[LLVMdev] Help with using LLVM to re-compile hot functions at run-time

Lang Hames lhames at gmail.com
Mon Jul 27 19:58:41 PDT 2015

Hi Revital,

What do you mean by "code cache"? Orc (and MCJIT) does have the concept of
an ObjectCache, which is a long-lived, potentially persistent, compiled
version of some IR. It's not a key component of the JIT though: Most
clients run without a cache attached and just JIT their code from scratch
in each session.

Recompilation is orthogonal to caching. There is no in-tree support for
recompilation yet. There are several ways that it could be supported,
depending on what security / performance trade-offs you're willing to make,
and how deep in to the LLVM code you want to get. As things stand at the
moment all function calls in the lazy JIT are indirected via function
pointers. We want to add support for patchable call-sites, but this hasn't
been implemented yet. The Indirect calls make recompilation reasonably
easy: You could add a transform layer on top of the CompileCallbackLayer
which would modify each function like this:

void foo$impl() {          void foo$impl() {
  // foo body        ->      if (trigger_condition) {
}                              auto fooOpt = jit_recompile_hot(&foo);
                             // foo body

You would implement the jit_recompile_hot function yourself in your JIT and
make it available to JIT'd code via the SymbolResolver. When the trigger
condition is met you'll get a call to recompile foo, at which point you:
(1) Add the IR for foo to a 2nd IRCompileLayer that has been configured
with a higher optimization level, (2) look up the address of the optimized
version of foo, and (3) update the function pointer for foo to point at the
optimized version. The process for patchable callsites should be fairly
similar once they're available, except that you'll trigger a call-site
update rather than rewriting a function pointer.

This neglects all sorts of fun details (threading, garbage collection of
old function implementations), but hopefully it gives you a place to start.

Regarding laziness, as Hal mentioned you'll have to provide some target
support for PowerPC to support lazy compilation. For a rough guide you can
check out the X86_64 support code in
llvm/include/llvm/ExecutionEngine/Orc/OrcTargetSupport.h and

There are two methods that you'll need to implement:
insertCompileCallbackTrampoline and insertResolverBlock. These work
together to enable lazy compilation. Both of these methods inject blobs of
target specific code in to the JIT process. To do this (at least for now) I
make use of a handy feature of LLVM IR: You can write raw assembly code
directly into a bitcode module ("module-level asm"). If you look at the X86
implementation of each of these methods you'll see they're written in terms
of string-streams building up a string of assembly which will be handed off
to the JIT to compile like any other code.

The first blob that you need to be able to output is the resolver block.
The purpose of the resolver block is to save program state and call back in
to the JIT to trigger lazy compilation of a function. When the JIT is done
compiling the function it returns the address of the compiled function to
the resolver block, and the resolver block returns to the compiled function
(rather than its original return address).

Because all functions share the same resolver block, the JIT needs some way
to distinguish them, which is where the trampolines come in. The JIT emits
one trampoline per function and each trampoline just calls the resolver
block. The return address of the call in each trampoline provides the
unique address that the JIT associates with the to-be-compiled functions.
The CompileCallbackManager manages this association between trampolines and
functions for you, you just need to provide the resolver/trampoline

In case it helps, here's what the output of all this looks like on X86.
Trampolines are trivial - they're emitted in blocks and proceeded by a
pointer to the resolver block:

module asm "Lorc_resolve_block_addr:"
module asm "  .quad 140439143575560"
module asm "orc_jcc_0:"
module asm "  callq *Lorc_resolve_block_addr(%rip)"
module asm "orc_jcc_1:"
module asm "  callq *Lorc_resolve_block_addr(%rip)"
module asm "orc_jcc_2:"
module asm "  callq *Lorc_resolve_block_addr(%rip)"

