JITLink and ORC’s ObjectLinkingLayer¶
Introduction¶
This document aims to provide a high-level overview of the design and API of the JITLink library. It assumes some familiarity with linking and relocatable object files, but should not require deep expertise. If you know what a section, symbol, and relocation are you should find this document accessible. If it is not, please submit a patch (Contributing to LLVM) or file a bug (How to submit an LLVM bug report).
JITLink is a library for JIT Linking. It was built to support the ORC JIT APIs and is most commonly accessed via ORC’s ObjectLinkingLayer API. JITLink was developed with the aim of supporting the full set of features provided by each object format; including static initializers, exception handling, thread local variables, and language runtime registration. Supporting these features enables ORC to execute code generated from source languages which rely on these features (e.g. C++ requires object format support for static initializers to support static constructors, eh-frame registration for exceptions, and TLV support for thread locals; Swift and Objective-C require language runtime registration for many features). For some object format features support is provided entirely within JITLink, and for others it is provided in cooperation with the (prototype) ORC runtime.
JITLink aims to support the following features, some of which are still under development:
Cross-process and cross-architecture linking of single relocatable objects into a target executor process.
Support for all object format features.
Open linker data structures (
LinkGraph
) and pass system.
JITLink and ObjectLinkingLayer¶
ObjectLinkingLayer
is ORCs wrapper for JITLink. It is an ORC layer that
allows objects to be added to a JITDylib
, or emitted from some higher level
program representation. When an object is emitted, ObjectLinkingLayer
uses
JITLink to construct a LinkGraph
(see Constructing LinkGraphs) and
calls JITLink’s link
function to link the graph into the executor process.
The ObjectLinkingLayer
class provides a plugin API,
ObjectLinkingLayer::Plugin
, which users can subclass in order to inspect and
modify LinkGraph
instances at link time, and react to important JIT events
(such as an object being emitted into target memory). This enables many features
and optimizations that were not possible under MCJIT or RuntimeDyld.
ObjectLinkingLayer Plugins¶
The ObjectLinkingLayer::Plugin
class provides the following methods:
modifyPassConfig
is called each time a LinkGraph is about to be linked. It can be overridden to install JITLink Passes to run during the link process.void modifyPassConfig(MaterializationResponsibility &MR, const Triple &TT, jitlink::PassConfiguration &Config)
notifyLoaded
is called before the link begins, and can be overridden to set up any initial state for the givenMaterializationResponsibility
if needed.void notifyLoaded(MaterializationResponsibility &MR)
notifyEmitted
is called after the link is complete and code has been emitted to the executor process. It can be overridden to finalize state for theMaterializationResponsibility
if needed.Error notifyEmitted(MaterializationResponsibility &MR)
notifyFailed
is called if the link fails at any point. It can be overridden to react to the failure (e.g. to deallocate any already allocated resources).Error notifyFailed(MaterializationResponsibility &MR)
notifyRemovingResources
is called when a request is made to remove any resources associated with theResourceKey
K for theMaterializationResponsibility
.Error notifyRemovingResources(ResourceKey K)
notifyTransferringResources
is called if/when a request is made to transfer tracking of any resources associated withResourceKey
SrcKey to DstKey.void notifyTransferringResources(ResourceKey DstKey, ResourceKey SrcKey)
Plugin authors are required to implement the notifyFailed
,
notifyRemovingResources
, and notifyTransferringResources
methods in
order to safely manage resources in the case of resource removal or transfer,
or link failure. If no resources are managed by the plugin then these methods
can be implemented as no-ops returning Error::success()
.
Plugin instances are added to an ObjectLinkingLayer
by
calling the addPlugin
method [1]. E.g.
// Plugin class to print the set of defined symbols in an object when that
// object is linked.
class MyPlugin : public ObjectLinkingLayer::Plugin {
public:
// Add passes to print the set of defined symbols after dead-stripping.
void modifyPassConfig(MaterializationResponsibility &MR,
const Triple &TT,
jitlink::PassConfiguration &Config) override {
Config.PostPrunePasses.push_back([this](jitlink::LinkGraph &G) {
return printAllSymbols(G);
});
}
// Implement mandatory overrides:
Error notifyFailed(MaterializationResponsibility &MR) override {
return Error::success();
}
Error notifyRemovingResources(ResourceKey K) override {
return Error::success();
}
void notifyTransferringResources(ResourceKey DstKey,
ResourceKey SrcKey) override {}
// JITLink pass to print all defined symbols in G.
Error printAllSymbols(LinkGraph &G) {
for (auto *Sym : G.defined_symbols())
if (Sym->hasName())
dbgs() << Sym->getName() << "\n";
return Error::success();
}
};
// Create our LLJIT instance using a custom object linking layer setup.
// This gives us a chance to install our plugin.
auto J = ExitOnErr(LLJITBuilder()
.setObjectLinkingLayerCreator(
[](ExecutionSession &ES, const Triple &T) {
// Manually set up the ObjectLinkingLayer for our LLJIT
// instance.
auto OLL = std::make_unique<ObjectLinkingLayer>(
ES, std::make_unique<jitlink::InProcessMemoryManager>());
// Install our plugin:
OLL->addPlugin(std::make_unique<MyPlugin>());
return OLL;
})
.create());
// Add an object to the JIT. Nothing happens here: linking isn't triggered
// until we look up some symbol in our object.
ExitOnErr(J->addObject(loadFromDisk("main.o")));
// Plugin triggers here when our lookup of main triggers linking of main.o
auto MainSym = J->lookup("main");
LinkGraph¶
JITLink maps all relocatable object formats to a generic LinkGraph
type
that is designed to make linking fast and easy (LinkGraph
instances can
also be created manually. See Constructing LinkGraphs).
Relocatable object formats (e.g. COFF, ELF, MachO) differ in their details, but share a common goal: to represent machine level code and data with annotations that allow them to be relocated in a virtual address space. To this end they usually contain names (symbols) for content defined inside the file or externally, chunks of content that must be moved as a unit (sections or subsections, depending on the format), and annotations describing how to patch content based on the final address of some target symbol/section (relocations).
At a high level, the LinkGraph
type represents these concepts as a decorated
graph. Nodes in the graph represent symbols and content, and edges represent
relocations. Each of the elements of the graph is listed here:
Addressable
– A node in the link graph that can be assigned an address in the executor process’s virtual address space.Absolute and external symbols are represented using plain
Addressable
instances. Content defined inside the object file is represented using theBlock
subclass.Block
– AnAddressable
node that hasContent
(or is marked as zero-filled), a parentSection
, aSize
, anAlignment
(and anAlignmentOffset
), and a list ofEdge
instances.Blocks provide a container for binary content which must remain contiguous in the target address space (a layout unit). Many interesting low level operations on
LinkGraph
instances involve inspecting or mutating block content or edges.Content
is represented as anllvm::StringRef
, and accessible via thegetContent
method. Content is only available for content blocks, and not for zero-fill blocks (useisZeroFill
to check, and prefergetSize
when only the block size is needed as it works for both zero-fill and content blocks).Section
is represented as aSection&
reference, and accessible via thegetSection
method. TheSection
class is described in more detail below.Size
is represented as asize_t
, and is accessible via thegetSize
method for both content and zero-filled blocks.Alignment
is represented as auint64_t
, and available via thegetAlignment
method. It represents the minimum alignment requirement (in bytes) of the start of the block.AlignmentOffset
is represented as auint64_t
, and accessible via thegetAlignmentOffset
method. It represents the offset from the alignment required for the start of the block. This is required to support blocks whose minimum alignment requirement comes from data at some non-zero offset inside the block. E.g. if a block consists of a single byte (with byte alignment) followed by a uint64_t (with 8-byte alignment), then the block will have 8-byte alignment with an alignment offset of 7.list of
Edge
instances. An iterator range for this list is returned by theedges
method. TheEdge
class is described in more detail below.
Symbol
– An offset from anAddressable
(often aBlock
), with an optionalName
, aLinkage
, aScope
, aCallable
flag, and aLive
flag.Symbols make it possible to name content (blocks and addressables are anonymous), or target content with an
Edge
.Name
is represented as anllvm::StringRef
(equal tollvm::StringRef()
if the symbol has no name), and accessible via thegetName
method.Linkage
is one of Strong or Weak, and is accessible via thegetLinkage
method. TheJITLinkContext
can use this flag to determine whether this symbol definition should be kept or dropped.Scope
is one of Default, Hidden, or Local, and is accessible via thegetScope
method. TheJITLinkContext
can use this to determine who should be able to see the symbol. A symbol with default scope should be globally visible. A symbol with hidden scope should be visible to other definitions within the same simulated dylib (e.g. ORCJITDylib
) or executable, but not from elsewhere. A symbol with local scope should only be visible within the currentLinkGraph
.Callable
is a boolean which is set to true if this symbol can be called, and is accessible via theisCallable
method. This can be used to automate the introduction of call-stubs for lazy compilation.Live
is a boolean that can be set to mark this symbol as root for dead-stripping purposes (see Generic Link Algorithm). JITLink’s dead-stripping algorithm will propagate liveness flags through the graph to all reachable symbols before deleting any symbols (and blocks) that are not marked live.
Edge
– A quad of anOffset
(implicitly from the start of the containingBlock
), aKind
(describing the relocation type), aTarget
, and anAddend
.Edges represent relocations, and occasionally other relationships, between blocks and symbols.
Offset
, accessible viagetOffset
, is an offset from the start of theBlock
containing theEdge
.Kind
, accessible viagetKind
is a relocation type – it describes what kinds of changes (if any) should be made to block content at the givenOffset
based on the address of theTarget
.Target
, accessible viagetTarget
, is a pointer to aSymbol
, representing whose address is relevant to the fixup calculation specified by the edge’sKind
.Addend
, accessible viagetAddend
, is a constant whose interpretation is determined by the edge’sKind
.
Section
– A set ofSymbol
instances, plus a set ofBlock
instances, with aName
, a set ofProtectionFlags
, and anOrdinal
.Sections make it easy to iterate over the symbols or blocks associated with a particular section in the source object file.
blocks()
returns an iterator over the set of blocks defined in the section (asBlock*
pointers).symbols()
returns an iterator over the set of symbols defined in the section (asSymbol*
pointers).Name
is represented as anllvm::StringRef
, and is accessible via thegetName
method.ProtectionFlags
are represented as a sys::Memory::ProtectionFlags enum, and accessible via thegetProtectionFlags
method. These flags describe whether the section is readable, writable, executable, or some combination of these. The most common combinations areRW-
for writable data,R--
for constant data, andR-X
for code.SectionOrdinal
, accessible viagetOrdinal
, is a number used to order the section relative to others. It is usually used to preserve section order within a segment (a set of sections with the same memory protections) when laying out memory.
For the graph-theorists: The LinkGraph
is bipartite, with one set of
Symbol
nodes and one set of Addressable
nodes. Each Symbol
node has
one (implicit) edge to its target Addressable
. Each Block
has a set of
edges (possibly empty, represented as Edge
instances) back to elements of
the Symbol
set. For convenience and performance of common algorithms,
symbols and blocks are further grouped into Sections
.
The LinkGraph
itself provides operations for constructing, removing, and
iterating over sections, symbols, and blocks. It also provides metadata
and utilities relevant to the linking process:
Graph element operations
sections
returns an iterator over all sections in the graph.findSectionByName
returns a pointer to the section with the given name (as aSection*
) if it exists, otherwise returns a nullptr.blocks
returns an iterator over all blocks in the graph (across all sections).defined_symbols
returns an iterator over all defined symbols in the graph (across all sections).external_symbols
returns an iterator over all external symbols in the graph.absolute_symbols
returns an iterator over all absolute symbols in the graph.createSection
creates a section with a given name and protection flags.createContentBlock
creates a block with the given initial content, parent section, address, alignment, and alignment offset.createZeroFillBlock
creates a zero-fill block with the given size, parent section, address, alignment, and alignment offset.addExternalSymbol
creates a new addressable and symbol with a given name, size, and linkage.addAbsoluteSymbol
creates a new addressable and symbol with a given name, address, size, linkage, scope, and liveness.addCommonSymbol
convenience function for creating a zero-filled block and weak symbol with a given name, scope, section, initial address, size, alignment and liveness.addAnonymousSymbol
creates a new anonymous symbol for a given block, offset, size, callable-ness, and liveness.addDefinedSymbol
creates a new symbol for a given block with a name, offset, size, linkage, scope, callable-ness and liveness.makeExternal
transforms a formerly defined symbol into an external one by creating a new addressable and pointing the symbol at it. The existing block is not deleted, but can be manually removed (if unreferenced) by callingremoveBlock
. All edges to the symbol remain valid, but the symbol must now be defined outside thisLinkGraph
.removeExternalSymbol
removes an external symbol and its target addressable. The target addressable must not be referenced by any other symbols.removeAbsoluteSymbol
removes an absolute symbol and its target addressable. The target addressable must not be referenced by any other symbols.removeDefinedSymbol
removes a defined symbol, but does not remove its target block.removeBlock
removes the given block.splitBlock
split a given block in two at a given index (useful where it is known that a block contains decomposable records, e.g. CFI records in an eh-frame section).
Graph utility operations
getName
returns the name of this graph, which is usually based on the name of the input object file.getTargetTriple
returns an llvm::Triple for the executor process.getPointerSize
returns the size of a pointer (in bytes) in the executor process.getEndinaness
returns the endianness of the executor process.allocateString
copies data from a givenllvm::Twine
into the link graph’s internal allocator. This can be used to ensure that content created inside a pass outlives that pass’s execution.
Generic Link Algorithm¶
JITLink provides a generic link algorithm which can be extended / modified at certain points by the introduction of JITLink Passes:
Phase 1
This phase is called immediately by the
link
function as soon as the initial configuration (including the pass pipeline setup) is complete.Run pre-prune passes.
These passes are called on the graph before it is pruned. At this stage
LinkGraph
nodes still have their original vmaddrs. A mark-live pass (supplied by theJITLinkContext
) will be run at the end of this sequence to mark the initial set of live symbols.Notable use cases: marking nodes live, accessing/copying graph data that will be pruned (e.g. metadata that’s important for the JIT, but not needed for the link process).
Prune (dead-strip) the
LinkGraph
.Removes all symbols and blocks not reachable from the initial set of live symbols.
This allows JITLink to remove unreachable symbols / content, including overridden weak and redundant ODR definitions.
Run post-prune passes.
These passes are run on the graph after dead-stripping, but before memory is allocated or nodes assigned their final target vmaddrs.
Passes run at this stage benefit from pruning, as dead functions and data have been stripped from the graph. However new content can still be added to the graph, as target and working memory have not been allocated yet.
Notable use cases: Building Global Offset Table (GOT), Procedure Linkage Table (PLT), and Thread Local Variable (TLV) entries.
Sort blocks into segments.
Sorts all blocks by ordinal and then address. Collects sections with matching permissions into segments and computes the size of these segments for memory allocation.
Allocate segment memory, update node addresses.
Calls the
JITLinkContext
’sJITLinkMemoryManager
to allocate both working and target memory for the graph, then updates all node addresses to their assigned target address.Note: This step only updates the addresses of nodes defined in this graph. External symbols will still have null addresses.
Run post-allocation passes.
These passes are run on the graph after working and target memory have been allocated, but before the
JITLinkContext
is notified of the final addresses of the symbols in the graph. This gives these passes a chance to set up data structures associated with target addresses before any JITLink clients (especially ORC queries for symbol resolution) can attempt to access them.Notable use cases: Setting up mappings between target addresses and JIT data structures, such as a mapping between
__dso_handle
andJITDylib*
.Notify the
JITLinkContext
of the assigned symbol addresses.Calls
JITLinkContext::notifyResolved
on the link graph, allowing clients to react to the symbol address assignments made for this graph. In ORC this is used to notify any pending queries for resolved symbols, including pending queries from concurrently running JITLink instances that have reached the next step and are waiting on the address of a symbol in this graph to proceed with their link.Identify external symbols and resolve their addresses asynchronously.
Calls the
JITLinkContext
to resolve the target address of any external symbols in the graph. This step is asynchronous – JITLink will pack the link state into a continuation to be run once the symbols are resolved.This is the final step of Phase 1.
Phase 2
This phase is called by the continuation constructed at the end of the external symbol resolution step above.
Apply external symbol resolution results.
This updates the addresses of all external symbols. At this point all nodes in the graph have their final target addresses, however node content still points back to the original data in the object file.
Run pre-fixup passes.
These passes are called on the graph after all nodes have been assigned their final target addresses, but before node content is copied into working memory and fixed up. Passes run at this stage can make late optimizations to the graph and content based on address layout.
Notable use cases: GOT and PLT relaxation, where GOT and PLT accesses are bypassed for fixup targets that are directly accessible under the assigned memory layout.
Copy block content to working memory and apply fixups.
Copies all block content into allocated working memory (following the target layout) and applies fixups. Graph blocks are updated to point at the fixed up content.
Run post-fixup passes.
These passes are called on the graph after fixups have been applied and blocks updated to point to the fixed up content.
Post-fixup passes can inspect blocks contents to see the exact bytes that will be copied to the assigned target addresses.
Finalize memory asynchronously.
Calls the
JITLinkMemoryManager
to copy working memory to the executor process and apply the requested permissions. This step is asynchronous – JITLink will pack the link state into a continuation to be run once memory has been copied and protected.This is the final step of Phase 2.
Phase 3.
This phase is called by the continuation constructed at the end of the memory finalization step above.
Notify the context that the graph has been emitted.
