Summary

SoftWire is a class library written in object-oriented C++ for compiling assembly code. It can be used in projects to generate x86 machine code at run-time as an alternative to self-modifying code. Scripting languages might also benefit by using SoftWire as a JIT-compiler back-end. It also allows to eliminate jumps for variables which are temporarily constant during run-time, like for efficient graphics processing by constructing an optimised pipeline. Because of its possibility for 'instruction rewiring' by run-time conditional compilation, I named it "SoftWire". It is targeted only at developers with a good knowledge of C++ and x86 assembly.

This document applies to SoftWire 4.0.1 and newer versions.

Content

1. Demo
2. Compilation
3. Syntax
4. Run-Time Intrinsics
5. Automatic Register Allocation
6. Design
7. Licence Conditions
8. Contributions & Credits
9. Bugs & Feature Requests
10. Acknowledgements

1. Demo

The demo application assembles seven test routines: HelloWorld.asm shows that it is possible to call external functions, like printf. SetBits.asm is a function to set a number of bits in a buffer, starting from a given bit. CrossProduct.asm shows the use of floating-point instructions, macros and inline functions. AlpahBlend.asm uses MMX instructions for blending 32-bit colors, and conditionally compiles for Katmai compatible or older processors. It also shows how to define static data. Factorial.asm calculates a factorial by recursively calling itself. Mandelbrot.asm draws the Mandelbrot fractal in ASCII. The last test shows the use of run-time intrinsics.

Execute SoftWire.exe at your own risk! It has been tested on many systems, but I make no guarantee that it will work on yours.

For more projects that use SoftWire, check out http://softwire.sourceforge.net/extra.

2. Compilation

This library was developed with Visual C++ .NET and has project and workspace files for this compiler included. Solution files for Visual C++ 6.0 and makefiles for Dev-C++ and GCC are also included. For GCC you will need a recent version which supports nameless structs. Should you have any problems compiling the code, please mail me at nicolas@capens.net.

Compilation with GCC requires the -fno-operator-names option. This is needed to avoid an error with functions named and, not, or and xor, which are according to the C++ standard reserved keyword. For more information see the Run-Time Intrinsics section.

3. Syntax

Before reading this detailed information it is best to take a look at Test.cpp and the .asm files to see the basics of what SoftWire has to offer...

The source line layout is similar to that from NASM and the Visual C++ inline assembler:
label: instruction operands ; comment

The assembler can accept a file with the .asm extension. This file is treated as one block of code, but subroutines and data can be created with labels. Execution will start at the lablel with the same name as the file, unless an other entry point has been defined. The assembler generates only 32-bit code for processors compatible with the 386 or above.

C/C++ comments are also supported. The assembler is case-sensitive, except for instructions and registers. Labels are like in inline assembler and cannot have special characters like $, #, @, ~, ?, etc.

Specifiers are always optional, but when the assembler has multiple possible instructions you can't predict the behaviour without using a specifier. For example, the assembler can't know if the code fld [esi] uses single or double precision floating-point numbers without a DWORD or QWORD specifier. The PTR keyword is optional. The NEAR or SHORT keywords can be used for jumps or calls and are equivalent.

The assembler supports the MMX, 3DNow!, Pentium Pro, SSE and SSE2 instruction set. Some specific instructions have been removed because they are unsafe and/or not useful for 32-bit protected mode:

- Privileged instructions
- FAR calls and jumps
- Undocumented instructions
- Extended precision 80-bit FPU instructions
- State save/restore instructions
- Segment/debug/control/table registers

This is very similar to the Visual C++ inline assembler. It should not limit you for 'normal' use of this run-time assembler. For more details, take a look at the InstructionSet.cpp file. You can always write your own machine-code by defining it as static data.

You can define static data with the DB, DW and DD keywords. By using a label, the address of this data can be referenced. The data will be created at the location of the definition, so it's advisable to put it after the return or before the function label. Since there is no standard way to declare local data, you should put everything on the stack yourself. Local data is not that usefull for a run-time assembler anyway. You could use the 'cdecl' calling convention (standard in Visual C++) to let the caller push the arguments on the stack and remove them after the function has been called. Subroutines can also be created by using labels. To create arrays of static data, you can use DB[#], DW[#] and DD[#]. All variables will be aligned on their natural boundaries.

To align data or code yourself, you can use the ALIGN keyword. For efficiency, jump labels should be 16 byte aligned, and for most SSE instructions the data also has to be 16 byte aligned. The assembler will use NOP instructions for padding for both data and code alignment.

External data can be declared by using the Assembler::defineExternal method in your C++ code. A handy macro defined in Assembler.hpp makes it possible to export a function like printf with ASM_EXPORT(printf). Externals can be any kind of data defined in your C++ application, and are treated like void pointers. Externals should be declared before assembling the file. They do not have to be re-declared in the assembly code.

