance