GONspell

[mono.git] / mono / mini / mini-porting.txt

                        Mono JIT porting guide.
                Paolo Molaro (lupus@ximian.com)

* Introduction

This documents describes the process of porting the mono JIT
to a new CPU architecture. The new mono JIT has been designed 
to make porting easier though at the same time enable the port
to take full advantage from the new architecture features and 
instructions. Knowledge of the mini architecture (described in the
mini-doc.txt file) is a requirement for understanding this guide,
as well as an earlier document about porting the mono interpreter
(available on the web site).

There are six main areas that a port needs to implement to
have a fully-functional JIT for a given architecture:

        1) instruction selection
        2) native code emission
        3) call conventions and register allocation
        4) method trampolines
        5) exception handling
        6) minor helper methods

To take advantage of some not-so-common processor features (for example
conditional execution of instructions as may be found on ARM or ia64), it may
be needed to develop an high-level optimization, but doing so is not a 
requirement for getting the JIT to work.

We'll see in more details each of the steps required, note, though,
that a new port may just as well start from a cut&paste of an existing
port to a similar architecture (for example from x86 to amd64, or from
powerpc to sparc).
The architecture specific code is split from the rest of the JIT,
for example the x86 specific code and data is all included in the 
following files in the distribution:

        mini-x86.h mini-x86.c
        inssel-x86.brg
        cpu-pentium.md
        tramp-x86.c 
        exceptions-x86.c 

I suggest a similar split for other architectures as well.

Note that this document is still incomplete: some sections are only
sketched and some are missing, but the important info to get a port 
going is already described.


* Architecture-specific instructions and instruction selection.

The JIT already provides a set of instructions that can be easily
mapped to a great variety of different processor instructions.
Sometimes it may be necessary or advisable to add a new instruction
that represent more closely an instruction in the architecture.
Note that a mini instruction can be used to represent also a short
sequence of CPU low-level instructions, but note that each
instruction represents the minimum amount of code the instruction 
scheduler will handle (i.e., the scheduler won't schedule the instructions
that compose the low-level sequence as individual instructions, but just
the whole sequence, as an indivisible block).
New instructions are created by adding a line in the mini-ops.h file,
assigning an opcode and a name. To specify the input and output for 
the instruction, there are two different places, depending on the context 
in which the instruction gets used.
If the instruction is used in the tree representation, the input and output
types are defined by the BURG rules in the *.brg files (the usual 
non-terminals are 'reg' to represent a normal register, 'lreg' to 
represent a register or two that hold a 64 bit value, freg for a
floating point register).
If an instruction is used as a low-level CPU instruction, the info
is specified in a machine description file. The description file is
processed by the genmdesc program to provide a data structure that
can be easily used from C code to query the needed info about the 
instruction.
As an example, let's consider the add instruction for both x86 and ppc:

x86 version:
        add: dest:i src1:i src2:i len:2 clob:1
ppc version:
        add: dest:i src1:i src2:i len:4

Note that the instruction takes two input integer registers on both CPU,
but on x86 the first source register is clobbered (clob:1) and the length
in bytes of the instruction differs.
Note that integer adds and floating point adds use different opcodes, unlike
the IL language (64 bit add is done with two instructions on 32 bit architectures,
using a add that sets the carry and an add with carry).
A specific CPU port may assign any meaning to the clob field for an instruction
since the value will be processed in an arch-specific file anyway.
See the top of the existing cpu-pentium.md file for more info on other fields:
the info may or may not be applicable to a different CPU, in this latter case
the info can be ignored.
The code in mini.c together with the BURG rules in inssel.brg, inssel-float.brg
and inssel-long32.brg provides general purpose mappings from the tree representation 
to a set of instructions that should be easily implemented in any architecture.
To allow for additional arch-specific functionality, an arch-specific BURG file
can be used: in this file arch-specific instructions can be selected that provide
better performance than the general instructions or that provide functionality
that is needed by the JIT but that cannot be expressed in a general enough way.
As an example, x86 has the special instruction "push" to make it easier to
implement the default call convention (passing arguments on the stack): almost
all the other architectures don't have such an instruction (and don't need it anyway),
so we added a special rule in the inssel-x86.brg file for it.

