[LLVMdev] Questions on llvm and jit

Howell, Nathan
Fri, 26 Mar 2010 21:16:56 -0000

If you really trying to avoid linking everything into a shared library (easiest
choice), consider splitting up the compilation into a few more steps:

1) Compile and optimize once as part of your build, target LLVM bitcode instead
of machine code
2) Ship .bc files out to each node
3) Fix-up call instructions in a BasicBlockPass, run JIT without any/many
additional IR optimizations enabled

This won't get you around all the optimization passes and the like but
it's still pretty fast and a lot easier than trying to patch a direct call
in machine code.

-n

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Shasank Chavan
Sent: Friday, March 26, 2010 1:52 PM
To: rnk at mit.edu
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Questions on llvm and jit

Hi Reid.  Thanks for your response.  We will be running this code in different
processes across different nodes.  Basically we have thousands of
"executor" processes that are solely responsible for executing this
code generated at compile-time across partitioned data.  Rather than each one of
these processes invoking the jit and compiling with full opts and all of that,
we believe it may be cheaper to do this once at compile-time and have the
processes simply point to it and execute.  As for a dll, compiling, linking,
sending over the dll to the different nodes, and then calling into the dll with
dlopen will not be cheap to do, and for that reason we're avoiding it.

So for all the reasons above, we prefer everything to be inlined where possible,
and, if need be, have a mechanism to fix up target addresses of call
instructions.  I'm more familiar with ia64 than x86, and so it's a
learning process for me to understand how easy it is to do this.  First off, is
there common code out there or in the dll that will allow me to walk through x86
instructions in search for a call?

Thanks again for your response.

- Shasank

> From: rnk at mit.edu
> Date: Fri, 26 Mar 2010 11:51:24 -0400
> Subject: Re: [LLVMdev] Questions on llvm and jit
> To: shanko_chavano at hotmail.com
> CC: llvmdev at cs.uiuc.edu
>
> On Tue, Mar 23, 2010 at 4:44 PM, Shasank Chavan
> <shanko_chavano at hotmail.com> wrote:
> > Hi.  I have more questions regarding llvm and using it as a jit for
our
> > purposes.  Also, let me confess that I haven't actually used llvm
yet (I'm
> > still prototyping using gnu's libjit).  Some of the issues that
have come up
> > from that work so far leads to me these questions:
> >
> > 1) We intend to use llvm as a jit in our expression compiler at
compile-time
> > only.  At runtime, the x86 code generated from the bitcode compiler
will be
> > packed and sent to our runtime facility for execution.  This implies a
need
> > to fixup things like external function calls made in the function we
> > generated at compile-time.  So suppose a call to "memcpy" is
made in our
> > function F.  The target address for memcpy will need to fixed up at
runtime
> > in the x86 code in memory to point to the actual memcpy available on
the
> > system.  Obviously we may have problems if memcpy is bound to a
different
> > load module (i.e. c runtime), so we may instead fix up the call to
point to
> > a wrapper call that's bounded to the app.  The question is, is it
possible
> > to walk through the x86 instructions in search for "call"
instructions to
> > then fix up the target?  Does llvm have a facility to walk through the
> > actual machine code instructions to do this?
> >
> > 2) Similarly to (1), any external calls made by llvm (e.g. calls to
> > intrinsic functions) will either need to be fixed up (as discussed in
(1))
> > or prevented all-together.  Is there a way to inline all intrinsic
calls so
> > that no calls are made other than those purposefully inserted by me.
> > Similarly, if a branch table is generated for a switch statement,
where in
> > memory will this sit relative to the generated x86 code for the
function?
> > Will it be possible to copy both the function and the jump table
together
> > into our memory buffer for packing and shipping to our runtime
facility?
> > Will there be wasted space?  Clearly the location of the jump table is
> > relevent in terms of ensuring that the branch target offsets are
correct
> > within the function, and for that reason this all matters.
>
> Why do you need this? Where are you running the code being generated,
> ie what is your "runtime facility"? Is it another process on the
same
> system or another machine on the network?
>
> In this case it might be easier for you to compile and link a shared
> library and then dlopen it in your runtime facility.
>
> > 3) I noticed with libjit that the reg-allocator and alias analysis
(assuming
> > this exists in libjit) is not the best.  I wonder if llvm will have a
> > problem.  If I make the appropriate llvm calls to redundandly load an
> > integer of a pointer several times (e.g. (goo == *a) .... ....  (foo
=> > *a)), will llvm make every effort to reusue the temp created to store
*a
> > initially in the second reference?  I tried this out with the llvm
demo, and
> > the C code generated automatically created a temp for the first load,
and
> > then reused that temp in the secondary reference.  I however would not
be
> > making such calls - I instead would probably generate the memory
reference
> > twice, and expect llvm to optimize this.  Is my expectation correct?
>
> It should.
>
> Reid
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