Mono Ahead Of Time Compiler
===========================

	The Ahead of Time compilation feature in Mono allows Mono to
	precompile assemblies to minimize JIT time, reduce memory
	usage at runtime and increase the code sharing across multiple
	running Mono application.

	To precompile an assembly use the following command:
	
	   mono --aot -O=all assembly.exe

	The `--aot' flag instructs Mono to ahead-of-time compile your
	assembly, while the -O=all flag instructs Mono to use all the
	available optimizations.

* Caching metadata
------------------

	Besides code, the AOT file also contains cached metadata information which allows
	the runtime to avoid certain computations at runtime, like the computation of
	generic vtables. This reduces both startup time, and memory usage. It is possible
	to create an AOT image which contains only this cached information and no code by
	using the 'metadata-only' option during compilation:

	   mono --aot=metadata-only assembly.exe

	This works even on platforms where AOT is not normally supported.

* Position Independent Code
---------------------------

	On x86 and x86-64 the code generated by Ahead-of-Time compiled
	images is position-independent code.  This allows the same
	precompiled image to be reused across multiple applications
	without having different copies: this is the same way in which
	ELF shared libraries work: the code produced can be relocated
	to any address.

	The implementation of Position Independent Code had a
	performance impact on Ahead-of-Time compiled images but
	compiler bootstraps are still faster than JIT-compiled images,
	specially with all the new optimizations provided by the Mono
	engine.

* How to support Position Independent Code in new Mono Ports
------------------------------------------------------------

	Generated native code needs to reference various runtime
	structures/functions whose address is only known at run
	time. JITted code can simple embed the address into the native
	code, but AOT code needs to do an indirection. This
	indirection is done through a table called the Global Offset
	Table (GOT), which is similar to the GOT table in the Elf
	spec.  When the runtime saves the AOT image, it saves some
	information for each method describing the GOT table entries
	used by that method. When loading a method from an AOT image,
	the runtime will fill out the GOT entries needed by the
	method.

   * Computing the address of the GOT

        Methods which need to access the GOT first need to compute its
	address. On the x86 it is done by code like this:

		call <IP + 5>
		pop ebx
		add <OFFSET TO GOT>, ebx
		<save got addr to a register>

	The variable representing the got is stored in
	cfg->got_var. It is allways allocated to a global register to
	prevent some problems with branches + basic blocks.

   * Referencing GOT entries

	Any time the native code needs to access some other runtime
	structure/function (i.e. any time the backend calls
	mono_add_patch_info ()), the code pointed by the patch needs
	to load the value from the got. For example, instead of:

	call <ABSOLUTE ADDR>
	it needs to do:
	call *<OFFSET>(<GOT REG>)

	Here, the <OFFSET> can be 0, it will be fixed up by the AOT compiler.
	
	For more examples on the changes required, see
	
	svn diff -r 37739:38213 mini-x86.c 

	* The Program Linkage Table

	As in ELF, calls made from AOT code do not go through the GOT. Instead, a direct call is
	made to an entry in the Program Linkage Table (PLT). This is based on the fact that on
	most architectures, call instructions use a displacement instead of an absolute address, so
	they are already position independent. An PLT entry is usually a jump instruction, which
	initially points to some trampoline code which transfers control to the AOT loader, which
	will compile the called method, and patch the PLT entry so that further calls are made
	directly to the called method.
	If the called method is in the same assembly, and does not need initialization (i.e. it
    doesn't have GOT slots etc), then the call is made directly, bypassing the PLT.

* Implementation
----------------

** The Precompiled File Format
-----------------------------
	
	We use the native object format of the platform. That way it
	is possible to reuse existing tools like objdump and the
	dynamic loader. All we need is a working assembler, i.e. we
	write out a text file which is then passed to gas (the gnu
	assembler) to generate the object file.
		
	The precompiled image is stored in a file next to the original
	assembly that is precompiled with the native extension for a shared
	library (on Linux its ".so" to the generated file). 

	For example: basic.exe -> basic.exe.so; corlib.dll -> corlib.dll.so

	To avoid symbol lookup overhead	and to save space, some things like the 
	compiled code of the individual methods are not identified by specific symbols
    like method_code_1234. Instead, they are stored in one big array and the
	offsets inside this array are stored in another array, requiring just two
	symbols. The offsets array is usually named 'FOO_offsets', where FOO is the
	array the offsets refer to, like 'methods', and 'method_offsets'.