Calls
JITLinkContext::notifyFinalized
and hands off theJITLinkMemoryManager::Allocation
object for this graph’s memory allocation. This allows the context to track/hold memory allocations and react to the newly emitted definitions. In ORC this is used to update theExecutionSession
instance’s dependence graph, which may result in these symbols (and possibly others) becoming Ready if all of their dependencies have also been emitted.
Passes¶
JITLink passes are std::function<Error(LinkGraph&)>
instances. They are free
to inspect and modify the given LinkGraph
subject to the constraints of
whatever phase they are running in (see Generic Link Algorithm). If a
pass returns Error::success()
then linking continues. If a pass returns
a failure value then linking is stopped and the JITLinkContext
is notified
that the link failed.
Passes may be used by both JITLink backends (e.g. MachO/x86-64 implements GOT
and PLT construction as a pass), and external clients like
ObjectLinkingLayer::Plugin
.
In combination with the open LinkGraph
API, JITLink passes enable the
implementation of powerful new features. For example:
Relaxation optimizations – A pre-fixup pass can inspect GOT accesses and PLT calls and identify situations where the addresses of the entry target and the access are close enough to be accessed directly. In this case the pass can rewrite the instruction stream of the containing block and update the fixup edges to make the access direct.
Code for this looks like:
Error relaxGOTEdges(LinkGraph &G) {
for (auto *B : G.blocks())
for (auto &E : B->edges())
if (E.getKind() == x86_64::GOTLoad) {
auto &GOTTarget = getGOTEntryTarget(E.getTarget());
if (isInRange(B.getFixupAddress(E), GOTTarget)) {
// Rewrite B.getContent() at fixup address from
// MOVQ to LEAQ
// Update edge target and kind.
E.setTarget(GOTTarget);
E.setKind(x86_64::PCRel32);
}
}
return Error::success();
}
Metadata registration – Post allocation passes can be used to record the address range of sections in the target. This can be used to register the metadata (e.g exception handling frames, language metadata) in the target once memory has been finalized.
Error registerEHFrameSection(LinkGraph &G) {
if (auto *Sec = G.findSectionByName("__eh_frame")) {
SectionRange SR(*Sec);
registerEHFrameSection(SR.getStart(), SR.getEnd());
}
return Error::success();
}
Record call sites for later mutation – A post-allocation pass can record the call sites of all calls to a particular function, allowing those call sites to be updated later at runtime (e.g. for instrumentation, or to enable the function to be lazily compiled but still called directly after compilation).
StringRef FunctionName = "foo";
std::vector<JITTargetAddress> CallSitesForFunction;
auto RecordCallSites =
[&](LinkGraph &G) -> Error {
for (auto *B : G.blocks())
for (auto &E : B.edges())
if (E.getKind() == CallEdgeKind &&
E.getTarget().hasName() &&
E.getTraget().getName() == FunctionName)
CallSitesForFunction.push_back(B.getFixupAddress(E));
return Error::success();
};
Memory Management with JITLinkMemoryManager¶
JIT linking requires allocation of two kinds of memory: working memory in the JIT process and target memory in the execution process (these processes and memory allocations may be one and the same, depending on how the user wants to build their JIT). It also requires that these allocations conform to the requested code model in the target process (e.g. MachO/x86-64’s Small code model requires that all code and data for a simulated dylib is allocated within 4Gb). Finally, it is natural to make the memory manager responsible for transferring memory to the target address space and applying memory protections, since the memory manager must know how to communicate with the executor, and since sharing and protection assignment can often be efficiently managed (in the common case of running across processes on the same machine for security) via the host operating system’s virtual memory management APIs.
To satisfy these requirements JITLinkMemoryManager
adopts the following
design: The memory manager itself has just one virtual method that returns a
JITLinkMemoryManager::Allocation
:
virtual Expected<std::unique_ptr<Allocation>>
allocate(const JITLinkDylib *JD, const SegmentsRequestMap &Request) = 0;
This method takes a JITLinkDylib*
representing the target simulated
dylib, and the full set of sections that must be allocated for this object.
JITLinkMemoryManager
implementations can (optionally) use the JD
argument to manage a per-simulated-dylib memory pool (since code model
constraints are typically imposed on a per-dylib basis, and not across
dylibs) [2]. The Request
argument, by describing all sections in the current
object up-front, allows the implementer to allocate those sections as a
single slab, either within a pre-allocated per-jitdylib pool or directly
from system memory.