So, one of the first things needed in a port is to write a cpu-$(arch).md machine
description file and fill it with the needed info. As a start, only a few
instructions can be specified, like the ones required to do simple integer
operations. The default rules of the instruction selector will emit the common
instructions and so we're ready to go for the next step in porting the JIT.


*) Native code emission

Since the first step in porting mono to a new CPU is to port the interpreter,
there should be already a file that allows the emission of binary native code
in a buffer for the architecture. This file should be placed in the 
        mono/arch/$(arch)/
directory.

The bulk of the code emission happens in the mini-$(arch).c file, in a function
called mono_arch_output_basic_block (). This function takes a basic block, walks the
list of instructions in the block and emits the binary code for each.
Optionally a peephole optimization pass is done on the basic block, but this can be
left for later, when the port actually works.
This function is very simple, there is just a big switch on the instruction opcode
and in the corresponding case the functions or macros to emit the binary native code
are used. Note that in this function the lengths of the instructions are used to
determine if the buffer for the code needs enlarging.

To complete the code emission for a method, a few other functions need
implementing as well:

        mono_arch_emit_prolog ()
        mono_arch_emit_epilog ()
        mono_arch_patch_code ()

mono_arch_emit_prolog () will emit the code to setup the stack frame for a method,
optionally call the callbacks used in profiling and tracing, and move the
arguments to their home location (in a caller-save register if the variable was 
allocated to one, or in a stack location if the argument was passed in a volatile 
register and wasn't allocated a non-volatile one). caller-save registers used by the
function are saved in the prolog as well.

mono_arch_emit_epilog () will emit the code needed to return from the function,
optionally calling the profiling or tracing callbacks. At this point the basic blocks
or the code that was moved out of the normal flow for the function can be emitted 
as well (this is usually done to provide better info for the static branch predictor).
In the epilog, caller-save registers are restored if they were used.
Note that, to help exception handling and stack unwinding, when there is a transition
from managed to unmanaged code, some special processing needs to be done (basically,
saving all the registers and setting up the links in the Last Managed Frame
structure).

When the epilog has been emitted, the upper level code arranges for the buffer of 
memory that contains the native code to be copied in an area of executable memory
and at this point, instructions that use relative addressing need to be patched
to have the right offsets: this work is done by mono_arch_patch_code ().


* Call conventions and register allocation

To account for the differences in the call conventions, a few functions need to
be implemented.

mono_arch_allocate_vars () assigns to both arguments and local variables
the offset relative to the frame register where they are stored, dead
variables are simply discarded. The total amount of stack needed is calculated.

mono_arch_call_opcode () is the function that more closely deals with the call
convention on a given system. For each argument to a function call, an instruction
is created that actually puts the argument where needed, be it the stack or a 
specific register. This function can also re-arrange th order of evaluation
when multiple arguments are involved if needed (like, on x86 arguments are pushed
on the stack in reverse order). The function needs to carefully take into accounts
platform specific issues, like how structures are returned as well as the
differences in size and/or alignment of managed and corresponding unmanaged 
structures.

The other chunk of code that needs to deal with the call convention and other
specifics of a CPU, is the local register allocator, implemented in a function
named mono_arch_local_regalloc (). The local allocator deals with a basic block 
at a time and basically just allocates registers for temporary
values during expression evaluation, spilling and unspilling as necessary.
The local allocator needs to take into account clobbering information, both
during simple instructions and during function calls and it needs to deal
with other architecture-specific weirdnesses, like instructions that take
inputs only in specific registers or output only is some.
Some effort will be put later in moving most of the local register allocator to 
a common file so that the code can be shared more for similar, risc-like CPUs.
The register allocator does a first pass on the instructions in a block, collecting
liveness information and in a backward pass on the same list performs the
actual register allocation, inserting the instructions needed to spill values,
if necessary.