	Generating code using an assembler and linker has some disadvantages:
	- it requires GNU binutils or an equivalent package to be installed on the
	  machine running the aot compilation.
	- it is slow.

	There is some support in the aot compiler for directly emitting elf files, but
	its not complete (yet).
	
	The following things are saved in the object file and can be
	looked up using the equivalent to dlsym:
	
		mono_assembly_guid
	
			A copy of the assembly GUID.
	
		mono_aot_version
	
			The format of the AOT file format.
	
		mono_aot_opt_flags
	
			The optimizations flags used to build this
			precompiled image.
	
		method_infos

			Contains additional information needed by the runtime for using the
			precompiled method, like the GOT entries it uses.

		method_info_offsets				

		    Maps method indexes to offsets in the method_infos array.
			
		mono_icall_table
	
			A table that lists all the internal calls
			references by the precompiled image.
	
		mono_image_table
	
			A list of assemblies referenced by this AOT
			module.

		methods
			
			The precompiled code itself.
			
		method_offsets
	
			Maps method indexes to offsets in the methods array.

		ex_info

			Contains information about methods which is rarely used during normal execution, 
			like exception and debug info.

		ex_info_offsets

			Maps method indexes to offsets in the ex_info array.

		class_info

			Contains precomputed metadata used to speed up various runtime functions.

		class_info_offsets

			Maps class indexes to offsets in the class_info array.

		class_name_table

			A hash table mapping class names to class indexes. Used to speed up 
			mono_class_from_name ().

		plt

			The Program Linkage Table

		plt_info

			Contains information needed to find the method belonging to a given PLT entry.

** Source file structure
-----------------------------

	The AOT infrastructure is split into two files, aot-compiler.c and 
	aot-runtime.c. aot-compiler.c contains the AOT compiler which is invoked by
	--aot, while aot-runtime.c contains the runtime support needed for loading
	code and other things from the aot files.

** Compilation process
----------------------------

	AOT compilation consists of the following stages:
	- collecting the methods to be compiled.
	- compiling them using the JIT.
	- emitting the JITted code and other information into an assembly file (.s).
	- assembling the file using the system assembler.
	- linking the resulting object file into a shared library using the system
	  linker.

** Handling compiled code
----------------------------

	  Each method is identified by a method index. For normal methods, this is
	equivalent to its index in the METHOD metadata table. For runtime generated
	methods (wrappers), it is an arbitrary number.
	  Compiled code is created by invoking the JIT, requesting it to created AOT
	code instead of normal code. This is done by the compile_method () function.
	The output of the JIT is compiled code and a set of patches (relocations). Each 
	relocation specifies an offset inside the compiled code, and a runtime object 
	whose address is accessed at that offset.
	Patches are described by a MonoJumpInfo structure. From the perspective
	of the AOT compiler, there are two kinds of patches:
	- calls, which require an entry in the PLT table.
	- everything else, which require an entry in the GOT table.
	How patches is handled is described in the next section.
	  After all the method are compiled, they are emitted into the output file into
	  a byte array called 'methods', The emission
	is done by the emit_method_code () and emit_and_reloc_code () functions. Each
	piece of compiled code is identified by the local symbol .Lm_<method index>. 
	While compiled code is emitted, all the locations which have an associated patch
	are rewritten using a platform specific process so the final generated code will
	refer to the plt and got entries belonging to the patches.
	The compiled code array 
can be accessed using the 'methods' global symbol. 

** Handling patches
----------------------------

	  Before a piece of AOTed code can be used, the GOT entries used by it must be
	filled out with the addresses of runtime objects. Those objects are identified
	by MonoJumpInfo structures. These stuctures are saved in a serialized form in
	the AOT file, so the AOT loader can deconstruct them. The serialization is done
	by the encode_patch () function, while the deserialization is done by the
	decode_patch_info () function.
	Every method has an associated method info blob inside the 'method_info' byte
	array in the AOT file. This contains all the information required to load the
	method at runtime:
	- the first got entry used by the method.
	- the number of got entries used by the method.
	- the serialized patch info for the got entries.
	Some patches, like vtables, icalls are very common, so instead of emitting their
	info every time they are used by a method, we emit the info only once into a 
	byte array named 'got_info', and only emit an index into this array for every
	access.