All subsequent operations are provided by the
JITLinkMemoryManager::Allocation
interface:
virtual MutableArrayRef<char> getWorkingMemory(ProtectionFlags Seg)
Should be overridden to return the address in working memory of the segment with the given protection flags.
virtual JITTargetAddress getTargetMemory(ProtectionFlags Seg)
Should be overridden to return the address in the executor’s address space of the segment with the given protection flags.
virtual void finalizeAsync(FinalizeContinuation OnFinalize)
Should be overridden to copy the contents of working memory to the target address space and apply memory protections for all segments. Where working memory and target memory are separate, this method should deallocate the working memory.
virtual Error deallocate()
Should be overridden to deallocate memory in the target address space.
JITLink provides a simple in-process implementation of this interface:
InProcessMemoryManager
. It allocates pages once and re-uses them as both
working and target memory.
ORC provides a cross-process JITLinkMemoryManager
based on an ORC-RPC-based
implementation of the orc::TargetProcessControl
API:
OrcRPCTPCJITLinkMemoryManager
. This API uses TargetProcessControl API calls
to allocate and manage memory in a remote process. The underlying communication
channel is determined by the ORC-RPC channel type. Common options include unix
sockets or TCP.
JITLinkMemoryManager and Security¶
JITLink’s ability to link JIT’d code for a separate executor process can be used to improve the security of a JIT system: The executor process can be sandboxed, run within a VM, or even run on a fully separate machine.
JITLink’s memory manager interface is flexible enough to allow for a range of trade-offs between performance and security. For example, on a system where code pages must be signed (preventing code from being updated), the memory manager can deallocate working memory pages after linking to free memory in the process running JITLink. Alternatively, on a system that allows RWX pages, the memory manager may use the same pages for both working and target memory by marking them as RWX, allowing code to be modified in place without further overhead. Finally, if RWX pages are not permitted but dual-virtual-mappings of physical memory pages are, then the memory manager can dual map physical pages as RW- in the JITLink process and R-X in the executor process, allowing modification from the JITLink process but not from the executor (at the cost of extra administrative overhead for the dual mapping).
Error Handling¶
JITLink makes extensive use of the llvm::Error
type (see the error handling
section of LLVM Programmer’s Manual for details). The link process itself, all
passes, the memory manager interface, and operations on the JITLinkContext
are all permitted to fail. Link graph construction utilities (especially parsers
for object formats) are encouraged to validate input, and validate fixups
(e.g. with range checks) before application.
Any error will halt the link process and notify the context of failure. In ORC, reported failures are propagated to queries pending on definitions provided by the failing link, and also through edges of the dependence graph to any queries waiting on dependent symbols.
Connection to the ORC Runtime¶
The ORC Runtime (currently under development) aims to provide runtime support for advanced JIT features, including object format features that require non-trivial action in the executor (e.g. running initializers, managing thread local storage, registering with language runtimes, etc.).
ORC Runtime support for object format features typically requires cooperation
between the runtime (which executes in the executor process) and JITLink (which
runs in the JIT process and can inspect LinkGraphs to determine what actions
must be taken in the executor). For example: Execution of MachO static
initializers in the ORC runtime is performed by the jit_dlopen
function,
which calls back to the JIT process to ask for the list of address ranges of
__mod_init
sections to walk. This list is collated by the
MachOPlatformPlugin
, which installs a pass to record this information for
each object as it is linked into the target.
Constructing LinkGraphs¶
Clients usually access and manipulate LinkGraph
instances that were created
for them by an ObjectLinkingLayer
instance, but they can be created manually:
By directly constructing and populating a
LinkGraph
instance.By using the
createLinkGraph
family of functions to create aLinkGraph
from an in-memory buffer containing an object file. This is howObjectLinkingLayer
usually createsLinkGraphs
.
createLinkGraph_<Object-Format>_<Architecture>
can be used whenboth the object format and architecture are known ahead of time.
createLinkGraph_<Object-Format>
can be used when the object format is known ahead of time, but the architecture is not. In this case the architecture will be determined by inspection of the object header.
createLinkGraph
can be used when neither the object format nor the architecture are known ahead of time. In this case the object header will be inspected to determine both the format and architecture.
JIT Linking¶
The JIT linker concept was introduced in LLVM’s earlier generation of JIT APIs, MCJIT. In MCJIT the RuntimeDyld component enabled re-use of LLVM as an in-memory compiler by adding an in-memory link step to the end of the usual compiler pipeline. Rather than dumping relocatable objects to disk as a compiler usually would, MCJIT passed them to RuntimeDyld to be linked into a target process.
This approach to linking differs from standard static or dynamic linking:
A static linker takes one or more relocatable object files as input and links them into an executable or dynamic library on disk.
