[LLVMdev] Using LLVM to generate x86 dynamically in memory
shanko_chavano at hotmail.com
Tue Feb 2 16:39:09 PST 2010
Hi Jim. Thanks for your speedy response. I'm not entirely sure if a JIT is what I'm looking for. I'm basically looking for a dll with an interface that takes a C program as input and compiles and optimizes it to native x86 instructions in an in-memory buffer. I don't want the dll the execute it, and I don't particularly want to translate our expressions into LLVM bitcode (although I can if the rest of the pieces are there). Also, I briefly read up on lli. This looks like a separate process will have to be spawned to invoke the JIT to execute programs in LLVM bytecode. This will definitely incur an overhead penalty that we wouldn't want to pay. Thanks in advance for your response.
BTW - I was doing some google searching just now on the topic again, and i was surprised to see how fast google had already found the log of this previous conversation recorded at uiuc. ;).
> Subject: Re: [LLVMdev] Using LLVM to generate x86 dynamically in memory
> From: grosbach at apple.com
> Date: Tue, 2 Feb 2010 15:24:23 -0800
> CC: shanko_chavano at hotmail.com
> To: llvmdev at cs.uiuc.edu
> On Feb 2, 2010, at 3:05 PM, Shasank Chavan wrote:
> > Hi. I'm very new to LLVM, and have read some of the documentation online. Before I invest too much time, however, in learning about LLVM, I'd like to know if it can be used for my purpose. We currently have a critical runtime component that evaluates expressions via interpretation. The bytecode that we generate goes through various optimization phases similarly to that of optimizing language compilers (although definitely not as complete) before being evaluated at runtime. For quite some time now we have been thinking about getting away from interpretation all-together and generating native code directly.
> > What I'd like to know is:
> > 1) Is there an LLVM backend library that can take either a) a C source program in memory or b) an LLVM program in memory, as input and generate x86 instructions in memory?
> LLVM does support a Just-In-Time (JIT) compiler interface, which is what it sounds like you're looking for. I would suggest looking more deeply into the llvm documentation at llvm.org, paying particular attention to JIT references and the lli utility.
> > 2) HOw "light-weight" would a mechanism like this be? I suppose that depends on what optimizations we decided to apply during this code-gen phase.
> It does indeed vary quite a bit. I suspect your best bet will be to create some test programs, or just use some of the examples from the llvm source tree, and look at the footprint and compare to what your needs are.
> > What we plan to do is invoke this in-memory compiler to generate the x86 instructions, and then ship them at runtime to our new expression evaluation engine that will simply set a function pointer to it and execute.
> > I heard of a compiler called TinyCC that basically has a library (libtcc) that can be invoked at runtime to generate x86 instructions directly from a C program stored in memory. That's what I would like, but with the aggressive opts provided in the LLVM infrastructure. Thanks so much for your time...
> > - Shasank
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