** The Procedure Linkage Table (PLT)
------------------------------------

	Our PLT is similar to the elf PLT, it is used to handle calls between methods.
	If method A needs to call method B, then an entry is allocated in the PLT for
	method B, and A calls that entry instead of B directly. This is useful because
	in some cases the runtime needs to do some processing the first time B is 
	called.
	There are two cases:
	- if B is in another assembly, then it needs to be looked up, then JITted or the
	corresponding AOT code needs to be found.
	- if B is in the same assembly, but has got slots, then the got slots need to be
	initialized.
	If none of these cases is true, then the PLT is not used, and the call is made
	directly to the native code of the target method.
	A PLT entry is usually implemented by a jump though a jump table, where the
	jump table entries are initially filled up with the address of a trampoline so
	the runtime can get control, and after the native code of the called method is
	created/found, the jump table entry is changed to point to the native code. 
	All PLT entries also embed a integer offset after the jump which indexes into
	the 'plt_info' table, which stores the information required to find the called
	method. The PLT is emitted by the emit_plt () function.

** Exception/Debug info
----------------------------

	Each compiled method has some additional info generated by the JIT, usable 
	for debugging (IL offset-native offset maps) and exception handling 
	(saved registers, native offsets of try/catch clauses). Since this info is
	rarely needed, it is saved into a separate byte array called 'ex_info'.

** Cached metadata
---------------------------

	When the runtime loads a class, it needs to compute a variety of information
	which is not readily available in the metadata, like the instance size,
	vtable, whenever the class has a finalizer/type initializer etc. Computing this
	information requires a lot of time, causes the loading of lots of metadata,
	and it usually involves the creation of many runtime data structures 
	(MonoMethod/MonoMethodSignature etc), which are long living, and usually persist
	for the lifetime of the app. To avoid this, we compute the required information
	at aot compilation time, and save it into the aot image, into an array called
	'class_info'. The runtime can query this information using the 
	mono_aot_get_cached_class_info () function, and if the information is available,
	it can avoid computing it.

** Full AOT mode
-------------------------

	Some platforms like the iphone prohibit JITted code, using technical and/or
	legal means. This is a significant problem for the mono runtime, since it 
	generates a lot of code dynamically, using either the JIT or more low-level
	code generation macros. To solve this, the AOT compiler is able to function in
	full-aot or aot-only mode, where it generates and saves all the neccesary code
	in the aot image, so at runtime, no code needs to be generated.
	There are two kinds of code which needs to be considered:
	- wrapper methods, that is methods whose IL is generated dynamically by the
	  runtime. They are handled by generating them in the add_wrappers () function,
	  then emitting them the same way as the 'normal' methods. The only problem is
	  that these methods do not have a methoddef token, so we need a separate table
	  in the aot image ('wrapper_info') to find their method index.
	- trampolines and other small hand generated pieces of code. They are handled
	  in an ad-hoc way in the emit_trampolines () function.

* Performance considerations
----------------------------

	Using AOT code is a trade-off which might lead to higher or
	slower performance, depending on a lot of circumstances. Some
	of these are:
	
	- AOT code needs to be loaded from disk before being used, so
	  cold startup of an application using AOT code MIGHT be
	  slower than using JITed code. Warm startup (when the code is
	  already in the machines cache) should be faster.  Also,
	  JITing code takes time, and the JIT compiler also need to
	  load additional metadata for the method from the disk, so
	  startup can be faster even in the cold startup case.

	- AOT code is usually compiled with all optimizations turned
	  on, while JITted code is usually compiled with default
	  optimizations, so the generated code in the AOT case should
	  be faster.

	- JITted code can directly access runtime data structures and
	  helper functions, while AOT code needs to go through an
	  indirection (the GOT) to access them, so it will be slower
	  and somewhat bigger as well.

	- When JITting code, the JIT compiler needs to load a lot of
	  metadata about methods and types into memory.

	- JITted code has better locality, meaning that if A method
	  calls B, then the native code for A and B is usually quite
	  close in memory, leading to better cache behaviour thus
	  improved performance. In contrast, the native code of
	  methods inside the AOT file is in a somewhat random order.
	
* Future Work
-------------

	- Currently, when an AOT module is loaded, all of its
	  dependent assemblies are also loaded eagerly, and these
	  assemblies need to be exactly the same as the ones loaded
	  when the AOT module was created ('hard binding'). Non-hard
	  binding should be allowed.

	- On x86, the generated code uses call 0, pop REG, add
	  GOTOFFSET, REG to materialize the GOT address. Newer
	  versions of gcc use a separate function to do this, maybe we
	  need to do the same.

	- Currently, we get vtable addresses from the GOT. Another
	  solution would be to store the data from the vtables in the
	  .bss section, so accessing them would involve less
	  indirection.