A dynamic linker applies relocations to executables and dynamic libraries that have been loaded into memory.
A JIT linker takes a single relocatable object file at a time and links it into a target process, usually using a context object to allow the linked code to resolve symbols in the target.
RuntimeDyld¶
In order to keep RuntimeDyld’s implementation simple MCJIT imposed some restrictions on compiled code:
It had to use the Large code model, and often restricted available relocation models in order to limit the kinds of relocations that had to be supported.
It required strong linkage and default visibility on all symbols – behavior for other linkages/visibilities was not well defined.
It constrained and/or prohibited the use of features requiring runtime support, e.g. static initializers or thread local storage.
As a result of these restrictions not all language features supported by LLVM worked under MCJIT, and objects to be loaded under the JIT had to be compiled to target it (precluding the use of precompiled code from other sources under the JIT).
RuntimeDyld also provided very limited visibility into the linking process itself: Clients could access conservative estimates of section size (RuntimeDyld bundled stub size and padding estimates into the section size value) and the final relocated bytes, but could not access RuntimeDyld’s internal object representations.
Eliminating these restrictions and limitations was one of the primary motivations for the development of JITLink.
The llvm-jitlink tool¶
The llvm-jitlink
tool is a command line wrapper for the JITLink library.
It loads some set of relocatable object files and then links them using
JITLink. Depending on the options used it will then execute them, or validate
the linked memory.
The llvm-jitlink
tool was originally designed to aid JITLink development by
providing a simple environment for testing.
Basic usage¶
By default, llvm-jitlink
will link the set of objects passed on the command
line, then search for a “main” function and execute it:
% cat hello-world.c
#include <stdio.h>
int main(int argc, char *argv[]) {
printf("hello, world!\n");
return 0;
}
% clang -c -o hello-world.o hello-world.c
% llvm-jitlink hello-world.o
Hello, World!
Multiple objects may be specified, and arguments may be provided to the JIT’d main function using the -args option:
% cat print-args.c
#include <stdio.h>
void print_args(int argc, char *argv[]) {
for (int i = 0; i != argc; ++i)
printf("arg %i is \"%s\"\n", i, argv[i]);
}
% cat print-args-main.c
void print_args(int argc, char *argv[]);
int main(int argc, char *argv[]) {
print_args(argc, argv);
return 0;
}
% clang -c -o print-args.o print-args.c
% clang -c -o print-args-main.o print-args-main.c
% llvm-jitlink print-args.o print-args-main.o -args a b c
arg 0 is "a"
arg 1 is "b"
arg 2 is "c"
Alternative entry points may be specified using the -entry <entry point
name>
option.
Other options can be found by calling llvm-jitlink -help
.
llvm-jitlink as a regression testing utility¶
One of the primary aims of llvm-jitlink
was to enable readable regression
tests for JITLink. To do this it supports two options:
The -noexec
option tells llvm-jitlink to stop after looking up the entry
point, and before attempting to execute it. Since the linked code is not
executed, this can be used to link for other targets even if you do not have
access to the target being linked (the -define-abs
or -phony-externals
options can be used to supply any missing definitions in this case).
The -check <check-file>
option can be used to run a set of jitlink-check
expressions against working memory. It is typically used in conjunction with
-noexec
, since the aim is to validate JIT’d memory rather than to run the
code and -noexec
allows us to link for any supported target architecture
from the current process. In -check
mode, llvm-jitlink
will scan the
given check-file for lines of the form # jitlink-check: <expr>
. See
examples of this usage in llvm/test/ExecutionEngine/JITLink
.
Remote execution via llvm-jitlink-executor¶
By default llvm-jitlink
will link the given objects into its own process,
but this can be overridden by two options:
The -oop-executor[=/path/to/executor]
option tells llvm-jitlink
to
execute the given executor (which defaults to llvm-jitlink-executor
) and
communicate with it via file descriptors which it passes to the executor
as the first argument with the format filedescs=<in-fd>,<out-fd>
.
The -oop-executor-connect=<host>:<port>
option tells llvm-jitlink
to
connect to an already running executor via TCP on the given host and port. To
use this option you will need to start llvm-jitlink-executor
manually with
listen=<host>:<port>
as the first argument.
Harness mode¶
The -harness
option allows a set of input objects to be designated as a test
harness, with the regular object files implicitly treated as objects to be
tested. Definitions of symbols in the harness set override definitions in the
test set, and external references from the harness cause automatic scope
promotion of local symbols in the test set (these modifications to the usual
linker rules are accomplished via an ObjectLinkingLayer::Plugin
installed by
llvm-jitlink
when it sees the -harness
option).
