[Lldb-commits] [PATCH] D90769: [lldb][ObjectFile] Relocate sections for in-memory objects (e.g. received via JITLoaderGDB)

[Stefan Gränitz via Phabricator via lldb-commits]

sgraenitz added a comment.

Thanks for having a look!

The JIT implementations in LLVM operate on relocatable object files, so someone needs to resolve them. MCJIT has a flag `ProcessAllSections` to control which sections will be loaded and relocated. As it is off by default, it will usually ignore sections that it doesn't need for execution. LLDB does it the other way around in ObjectFileELF::RelocateSection(), it only processes sections with the ".debug" prefix. This seems to be a reasonable distribution of tasks. In general, relocations are resolved in place, so it doesn't rise memory consumption.

> I do wonder though if the jit could be changed to avoid relocation.

Well, I don't think we are anywhere close to optimizations like this, but it would be nice indeed. If we compile from bitcode on the JIT side, we could lookup external symbols at compile-time and don't produce relocations for them in the first place. I guess it would heavily reduce the number of relocations and potentially save time. On the other hand, thinking about concurrent compile jobs and cross-dependencies.. I can imagine it gets hairy quickly. Plus: the way it is now, we can cache the object files and reuse them thanks to position-independent code.


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https://reviews.llvm.org/D90769