dmr_C

The aim of the dmr_C project is to create a JIT compiler for C. dmr_C is based on the the Linux Sparse library originally written by Linus Torvalds.

The name dmr_C is a homage to Dennis M Ritchie.

Overview

dmr_C is a fork of Sparse. The main changes are:

Global state has been removed
Functions with external linkage have been renamed to avoid polluting the global namespace
The library can be built on Windows using MSVC
WIP Ensure that code can be compiled using a C++ compiler
The LLVM backend has been converted to a JIT compiler - i.e. it is exposed as a library API
WIP Work is ongoing on additional backend based on NanoJIT.
A new backend based on Eclipse OMR derived [JIT].

Current status

The code builds on Windows, Linux and Mac OSX. However there are platform specific limitations - see below for details.
The Sparse code generator for LLVM has had many fixes and is able to compile real programs. See details below for what works and what doesn't.
The NanoJIT code generator is now able to handle a reasonable subset of C, although it is not yet as well tested as the code generator targetting LLVM.
A new backend using Eclipse OMR is now available.

News

Jun-2018 - New backend JIT using Eclipse OMR
Sep-2017 - a lot more functional NanoJIT backend
Aug-2017 - synced with Sparse 0.5.1 release

Build instructions

The build is pretty standard CMake build. There are no external dependencies except the JIT backend. To build without a JIT backend just try:

mkdir build
cd build
cmake ..

This will generate appropriate build files that can then be used to build the project.

LLVM Backend

To build with LLVM support, additional arguments are needed. Following instructions are for LLVM 3.9 on Windows 10.

mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/path/to/install -DLLVM_JIT=ON -DLLVM_DIR=$LLVM_INSTALL_DIR\lib\cmake\llvm -G "Visual Studio 15 2017 Win64" ..

Here $LLVM_INSTALL_DIR refers to the path where LLVM is installed.

Generation of build scripts follows a similar process on Linux and Mac OSX platforms. Note that on Ubuntu the standard LLVM package has broken CMake files hence the recommended approach is to download and build LLVM before attempting to build dmr_C.

Following steps are how I build on Linux:

mkdir build
cd build
cmake  -DCMAKE_INSTALL_PREFIX=/path/to/install -DLLVM_JIT=ON -DLLVM_DIR=$HOME/Software/llvm501/lib/cmake/llvm -G "Unix Makefiles" ..

In my setup LLVM is installed at $HOME/Software/llvm501.

Once the build files are generated you can use the normal build tools i.e. Visual Studio on Windows and make on UNIX or Mac OSX platforms.

Assuming you specified the CMAKE_INSTALL_PREFIX you can install the header files and the library using your build script. For example, on Linux, just do:

make install

OMRJIT Backend

First follow instructions to build a OMRJIT library.
You may need to amend the script FindOMRJIT.cmake to match your installation details. By default it expects the find OMRJIT at ~/Software/omr on Linux/Mac OSX and /Software/omr on Windows.
Next you need to generate the CMake build scripts. I use following on Windows 10.

mkdir build
cd build
cmake -DOMR_JIT=ON -G "Visual Studio 15 2017 Win64" ..

On Linux, I use:

mkdir build
cd build
cmake -DOMR_JIT=ON ..

Assuming you specified the CMAKE_INSTALL_PREFIX you can install the header files and the library using your build script. On Linux just do:

make install

NanoJIT Backend

First follow instructions to build a NanoJIT library.
You may need to amend the script FindNanoJIT.cmake to match your installation details.
Next you can generate the CMake build scripts. I use following on Windows 10.

mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/path/to/install -DNANO_JIT=ON -G "Visual Studio 15 2017 Win64" ..

On Linux, I use:

mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=$HOME/ravi -DNANO_JIT=ON -G "Unix Makefiles" ..

Assuming you specified the CMAKE_INSTALL_PREFIX you can install the header files and the library using your build script. On Linux just do:

make install

Using dmr_C as a JIT

dmr_C has three alternative backend JIT engines, LLVM, OMRJIT and NanoJIT. The LLVM backend is better tested and has evolved from sparse-llvm tool that comes with Sparse. The NanoJIT and OMRJIT backends are entirely new, have had less testing, and are also more limited in the features supported.

Using the LLVM backend

To use it as a LLVM based JIT you only need to invoke following function declared in header file dmr_c.h:

extern bool dmrC_llvmcompile(int argc, char **argv, LLVMModuleRef module,
			     const char *inputbuffer);

The call accepts the arguments passed to a main() function, an LLVMModuleRef, and an optional buffer to be compiled. It will preprocess files if needed, and compile each of the source files given in the argument list. It will finally compile the supplied input buffer. The results of the compilation will be in the supplied LLVMModuleRef for the calling program to use as it wishes.

