Disable IPO by default on debug builds in CMake

[sleweke-bayer]

Debug builds produce slow binaries. We do not gain anything by using IPO here. On the contrary, IPO increases build times considerably. So we disable IPO by default on debug builds to save some developer time.

[schmoelder]

Hey everyone,

could someone please explain what IPO means? I guess it's not initial public offering, right? 🤑

[ronald-jaepel]

Googling "IPO c compilation" gives https://en.wikipedia.org/wiki/Interprocedural_optimization . Is that correct?

[sleweke-bayer]

What @ronald-jaepel says: IPO = Interprocedural optimization (mostly link time optimization = LTO). Originally, every compilation unit (roughly a .cpp file) is compiled and optimized individually. The resulting object files are then linked together to form the final binary.
With IPO or LTO, the compilation pass is rather thin since only local optimizations are performed. Most of the heavy-lifting and optimization is postponed to link time, where information from all compilation units (say, .cpp files) is available. This enables optimizations across compilation units.
Therefore, IPO results in faster binaries but has high costs in compile time and memory usage during compile time.

[ronald-jaepel]

When building this PR on Windows 11 with Visual Studio 17.8.5 and MSBuild I get the error:

C1128 number of sections exceeded object file format limit: compile with /bigobj ...\CADET\src\libcadet\model\GeneralRateModelDG.cpp

I therefore replaced

target_compile_options(CADET::CompileOptions INTERFACE $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>,$<CXX_COMPILER_ID:GNU>>:
		-Wall -pedantic-errors -Wextra -Wno-unused-parameter -Wno-unused-function> #-Wconversion -Wsign-conversion
	$<$<CXX_COMPILER_ID:MSVC>:
		/W4 /wd4100>
)

in the CMakeLists.txt

with a check if "Debug" and then an added /bigobj and no change if CMAKE_BUILD_TYPE is not "Debug":

if (CMAKE_BUILD_TYPE STREQUAL "Debug")
	target_compile_options(CADET::CompileOptions INTERFACE $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>,$<CXX_COMPILER_ID:GNU>>:
		-Wall -pedantic-errors -Wextra -Wno-unused-parameter -Wno-unused-function> #-Wconversion -Wsign-conversion
	$<$<CXX_COMPILER_ID:MSVC>:
		/W4 /wd4100 /bigobj>
)
else()
	target_compile_options(CADET::CompileOptions INTERFACE $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>,$<CXX_COMPILER_ID:GNU>>:
		-Wall -pedantic-errors -Wextra -Wno-unused-parameter -Wno-unused-function> #-Wconversion -Wsign-conversion
	$<$<CXX_COMPILER_ID:MSVC>:
		/W4 /wd4100>
)
endif()

After that it builds. I rebased onto master and compared the two and with IPO it takes 2:39.6 (minutes:seconds.milliseconds) and without IPO it takes 2:39.1 . Is the difference larger on your machine Sam?

[sleweke-bayer]

It doesn't make a noticeable difference on my current machine. It makes a difference on my old one, though.

[github-actions]

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[sleweke-bayer]

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[sleweke-bayer]

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