When this part of code is implemented, some testing can be done with the generated 
code for the new architecture. Most helpful is the use of the --regression
command line switch to run the regression tests (basic.cs, for example).
Note that the JIT will try to initialize the runtime, but it may not be able yet to
compile and execute complex code: commenting most of the code in the mini_init()
function in mini.c is needed to let the JIT just compile the regression tests.
Also, using multiple -v switches on the command line makes the JIT dump an 
increasing amount of information during compilation.


* Method trampolines

To get better startup performance, the JIT actually compiles a method only when
needed. To achieve this, when a call to a method is compiled, we actually emit a 
call to a magic trampoline. The magic trampoline is a function written in assembly
that invokes the compiler to compile the given method and jumps to the newly compiled
code, ensuring the arguments it received are passed correctly to the actual method.
Before jumping to the new code, though, the magic trampoline takes care of patching
the call site so that next time the call will go directly to the method instead of the
trampoline. How does this all work?
mono_arch_create_jit_trampoline () creates a small function that just
preserves the arguments passed to it and adds an additional argument (the method
to compile) before calling the generic trampoline. This small function is called 
the specific trampoline, because it is method-specific (the method to compile
is hard-code in the instruction stream).
The generic trampoline saves all the arguments that could get clobbered
and calls a C function that will do two things: 

*) actually call the JIT to compile the method
*) identify the calling code so that it can be patched to call directly
the actual method

If the 'this' argument to a method is a boxed valuetype that is passed to
a method that expects just a pointer to the data, an additional unboxing
trampoline will need to be inserted as well.


* Exception handling

Exception handling is likely the most difficult part of the port, as it needs
to deal with unwinding (both managed and unmanaged code) and calling
catch and filter blocks. It also needs to deal with signals, because mono
takes advantage of the MMU in the CPU and of the operation system to
handle dereferences of the NULL pointer. Some of the function needed
to implement the mechanisms are:

mono_arch_get_throw_exception () returns a function that takes an exception object
and invokes an arch-specific function that will enter the exception processing.
To do so, all the relevant registers need to be saved and passed on.

mono_arch_handle_exception () this function takes the exception thrown and
a context that describes the state of the CPU at the time the exception was 
thrown. The function needs to implement the exception handling mechanism,
so it makes a search for an handler for the exception and if none is found,
it follows the unhandled exception path (that can print a trace and exit or
just abort the current thread). The difficulty here is to unwind the stack 
correctly, by restoring the register state at each call site in the call chain,
calling finally, filters and handler blocks while doing so.

As part of exception handling a couple of internal calls need to be implemented
as well.
ves_icall_get_frame_info () returns info about a specific frame.
mono_jit_walk_stack () walks the stack and calls a callback with info for
each frame found.
ves_icall_get_trace () return an array of StackFrame objects.


* Minor helper methods

A few minor helper methods are referenced from the arch-independent code.
Some of them are:

*) mono_arch_cpu_optimizations ()
        This function returns a mask of optimizations that should be enabled for the
        current CPU and a mask of optimizations that should be excluded, instead.

*) mono_arch_regname ()
        Returns the name for a numeric register.

*) mono_arch_get_allocatable_int_vars ()
        Returns a list of variables that can be allocated to the integer registers
        in the current architecture.

*) mono_arch_get_global_int_regs ()
        Returns a list of caller-save registers that can be used to allocate variables 
        in the current method.

*) mono_arch_instrument_mem_needs ()
*) mono_arch_instrument_prolog ()
*) mono_arch_instrument_epilog ()
        Functions needed to implement the profiling interface.


* Writing regression tests

Regression tests for the JIT should be written for any bug found in the JIT
in one of the *.cs files in the mini directory. Eventually all the operations
of the JIT should be tested (including the ones that get selected only when 
some specific optimization is enabled).


* Platform specific optimizations

An example of a platform-specific optimization is the peephole optimization:
we look at a small window of code at a time and we replace one or more 
instructions with others that perform better for the given architecture or CPU.