Note that the LLVM backend only generates LLVM IR - compiling the IR to machine code is the client's responsibility.

A very simple use is below:

#include <dmr_c.h>
#include <stdio.h>

int main(int argc, char **argv)
{
	int rc = 1;

	LLVMContextRef context = LLVMGetGlobalContext();
	LLVMModuleRef module =
	    LLVMModuleCreateWithNameInContext("dmrC", context);
	if (module) {
		if (dmrC_llvmcompile(argc, argv, module, NULL))
			rc = 0;
		LLVMDisposeModule(module);
	}
	return rc;
}

This is basically what the sparse-llvm command (see below) does.

Limitations of LLVM backend

The code generator targetting LLVM has some limitations and is unable to correctly handle following scenarios:

Initializers on static and globals do not work yet except for simple strings and scalar variables. The C parser and Sparse IR handle these but the code generator does not.
Aggregate assignments are not yet supported by the code generator, i.e. you cannot assign a struct by value.
Initialization of char[] doesn't work as it is treated as an aggregate assignment.
The va_arg mechanism is not supported - this is not supported in the Sparse IR generation.
There is no support for computed gotos in the code generator.
The front-end C parser and pre-processor knows about many Linux constructs hence it can process C header files on Linux. However, it doesn't know about Windows or Mac OSX features. As typically the vendor supplied header files have many platform specific extensions, unfortunately this means that you cannot process vendor supplied header files on these platforms.

Using the OMRJIT backend

This is very similar to how the LLVM backend is used. There is again a single API call to compile C code; this is declared in header file dmr_c.h. Additional steps are needed to execute the compiled code.

bool dmrC_omrcompile(int argc, char **argv, JIT_ContextRef module,
		      const char *inputbuffer);

Below is what the sparse-omrjit tool does:

#include <dmr_c.h>

#include <stdbool.h>
#include <stdio.h>

int main(int argc, char **argv)
{
	JIT_ContextRef module = JIT_CreateContext();

	int rc = 0;
	if (!dmrC_omrcompile(argc, argv, module, NULL))
		rc = 1;

	int (*fp)(void) = NULL;
	if (rc == 0) {
		/* To help with testing check if the source defined a function
		 * named TestNano() and if so, execute it
		 */
		fp = JIT_GetFunction(module, "TestNano");
		if (fp) {
			int fprc = fp();
			if (fprc != 0) {
				printf("TestNano Failed (%d)\n", fprc);
				rc = 1;
			} else {
				printf("TestNano OK\n");
			}
		}
	}
	JIT_DestroyContext(module);

	return rc;
}

In the example above, we check if there is a compiled function named 'TestNano'. If it is then we invoke it.

Note that you can supply optimzation option -O<n> to the compiler. The setting of 0 disables optimization, 1 enables some optimizations, and 2 is the highest level with additional optimizations enabled. Some programs may fail to compile with level 2 at present.

Limitations of OMR JIT backend

You cannot have static or global data in JIT code
String literals are not supported as they require static storage
Computed gotos are not supported
Bitfield access is implemented but not well tested - the sign of bitfield values is a problem.
Passing local (automatic) storage scalar values by reference (i.e. through a pointer) may cause subsequent code to not realize that the value may have changed; this is being worked on and a tentative fix is in place, but testing shows that this still fails under -O2.
Aggregate initializers are not supported as yet - mainly because we need to zero initialize any values not explicitly assigned, and this is not done yet.
The va_arg mechanism is not supported
Recursive function calls are not possible at present due to the way functions are resolved; OMRJIT supports recursive calls so I expect to add these at some point in the future.
String initializers are not supported
There has been limited amount of testing of this backend, so expect it to fail! See the tests in tests/omrjit for examples of what works.
Note that a version of the OMRJIT backend is being used in the Ravi project. It is my plan to make this the main JIT backend for Ravi.

