[LLVMdev] Re: questions about LLVM

[John T. Criswell]

Shuo Chen wrote:

>Prof. Adve,
>
>The idea is to develop a memory model where each byte is extended with 
>3 extra bits. Programs are running on this memory model.
>Load/store instructions, including those in LibC functions, 
>needs to deal with the extra bits in a certain manner. Basically, my 
>questions are:
>(1) Is it feasible to implement the memory model where each byte is 
>extended with 3 extra bit?
>  
>
I imagine that you could do this by modifying the interpretor or the 
JIT/code generators.  Can you give a more complete description of what 
the memory model does (or what you have to do with these three bits)?  
Depending on what you're doing, you might just be able to get away with 
writing an LLVM transformation pass.

>(2) Is there a LLVM version of LibC (in its VM code format) currently?
>  
>
Currently, no.  We do not have a complete libc implementation compiled 
into LLVM bytecode, although someone might be working on that.

In theory, the only difficult part about getting a C library to work is 
the interface to system calls.  In a traditional libc, assembly code is 
used to provide that functionality and is part of the C library source 
code.  Since LLVM cannot use assembly code, there are two options:

o Add an LLVM syscall intrinsic
o Write a small asm library that wraps the system calls

I believe the latter would be your best option.  It would be fairly easy 
to write and could be loaded by the JIT or linked into native code 
generated from the LLVM bytecode.  In essence, everything above the 
system calls (i.e. fread, printf, etc) would be compiled into LLVM 
bytecode, but system calls  (i.e. read/write/fork) would be native code 
that would be linked in.  For your project, that should not be a problem.

In practice, getting libc compiled with LLVM is painful because C 
libraries tend to have terrible build environments.  For example, glibc 
(used on Linux) assumes that you're compiling for ELF or a.out format 
and uses every GCC feature that could possibly exist.  Other C libraries 
have had strange configuration systems, or make assumptions about the 
operating system, etc.

You can get this to work; it may just be time consuming.

>(3) Is LLVM able to compile HTTP servers, FTP servers and SSH servers to 
>VM code so that every single VM instruction (include LibC code) is 
>executed by the VM interpretor?
>  
>

Well, we've compiled httpd (Apache), ftpd, telnetd, fingerd, the GNU NIS 
daemon, and maybe more.  We haven't compiled sshd yet, but it would 
probably work (assuming that you can easily disable the inline asm it 
will use for quick compression/encryption).

In theory, these should work in the interpreter, but the interpreter 
doesn't get as much attention as the JIT or native code generators.

-- John T.


>thank you very much
>-Shuo
>   
>On Wed, 17 Nov 2004, Vikram S. Adve wrote:
>
>  
>
>>Shuo,
>>
>>    
>>
>>>>I have a few questions about LLVM:
>>>>(1) The LLVM tutorial says LLVM can be used in architecture research. 
>>>>If I want to run my program on an instruction set defined by myself, 
>>>>is LLVM a right tool to do that?
>>>>        
>>>>
>>What kind of instruction set do you have in mind?  The closer it is to 
>>one we already target, the easier this is, but it is quite possible to 
>>write a back-end for a relatively new one.
>>
>>
>>    
>>
>>>>In this aspect, is LLVM similar to SimpleScalar simulator?
>>>>        
>>>>
>>You can use the interpreter as a simulator for a very abstract machine 
>>(and extend it with any performance metrics you want).   How do you 
>>want to use it?
>>
>>--Vikram
>>http://www.cs.uiuc.edu/~vadve
>>http://llvm.cs.uiuc.edu/
>>
>>
>>    
>>
>
>  
>