[LLVMdev] Another way to JIT: "dlopen from memory"

[Félix Cloutier]

Wouldn't it mean each time a function is compiled it would need to be bundled to its own dylib? How well would that scale?

Félix

Le 2010-07-07 à 15:41:18, Reid Kleckner a écrit :

> On Tue, Jul 6, 2010 at 9:08 AM, Greg Clayton <gclayton at apple.com> wrote:
>>> 2) Can I easily debug JIT'd code with LLDB?
>> 
>> Are you running JIT'd code within another process that can be debugged (i.e. the simulation binary)? If so, you might want to have LLVM generate a full blown dylib, not just a JIT'd chunk of code and load the dylib using the standard shared library load/unload calls. Why? This gets you around the fact that JIT'd code isn't recognized by the dynamic linker on most systems. /usr/bin/dyld on Mac OS X has some provisions for JIT'd code, but nothing that gives debuggers visibility into that code. So going the dylib route has a few benefits:
>> - code will work with any debugger (gdb and lldb) in a standard kind of way
>> - dylib's can have debug information generated in the standard way
>> - no need to manage memory chunks for JIT'd code
> 
> This idea has been kicked around a few times on IRC and elsewhere, and
> I think it's pretty clever.  The idea is to JIT by making a valid
> dylib/dll/so (heceforth, dylib) with the JITed code in memory, and
> then convincing the loader to load it.  This is nice for the reasons
> listed above.
> 
> I think it would be neat if LLVM could package up this functionality,
> since it fits most use cases for the current LLVM JIT.  LLVM
> compilation is already heavyweight, so the overhead of linking and
> loading the library shouldn't be too bad.
> 
> For now, this can be achieved by rolling your own compilation pipeline of:
> 1. run llc or the equivalent to make a .s in a temp dir
> 2. assemble it
> 3. link it to make a dylib in the temp dir
> 4. dlopen it
> 5. use dladdr to get the function pointer for your code
> 
> 1 and 2 can already be done in memory with LLVM.
> 
> 3 cannot, and doesn't look like it is on anyone's roadmap.
> 
> I think 4 may become possible on OS X, and there's this feature
> request for glibc that essentially implements "dlopen from memory":
> http://sourceware.org/bugzilla/show_bug.cgi?id=11767
> 
> 5 is trivial.
> 
> Even a prototype of this initial JIT model that uses temporaries on
> the filesystem would be useful for JIT users doing debugging or
> profiling.
> 
> I don't really have time to implement this.  I'm just kicking the
> proverbial can down the road.  I may file a feature request PR or
> write up something in the style of sabre's LLVMNotes:
> http://nondot.org/sabre/LLVMNotes/
> 
> Reid
> 
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