llvm dev - Mar 2009 - [LLVMdev] n00b question: From module/bitcode to Mach-O dylib file directly?

I'm a total LLVM n00b, and have just started to work through some of  
the tutorials with the intention of gathering a clear picture of what  
LLVM does and doesn't do for a possible project.

I'm on the Mac, and would like to have my code dynamically create and  
load new functions into a process.  I believe I can do this, but I'm  
not sure yet how 'direct' things will be in terms of both the tool  
chain, and how much I need to emit intermediate files.

I think I will have to ultimately dynamically load from a Mach-O dylib  
file (dopen), but in any case I'm interested in whether I can go  
straight from my in-memory linked module(s) to a Mach-O file that I  
can dynamically load, using the LLVM API as it exists at V2.5.

Before I even worry about coding this up directly, I've been playing  
with doing this the available tools to see what the various steps are  
likely to be.  From comments on this list, it looks like going from  
bitfile to a native Mach-O file ought to be possible, but I've tried  
"llc -filetype=dynlib" (and indeed -filetype=obj) and get the message
"target does not support generation of this file type!".  BTW, I  
compiled the LLVM stuff myself, but assume it does the right thing  
w.r.t. building in appropriate knowledge of the native platform.

I can see how to generate an assembly file, then use the platform  
tools to compile and link this, but I get the impression this is  
unnecessary with the current state-of-play.

Would anyone be kind enough to provide clue?  I couldn't find any  
samples or docs that directly speak to this - probably because it's a  
minor part of the overall LLVM system.

Cheers

Luke

On Mar 15, 2009, at 3:20 PM, Luke Evans wrote:
> I'm on the Mac, and would like to have my code dynamically create and
> load new functions into a process.  I believe I can do this, but I'm
> not sure yet how 'direct' things will be in terms of both the tool
> chain, and how much I need to emit intermediate files.
Are you just loading functions into the current process?  There's no  
need
to create temporary object files and dynamically link them in;  LLVM is
perfectly capable of assembling a function in memory.  Look up the
docs for llvm::ExecutionEngine::create and go from there.

John.

Thanks John.

I had passed over ExecutionEngine as it looked like it offered a JIT.   
Maybe there's more to it than meets the (hasty) eye though.

I'm interested in getting a native image.  Ultimately, I'd like to do  
things like emitting Objective-C IMPs and building Objective-C classes  
around them.
However, I'm going one step at a time (there's probably much to learn  
and maybe gotchas to discover before I get to this).
To begin with, I figured I'd try to get a Mach-O file written out,  
dload'ed and then make a regular C call to a loaded function.

So, I'll take a look at ExecutionEngine, but I'm still curious if
I'm
currently able to emit Mach-O dylibs directly from the LLVM tool  
chain, or if I have to go a little more round-the-houses (perhaps  
emitting .s and then using platform tools to get to the dylib).

-- lwe


On 15-Mar-09, at 6:09 PM, John McCall wrote:
> Are you just loading functions into the current process?  There's no
> need
> to create temporary object files and dynamically link them in;  LLVM  
> is
> perfectly capable of assembling a function in memory.  Look up the
> docs for llvm::ExecutionEngine::create and go from there.
>
> John.
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On Mar 15, 2009, at 4:23 PM, Luke Evans wrote:
> I had passed over ExecutionEngine as it looked like it offered a  
> JIT.  Maybe there's more to it than meets the (hasty) eye though.
>
It does...
> I'm interested in getting a native image.  Ultimately, I'd like to
> do things like emitting Objective-C IMPs and building Objective-C  
> classes around them.
> However, I'm going one step at a time (there's probably much to  
> learn and maybe gotchas to discover before I get to this).
> To begin with, I figured I'd try to get a Mach-O file written out,  
> dload'ed and then make a regular C call to a loaded function.
>
Aaah. Wasn't quite sure what you were doing here. I'm not positive  
what llvm can emit via the writers (there's support for all parts of  
the file format), but it could be extended to write one out. I'm just  
not quite sure why :)

-eric

> Aaah. Wasn't quite sure what you were doing here. I'm not positive
> what llvm can emit via the writers (there's support for all parts of
> the file format), but it could be extended to write one out. I'm just
> not quite sure why :)
>
> -eric
Well, ultimately I'm curious about what it would take to port a JVM  
based language (http://openquark.org) to LLVM.
A main motivator though is language/library/platform integration on  
the Mac (Cocoa, Objective-C, autozone).  I'm not too interested in a  
JIT at this point, but rather native code generation.

So far I can see a MachOWriter (with an "AddMachOWriter" in  
FileWriters.h) and it looks like I get to pass an appropriate  
TargetMachine to this.
I'm wondering if this is expected to be sufficient (added to a pass  
manager) to be able to output a library on the Mac - whether or not it  
actually works.

I only picked LLVM up yesterday, so I'm still trying to understand how  
the parts work together.  With my current lack of orientation I have  
very little
intuition as to whether I'm on the right track, and if it fails, how  
close (or otherwise) I might have been to getting something going.  So  
I suppose I've
been looking for hints at the vague 'shape' of the code I would expect  
to connect together to get the output I want, and while there are some  
nice samples for some parts of LLVM, I haven't found much to help  
learn how to emit native .o or .dylib (assuming this is possible).

In the meantime I'll spend some time trying to bang some of the  
aforementioned pieces together and see if I can get it to do anything  
interesting with a minimal module.

Thanks.

-- lwe






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