For constants, only numbers and character constants are supported. They can be in binary, octal, decimal or hexadecimal base, with the usual pre- or postfixes. All constant expressions are evaluated, inclusing those of data definitions and memory references. String literals can be created by using DB and double quotation marks.

Conditional compilation can be controlled with the #if, #elif, #else and #endif precompiler directives. The ASM_DEFINE macro can be used to send an integer to the assembler which can be used after the #if and #elif directives. Boolean expressions are evaluated as in C/C++.

This powerful feature can be used to generate many different specific functions without having to code completely new ones. It can eliminate jumps in the assembly code to generate exactly the optmized function you need. An example of this is an SIMD optimized vertex pipeline for 3D graphics. Without conditional compilation, many comparisons and jumps would be needed per vertex to transform and light it correctly for the current settings. With run-time conditional compilation, these instructions can be eliminated, leaving only the wanted instructions, while still being able to handle thousands of setting combinations. It also allows to write different code for other processors, without needing a control statement in your high-performance assembly code and without the need to write it as separate functions which are difficult to maintain.

The preprocessor also supports #include and #define. There is also an inline keyword, which behaves like #define but produces less error-prone code (caused by nested macros) and has a nicer syntax for multiple lines:

inline macroName(argument1, argument2, ...)
{
code block
}

It is a nice way of defining new instructions, and it even allows to define 'instructions' to be emulated with x86 assembly, like DirectX shader instructions! With normal macros, the only problem for doing this would be that you need parenthesis around the arguments. But even this is solved with the inline macros. You can simply use the above macro like this:

    macroName argument1, argument2, ...

When you don't write an open parenthesis, SoftWire will automatically assume that you want to use this 'implicit' argument list. The argument list stops at the end of the line, so it is not possible to nest multiple macros without using parenthesis.

Sometimes it can be rather unhandy to work with the Assembler class if all you need is the machine code. To have the control over the code and safely delete the Assembler instance without destroying the code, use the Assembler::aquire() method. This method returns the pointer to the machine code, and tells the Assembler not to delete it at the destructor. Note that it is still necessary to first use Assembler::callable(entryLabel) to get the pointer(s) to your function(s). If aquire() is called before any call to callable(), it will return zero.

To minimize memory usage, call Assembler::finalize() after retrieving the pointers to the functions. This is especially useful for sub-classing Assembler. It will delete all temporary memory used by the scanner, parser, instruction table and intermediate code. Any futher assembling tasks like using a run-time intrinsic will result in an exception to be thrown. The generated machine code is still property of Assembler unless you also call aquire().

4. Run-Time Intrinsics

SoftWire also supports another form of run-time code generation. With every assembly instruction corresponds a member function of Assembler with the same name. These functions encode the corresponding instruction and put it in the Loader so it is ready to execute. These run-time intrinsics are ideal for writing a compiler back-end. Because it is all written in C++, things like conditional compilation become trivial.

First you need to construct an Assembler, without providing any arguments (like you had to do when assembling a file).

You can use the usual register names directly. For example add(eax, ebx); is a valid member function call of Assembler. For memory operands, you need to use dword_ptr [...] and the like. Note that all syntax checks should happen at compile-time. An exception is that it's impossible to check the scale factor in a memory reference.

To add labels, you need to use the Assembler::label() method. For example to create an array of 100 uninitialized bytes:

    x86.label("table");
    x86.db(byte [100]);

Note that here it doesn't matter if you use byte or any other specifier, it is only needed to have a valid C++ syntax. To reference the data or function at a label, also use a string literal. This works on any instruction which can accept a 32-bit immediate, like jmp, call, push, dd, etc.

Take a look at testIntrinsics() in Test.cpp for some simple working example code.

Because and, not, or and xor are both assembly instructions and reserved C++ keywords, the compiler should ignore them. In Visual C++ they are not recognised as keywords, but with GCC you need the -fno-operator-names option to allow run-time intrinsics.

To simplify debugging of a code generator which uses intrinsics, there is an Assembler::setEchoFile method. This function takes the name of a file where SoftWire will copy the textual assembly code corresponding with the intrinsic. This way you can check the output without using a disassembler. There's also an Assembler::annotate method to comment your generated code. It will put a semicolon at the start of the line, so the echo file can also be used as the input for SoftWire.

The Intrinsics.hpp file can take a while to compile. If run-time intrinsics are not needed in your project but fast compilation is a must, define SOFTWIRE_NO_INTRINSICS before including Assembler.hpp.

5. Automatic Register Allocation

Now that you know how run-time intrinsics work, let's take this to the next level. I already mentioned that things like conditional compilation become trivial because it's all written in C++, but we can do lot's more like eliminating redundant operation with peephole optimization, and automatic register allocation.

The latter has been integrated into SoftWire 4.0.0 and newer versions. Basically what it does is mapping variables in memory to registers. When there are more variables than physical registers, it automatically spills registers. This enables you to implement any kind of compiler, from a full-blown C++ compiler to a simple scripting language without worrying about the low-level stuff. It's almost like directly writing intermediate code!

