This is one of the things which cheeses me the most about LLVM. I can't build LLVM on less than 16GB of RAM without it swapping to high heaven (and often it just gets OOM killed anyways). You'd think that LLVM needing >16GB to compile itself would be a signal to take a look at the memory usage of LLVM but, alas :)

The thing that causes you run out of memory isn't actually anything in LLVM, it's all in ld. If you're building with debugging info, you end up pulling in all of the debug symbols for deduplication purposes during linking, and that easily takes up a few GB. Now link a dozen small programs in parallel (because make -j) and you've got an OOM issue. But the linker isn't part of LLVM itself (unless you're using lld), so there's not much that LLVM can do about it.

(If you're building with ninja, there's a cmake option to limit the parallelism of the link tasks to avoid this issue).