[LLVMdev] C++ to C?

Daniel Dunbar daniel at zuster.org
Sat Oct 11 13:18:08 PDT 2008

Hi Michael,

On Sat, Oct 11, 2008 at 12:44 PM, Michael McDonnell <
michaeldmcdonnell at yahoo.com> wrote:
> My assumption has been that LLVM generates machine code for a virtual
> machine, and that you supply an interpreter that will execute the code.

The name can be somewhat confusing. LLVM is a lot of things, the web page
gives some important areas (http://llvm.org/). In your case it sounds like
you are mainly interested in the "virtual instruction set" aspect. In this
case, yes, llvm-gcc does generate "machine code" (LLVM intermediate
representation (IR)) for the virtual instruction set, which lli can
interpret directly. Additionally, LLVM supplies a variety of tools for
working with .bc files (serialized versions of this formation), i.e. for
linking, archiving, etc.

> I'm interested in this from an educational standpoint. What I'd like is a
> C/C++ compiler that generates machine code for a virtual software machine.
> Ideally the machine would support interrupts, timers, DMA controllers,
> etc.. I know that your interpreter does not, but I thought I might add these
> peripherals in.

Using LLVM is a viable strategy for this. However, it is a question of how
much support you are expecting. The main benefit you are getting is precise
semantics for LLVM IR and the tool chain for working with .bc files. This
allows you to avoid dealing with many nitty particulars of x86 (assuming
that is your target). On the other hand, the current interpreter makes no
pretense of running on a "virtual machine", so if this is your goal you will
need to build those facilities yourself. Finally, using LLVM IR directly may
pose some issues depending on what level of precision you want. Since a
significant amount of work is done in code generation for the particular
target, the actual x86 instructions which are generated may access memory
"differently" than your interpretation of the LLVM IR would; generally this
would be because the source code didn't constrain things appropriately
(volatile) but it is something to be cognizant of.

 - Daniel

If you have any suggestions I'd appreciate hearing them. I know about the
> various PC emulators like BOCHS, but they're doing a lot more than I need.
> Thanks,
> M. McDonnell
> --- On *Sat, 10/11/08, Duncan Sands <baldrick at free.fr>* wrote:
> From: Duncan Sands <baldrick at free.fr>
> Subject: Re: [LLVMdev] C++ to C?
> To: michaeldmcdonnell at yahoo.com
> Cc: llvmdev at cs.uiuc.edu
> Date: Saturday, October 11, 2008, 12:25 PM
> Hi Michael,
> > Thank you very much for your help. I have a few more questions if you have
> a moment...
> >
> > * Are there executables available for windows?
> I think so, but since I don't use windows I can't say for sure.
> > * Is the source code for the interpreter available, and if so, what is/are
> the filename(s)?
> Sure, all source code is available: this is an open source project!
> Do you really mean the interpreter?  You seemed more interested in
> the C backend.  In any case, you can find source code here:
>   http://llvm.org/releases/
> For the 2.3 release:
>   http://llvm.org/releases/download.html#2.3
> > * Is there an IDE available?
> LLVM is not a compiler.  It is used by various compilers such
> as llvm-gcc and clang.  One of those might have an IDE, but I
> wouldn't know since I never use IDE's myself.
> Ciao,
> Duncan.
> PS: Please don't send messages just to me: CC to mailing
> list too.  That way others can answer you too, and the
> discussion is recorded in the archives where others with
> the same questions can find it.
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