The resolver block is more complicated and I won't provide the full code
for it here. You can find it by running:

lli -jit-kind=orc-lazy -orc-lazy-debug=mods-to-stderr <hello_world.ll>

and looking at the initial output. In pseudo-asm though, it looks like this:

module asm "jit_callback_manager_addr:"
module asm "  .quad 0x46fc190" // <- address of callback manager object
module asm "orc_resolver_block:"
module asm "  // save register state."
module asm "  // load jit_callback_manager_addr into %rdi
module asm "  // load the return address (from the trampoline call) into
module asm "  // %rax = call jit(%rdi, %rsi)
module asm "  // save %rax over the return address
module asm "  //  restore register state
module asm "  //  retq"

So, that's a whirlwind intro to implementing lazy JITing support for a new
architecture in Orc. I'll try to answer any questions you have on the
topic, though I'm not familiar with PowerPC at all. If you're comfortable
with PowerPC assembly I think it should be possible to implement once you
grok the concepts.

Hope this helps!


On Jul 26, 2015, at 11:17 PM, Revital1 Eres <ERES at il.ibm.com> wrote:

Hi Again,

I'm a little confused regarding what is the exact Orc's functions I should
in order to save the functions code in a code cache so it could be later
replaced with different versions of it and I appreciate your help.

Just a reminder I want to dynamically recompile the program based on profile
 collected at the run-time. I would like to start executing the program from
the code-cache and at some point be able to replace a function body with
new compiled version; this can be done by replacing the entry in the
 code with a trampoline to It's new version so that future calls to it will
call the new version code.

Does the CompileOnDemandLayer executes the program from a code cache
and holds pointers to the code of the functions it executes? I am compiling
for Power machine.
Is there a target specific pieces that I should implement for making Orc
work on Power?

Thanks again,

From:        Lang Hames <lhames at gmail.com>
To:        Revital1 Eres/Haifa/IBM at IBMIL
Cc:        LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
Date:        20/07/2015 08:41 PM
Subject:        Re: [LLVMdev] Help with using LLVM to re-compile hot
functions at run-time

Hi Revital,

The CompileOnDemand layer is used by the lazy bitcode JIT in the lli tool.
You can find the code in llvm/tools/lli/OrcLazyJIT.* .


On Mon, Jul 20, 2015 at 2:32 AM, Revital1 Eres <*ERES at il.ibm.com*
<ERES at il.ibm.com>> wrote:
Hello Lang,

Thanks for your answer.

I am now looking for an example of the usage of CompileOnDemandLayer. Is
there an example available for that (could not find one in llvm/examples)?


From:        Lang Hames <*lhames at gmail.com* <lhames at gmail.com>>
To:        Revital1 Eres/Haifa/IBM at IBMIL
Cc:        LLVM Developers Mailing List <*llvmdev at cs.uiuc.edu*
<llvmdev at cs.uiuc.edu>>
Date:        10/07/2015 12:10 AM
Subject:        Re: [LLVMdev] Help with using LLVM to re-compile hot
functions at run-time

Hi Revital,

LLVM does have an IR interpreter, but I don't think it's maintained well
(or possibly at all). The interpreter is also not designed to interact with
the LLVM JITs.

We generally encourage people to just JIT LLVM IR, rather than interpreting
it. For the use-case you have described, you could JIT IR with no
optimizations to begin with, then re-JIT hot functions at a higher level.

The Orc JIT APIs (LLVM's newer JIT APIs) were written with this kind of
use-case in mind, and are probably a better fit for this than MCJIT. There
is no built-in hot-function detection or recompilation yet, but I think
this would be *fairly* easy to write in terms of Orc's callback API.


On Thu, Jul 9, 2015 at 4:19 AM, Revital1 Eres <*ERES at il.ibm.com*
<ERES at il.ibm.com>> wrote:

I am new to LLVM and a I appreciate your help with the following:

I want to run the LLVM IR through virtual machine (LLVM interpreter?) and
compile the hot functions (using MCJIT).

This task will require amongst other identifying the hot functions and
having a
code cache that should be patched with the native code of the functions
they are jitted.

I've read so far about MCJIT and lli however I have not seen that the LLVM
interpreter can be used as a VM the way I was looking for; meaning
execute the code one instruction at a time; have a profiling mode to
identify hot functions and call jit to compile the hot functions.

I appreciate any advice/starting points for this project.


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