With these modifications in place we can selectively test functions in an object
file by mocking those function’s callees. For example, suppose we have an object
file, test_code.o
, compiled from the following C source (which we need not
have access to):
void irrelevant_function() { irrelevant_external(); }
int function_to_mock(int X) {
return /* some function of X */;
}
static void function_to_test() {
...
int Y = function_to_mock();
printf("Y is %i\n", Y);
}
If we want to know how function_to_test
behaves when we change the behavior
of function_to_mock
we can test it by writing a test harness:
void function_to_test();
int function_to_mock(int X) {
printf("used mock utility function\n");
return 42;
}
int main(int argc, char *argv[]) {
function_to_test():
return 0;
}
Under normal circumstances these objects could not be linked together:
function_to_test
is static and could not be resolved outside
test_code.o
, the two function_to_mock
functions would result in a
duplicate definition error, and irrelevant_external
is undefined.
However, using -harness
and -phony-externals
we can run this code
with:
% clang -c -o test_code_harness.o test_code_harness.c
% llvm-jitlink -phony-externals test_code.o -harness test_code_harness.o
used mock utility function
Y is 42
The -harness
option may be of interest to people who want to perform some
very late testing on build products to verify that compiled code behaves as
expected. On basic C test cases this is relatively straightforward. Mocks for
more complicated languages (e.g. C++) are much trickier: Any code involving
classes tends to have a lot of non-trivial surface area (e.g. vtables) that
would require great care to mock.
Tips for JITLink backend developers¶
Make liberal use of assert and
llvm::Error
. Do not assume that the input object is well formed: Return any errors produced by libObject (or your own object parsing code) and validate as you construct. Think carefully about the distinction between contract (which should be validated with asserts and llvm_unreachable) and environmental errors (which should generatellvm::Error
instances).Don’t assume you’re linking in-process. Use libSupport’s sized, endian-specific types when reading/writing content in the
LinkGraph
.
As a “minimum viable” JITLink wrapper, the llvm-jitlink
tool is an
invaluable resource for developers bringing in a new JITLink backend. A standard
workflow is to start by throwing an unsupported object at the tool and seeing
what error is returned, then fixing that (you can often make a reasonable guess
at what should be done based on existing code for other formats or
architectures).
In debug builds of LLVM, the -debug-only=jitlink
option dumps logs from the
JITLink library during the link process. These can be useful for spotting some bugs at
a glance. The -debug-only=llvm_jitlink
option dumps logs from the llvm-jitlink
tool, which can be useful for debugging both testcases (it is often less verbose than
-debug-only=jitlink
) and the tool itself.
The -oop-executor
and -oop-executor-connect
options are helpful for testing
handling of cross-process and cross-architecture use cases.
Roadmap¶
JITLink is under active development. Work so far has focused on the MachO implementation. In LLVM 12 there is limited support for ELF on x86-64.
Major outstanding projects include:
Refactor architecture support to maximize sharing across formats.
All formats should be able to share the bulk of the architecture specific code (especially relocations) for each supported architecture.
Refactor ELF link graph construction.
ELF’s link graph construction is currently implemented in the ELF_x86_64.cpp file, and tied to the x86-64 relocation parsing code. The bulk of the code is generic and should be split into an ELFLinkGraphBuilder base class along the same lines as the existing generic MachOLinkGraphBuilder.
Implement ELF support for arm64.
Once the architecture support code has been refactored to enable sharing and ELF link graph construction has been refactored to allow re-use we should be able to construct an ELF / arm64 JITLink implementation by combining these existing pieces.
Implement support for new architectures.
Implement support for COFF.
There is no COFF implementation of JITLink yet. Such an implementation should follow the MachO and ELF paths: a generic COFFLinkGraphBuilder base class that can be specialized for each architecture.
Design and implement a shared-memory based JITLinkMemoryManager.
One use-case that is expected to be common is out-of-process linking targeting another process on the same machine. This allows JITs to sandbox JIT’d code. For this use case a shared-memory based JITLinkMemoryManager would provide the most efficient form of allocation. Creating one will require designing a generic API for shared memory though, as LLVM does not currently have one.
JITLink Availability and Feature Status¶
Architecture |
ELF |
COFF |
MachO |
---|---|---|---|
arm64 |
Partial (small code model, PIC relocation model only) |
||
x86-64 |
Partial |
Full (except TLV and debugging) |