Error while making LLVM on ubuntu 19.10

[sabinaya]

I'm attempting to build and install LLVM and clang on my ubuntu 19.10 machine. I'm facing the below issue.

On my further research, I figured out that it's because my system doesn't have enough RAM for building.
Below is a screenshot of my system's RAM availability.

Can anybody help me with the minimum system requirements for the LLVM build and install?
Is there any build system for usage?
Any workarounds for this problem?

[badenh]

Hi

I'm not aware of a hard lower limit for building, but most people say OOB that you need 4Gb to build llvm and clang.

Of course, you could install a precompiled binary for Ubuntu. See https://apt.llvm.org/
The repos to add are:

# i386 not available
deb http://apt.llvm.org/eoan/ llvm-toolchain-eoan main
deb-src http://apt.llvm.org/eoan/ llvm-toolchain-eoan main
# 9
deb http://apt.llvm.org/eoan/ llvm-toolchain-eoan-9 main
deb-src http://apt.llvm.org/eoan/ llvm-toolchain-eoan-9 main
# 10

If you need to build then there's still some other options.

If you can't add more RAM, there's a few things you can do to reduce the resources requirements to build clang and llvm.

1. You can use gold instead of ld as a linker, by passing the parameter
   -DLLVM_USE_LINKER=gold

2. You can reduce the set of artefacts created from the build.
   https://llvm.org/docs/CMake.html provides a list of things you can turn off by editing your CMakeCache.txt. For example, using CMAKE_BUILD_TYPE:STRING=Release will avoid producing Debug artefacts and using LLVM_TARGETS_TO_BUILD:STRING=ARM (or whatever suits your requirement) will avoid producing targets for other architectures.

Note: As an alternative to 1), you can also change the linker in the same CMakeCache.txt file eg CLANG_DEFAULT_LINKER:STRING=gold

3. You can reduce the number of parallel jobs that are kicked off by Make, by specifying the number explicitly make -j1 (or some small number)

The combination of these can mean that a build takes a long time (like 8+ hours) but it succeeds on relatively low RAM systems.

Hope this helps

[zmodem]

Can anybody help me with the minimum system requirements for the LLVM build and install?
Is there any build system for usage?
Any workarounds for this problem?

Did you make sure to do a release build and not a debug build? Release builds require much less memory for linking (and also build faster):

cmake -DCMAKE_BUILD_TYPE=Release ...

[sabinaya]

@badenh @zmodem Thanks for the comments.
I'm able to build LLVM with
cmake -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_TARGET_ARCH=X86 -DCMAKE_BUILD_TYPE="Release" -DLLVM_BUILD_EXAMPLES=1 -DCLANG_BUILD_EXAMPLES=1 -DLLVM_USE_LINKER=gold -G "Unix Makefiles" ../source/llvm-project/llvm
make -j 8
But, I also want to build clang along with it. Passing -DLLVM_ENABLE_PROJECTS=clang again results in either a system freeze or error.

[uenoku]

Please try to use Ninja as a build system generator and LLD as a linker.
It is known that these are efficient.

[abrachet]

+1 to not using -j 8 if the build keeps failing with that. Doing this in just one thread may be excruciating but at least you'll get there. You could also increase the amount of swap space you have and the build won't fail it'll just take super long. People also swear by lld but gold is the next best if you don't have it.

I'm guessing because you are building from source and not just installing you want to hack on clang and not just use it? In which case a Release build might not be practical but you could try -DLLVM_USE_SPLIT_DWARF=ON