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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.
* 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 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
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.
method_offsets
The equivalent to a procedure linkage table.
* 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, the runtime needs to setup some data structures and fill out
GOT entries before a method is first called. This means that even calls to
a method whose code is in the same AOT image need to go through the GOT,
instead of using a direct call.
- 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.
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