[LLVMdev] [MCJIT] Why does it allocate function by function?

Jim Grosbach grosbach at apple.com
Wed Jul 6 08:28:23 PDT 2011

On Jul 5, 2011, at 6:04 PM, Yuri wrote:

> I am implementing ELFObject class for MCJIT to be able to run ELFs on ELF-platforms.
> One thing bothers me: I see that the RTDyldMemoryManager-based allocator is always passed to MCJIT and to RuntimeDyld classes from outside. This enforces the approach that memory will be allocated function by function with startFunctionBody/endFunctionBody. This maybe was good for the old JIT, but not for MCJIT.
> For ELF for example this isn't right. ELF is designed to be loaded into memory as a whole. I don't know much about MachO, but most likely it should be normally loaded into memory by OS in one block too, as a consequence of efficiency considerations.
> So is this a design flaw that should be fixed? Or am I missing something?
> I think I can lower the allocator from MCJIT into inside of RuntimeDyld. So at least ELF will be able to use the bulk allocator. But I though I ask here first.

Hi Yuri,

You're correct that the current JITMemoryManager interface isn't a very good fit for the MCJIT. For the time being though, the intent is to work with it as much as possible so that the MCJIT can be a drop-in replacement for the old JIT. If we change the JITMemoryManager API right off the bat, we'll make it that much harder for clients to migrate to the new JIT, slowing adoption and reducing testing. If we run into situations where it's just unreasonable to do that, we'll need to re-evaluate, but at least for basic things, it should be reasonably feasible.


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