Hardware requirements to build LLVM

Beginners
1

Hi,

Building LLVM in Debug mode takes more than 50 GB of disk space on my machine and then fails due to insufficient RAM (I have 32 GB). Am I missing a tip that could allow me to build on my 2021 laptop?

I just did:

cmake -S llvm -B build -G 'Unix Makefiles' -DLLVM_ENABLE_PROJECTS=clang\;clang-tools-extra -DCMAKE_BUILD_TYPE=Debug
cd build
make -j8
2

Debug builds certainly take a lot of extra space; there is no easy way around that.

The first thing I would experiment with is the LLVM_PARALLEL_LINK_JOBS CMake variable: -DLLVM_PARALLEL_LINK_JOBS=1

Linking tends to use the most RAM. You can try increasing it incrementally to see what works best for you.

3

Depending on what system that is, you might be using ld.bfd to link. That always fails for me with a debug build of LLVM, even on systems with exorbitant amounts of ram. Try using LLD instead.

4

Thanks, I used -DLLVM_PARALLEL_LINK_JOBS=1 and I build with clang, which is much faster. I no longer run out of RAM. But it looks like building with clang does not work in the end?

$ cmake -S llvm -B build -G 'Unix Makefiles' -DLLVM_ENABLE_PROJECTS=clang -DLLVM_ENABLE_RUNTIMES=compiler-rt -DCMAKE_BUILD_TYPE=Debug -DLLVM_PARALLEL_LINK_JOBS=1
[...]
[ 95%] Built target sanstats
[ 95%] Built target split-file
[ 95%] Built target verify-uselistorder
[ 97%] Built target yaml2obj
[ 97%] Built target builtins-clobber
[ 97%] Performing build step for 'builtins'
[  0%] Building C object CMakeFiles/clang_rt.builtins-x86_64.dir/emutls.c.o
/home/olivier/other_projects/llvm-project/compiler-rt/lib/builtins/emutls.c:10:10: fatal error: 'stdlib.h' file not found
#include <stdlib.h>
         ^~~~~~~~~~
1 error generated.
make[5]: *** [CMakeFiles/clang_rt.builtins-x86_64.dir/build.make:1672: CMakeFiles/clang_rt.builtins-x86_64.dir/emutls.c.o] Error 1
make[4]: *** [CMakeFiles/Makefile2:416: CMakeFiles/clang_rt.builtins-x86_64.dir/all] Error 2
make[3]: *** [Makefile:136: all] Error 2
make[2]: *** [runtimes/CMakeFiles/builtins.dir/build.make:86: runtimes/builtins-stamps/builtins-build] Error 2
make[1]: *** [CMakeFiles/Makefile2:94178: runtimes/CMakeFiles/builtins.dir/all] Error 2
make: *** [Makefile:156: all] Error 2
5

@OlivierNicole If you don’t need an LLVM with all targets, this might also reduce the resources needed for a compilation.

LLVM_TARGETS_TO_BUILD:STRING
    Control which targets are enabled. For example you may only need to enable your native target with, for example, -DLLVM_TARGETS_TO_BUILD=X86.

Find more options here: Building LLVM with CMake — LLVM 17.0.0git documentation

6

Thanks for your advice. I can almost build without OOM now. Looks like the rest of my build issues are 32-bit-related (and so unrelated to this thread).

7

I usually don’t build debug builds, because they are so big. And because I generally don’t need to debug LLVM builds. What I do instead, is that I make a release build with assertions enabled. This will catch most simple mistakes and still produces reasonably usable stack trace when hitting an assertion.

(The other suggestions are also a good idea, especially using lld because it’s so much faster).