Using the NanoJIT backend

bool dmrC_nanocompile(int argc, char **argv, NJXContextRef module,
		      const char *inputbuffer);

Below is what the sparse-nanojit tool does:

#include <dmr_c.h>

#include <stdbool.h>
#include <stdio.h>

int main(int argc, char **argv)
{
	NJXContextRef module = NJX_create_context(true);

	int rc = 0;
	if (!dmrC_nanocompile(argc, argv, module, NULL))
		rc = 1;

	int (*fp)(void) = NULL;
	if (rc == 0) {
		/* To help with testing check if the source defined a function
		 * named TestNano() and if so, execute it
		 */
		fp = NJX_get_function_by_name(module, "TestNano");
		if (fp) {
			int fprc = fp();
			if (fprc != 0) {
				printf("TestNano Failed (%d)\n", fprc);
				rc = 1;
			} else {
				printf("TestNano OK\n");
			}
		}
	}
	NJX_destroy_context(module);

	return rc;
}

In the example above, we check if there is a compiled function named 'TestNano'. If it is then we invoke it.

Limitations of NanoJIT backend

Note that NanoJIT is a tiny library that aims to generate code quickly - but the generated code is not much optimized. In particular straight line code without branching is better, and the code generator has some ability to optimize loops; but as the register allocator does local register allocation only, there is too much stack spilling if there are branches in the code.
String literals are not supported as they require static storage
Computed gotos are not supported
Bitfield access is implemented but not well tested
Calling functions through function pointers is not yet supported
Aggregate initializers are not supported
The va_arg mechanism is not supported
Recursive function calls are not possible at present due to the way functions are resolved
NanoJIT only allows 4 (Win32) or 6 (Linux) integer/pointer parameters (on X86-64) inside a JIT function, a JITed function only processes arguments passed via registers
You cannot have static or global data in JIT code
String initializers are not supported
Generated code for switch statements uses if (cond) branching rather than a jump table. NanoJIT does have a jump table instruction but it requires the indices to be consecutive starting from zero, and there is no support for a default case. In future this instruction may be used when possible.
There has been limited amount of testing of this backend, so expect it to fail!

Using dmr_C command line tools

The following command line tools are built:

sparse-llvm

The sparse-llvm tool takes in a source C file and generates an LLVM module. It writes the LLVM Module in LLVM bitcode format. Note that this command line tool is only built when LLVM backend is being used.

The tool will output the preprocesed source code if -E option is given.
The tool will save the output to specified file if -o option is given, otherwise output is saved to file named 'out.bc' in current directory.
The -O1 option can be used to enable internal IR simplifications. Note that these simplifications sometimes cause issues - see the tests/bugs folder for known issues.
To run the generated module, you typically need to generate assembly code and then link it to produce an executable. Example:

sparse-llvm test.c -o test.bc
llc test.bc
gcc -o test test.s

Alternatively you can also run the bitcode directly using the LLVM command lli.

sparse-llvm test.c -o test.bc
lli test.bc

You can generate LLVM IR by running the llvm-dis command.

sparse-llvm test.c -o test.bc
llvm-dis test.bc

linearize

The linearize tool outputs the Sparse IR generated by the parser and compiler front-end.

showsymbols

The showsymbols tool shows the global symbols found in a source file, output is generated in XML format.

showparsetree

This tool dumps the parse tree as built by the C parser - note that this tool is experimental and the output format is evolving.

sparse

The sparse tool checks C code and outputs warnings or error messages for certain conditions. For details please see Linux Sparse man page.

Bugs

Many bugs have been fixed in the code generators and the tool is able to compile and run real programs. However there are still bugs that mean that the generated code is sometimes not correct. See the tests/bugs folder for examples of programs that fail to compile successfully. If you hit a problem, please submit a bug report with a minimal example of program that fails.

Note that the NanoJIT backend is in early phase of development, and has had much less testing.

Using dmr_C as a library

The dmr_C is also a library and can be linked and used by application programs. As a library it is made up of several components:

Tokenizer - this converts the input stream into tokens
Preprocesser - this carries out C pre-processing
Parser - this parses the token stream and creates parse tree and symbols
Linearizer - this takes the parsed tree and symbols and generates SSA IR
Optimizer - this performs various optimizations on the SSA IR. Note that this is not very robust and is now disabled.

The JIT backends

LLVM backend - this takes the Sparse IR output and converts this to LLVM IR
OMR JIT backend - uses Eclipse OMR JIT engine to generate machine code
NanoJIT backend - alternative backend that generates machine code using NanoJIT
The JIT backends take the Sparse IR generated by the linearizer in sparse and convert these to JIT IR instructions.
You can view the Sparse IR output by running the linearize command.
The Sparse IR is lower level than LLVM IR - in particular it has no concept of types other than primitive integer, floating point and pointer types, and is similar to NanoJIT IR in this respect. The generated LLVM IR therefore relies upon casting the values where required to types expected by LLVM.

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