Because this adds an extra level of abstraction it is not implemented in the Assembler class, but in a subclass called CodeGenerator . The main reason it was not added to Assembler is because if you only need a simple assembler you can define SOFTWIRE_NO_INTRINSICS to keep things lightweight. The CodeGenerator class could later also feature optimizations like scheduling which do not guarantee a 1-to-1 mapping of the assembly code to the machine code.

The first and most important new method is CodeGenerator::r32(). It takes a memory reference as argument and returns a general purpose register (not including esp and ebp). For MMX and SSE registers you can use CodeGenerator::r64() and CodeGenerator::r128() respectively. It is advisable to derive your own code generator from CodeGenerator so the class name can be ommited. Example:

class ScriptCompiler : pulic CodeGenerator
{
public:
void generateCode()
{
static int foo;
mov(r32(&foo), 0x12345678);
...

It works just as well with non-static variables, like variables on the stack:

    mov(r32(esp+12), 0x12345678);

Or if you're indexing an array of ints 'array' with an index 'index' and store that in 'value':

    mov(r32(&value), dword_ptr[r32(&array)+r32(&index)*4]);

So there is no need to worry anymore what register is used where or if it is available. You treat them all like variables in memory and SoftWire automatically places them into registers.

After the operations on the variable are done, we would like to write it back to memory. This can be done with the spill() method, which takes a register or reference as argument. The register to which the variable is allocated then becomes available again. Another method that does this is free(). The difference with spill is that free does not write the variable's data back to memory. This is useful for registers that are only needed temporarily, more about that later. When an instruction can only work on for example eax, you should spill or free eax before using this instruction. Also beware that at branches you have to spill all used variables. This can also be done with the spillAll() method, or with freeAll() if you've written back all results yourself.

The above example is not optimal, since SoftWire will copy the data from [esp+12] to the allocated register, and it then immediately gets overwritten with 0x12345678. To eliminate this redundant copying, use the x32() method instead, or x64/x128 for MMX/SSE. This function works just like r32 but does not copy the data from the memory reference to the register. This is mainly useful for temporary registers. Have a look at the TestRegisterAllocator class in Test.cpp for a working example.

Because some intermediate instructions can be complex and require temporary variables in physical registers for speed, there's also the t32() method, or t64/t128 for MMX/SSE. This function does not take a reference to a memory location as argument, but only an index. The function returns a register that will never be spilled. Therefore you can only use indices 0 to 5 for 32-bit registers and 0 to 7 for MMX/SSE registers. The difference with using the registers directly is that it does not interfere with the regiser allocator, and if necessary it spills the register with lowest priority first. So there is no 1-to-1 mapping of t32 registers to the physical registers. Free them as soon as possible to avoid unnecessary spilling or running out of registers! To avoid this complexity, just stick to the r32 or x32 functions. The only advantage of t32 is that you don't need a memory location where the register can be written to if it needs to be spilled.

To optimize memory accesses, there is also a m32() method, or m64/m128 for MMX/SSE. This function returns either a register or a memory reference. many instructions can accept a r/m32 argument, and of course registers are more efficient. If the variable is already located in a register, m32 will return that register, else it will return the memory reference.

The register allocator assumes that all six general purpose registers are available, and all MMX and SSE registers. It is your task to save and restore registers before using automatic register allocation.

The spilling heuristic uses the access frequency to find the best candidate for spilling. When a register is allocated, it has the highest priority, meaning that it is the worst candidate for the next spill. For every access of a register, all other registers loose priority. This also means that less recently used registers have a lower priority. When spilling is needed, the register with lowest priority is written back to its memory location.

The CodeGenerator class automatically does some trivial peephole optimizations. To disable this behaviour, define SOFTWIRE_NO_PEEPHOLE before including Codegenerator.hpp.

6. Design

The whole library is encapsulated in a namespace called SoftWire. This is to prevent name clashes with other projects.

The only class you'll need for assembling a file and getting a pointer to the callable code is Assembler. It has to be constructed with the name of the .asm file which contains the assembly code. The assembler treats it as one block of code, and you can get a void pointer to the assembled code by calling the callable() method. By default the entry point will be a label with the same name as the file. You can also pass the name of a label as the entry point if you want to start excecution from another line. To effectively call the function, you first need to cast it to an appropriate function pointer. When the Assembler is destructed, it also deletes the assembled code. To prevent this and control the lifetime of the code yourself, call the acquire() method. This will return a pointer to the start of the code. This is not necessarily the entry point of the code, so you still need to use callable().

The first class the assembler will use for processing the assembly file is the Scanner. This class has the task to break up the source code into words, called tokens. It is also resposible for the preprocessing tasks like file inclusion, conditional compilation and macro expansion. The Macro class helps with this last task.

The tokens are stored in a Token class. The scanner also recognizes tokens as being identifiers, constants or punctuators. The scanner does not recognize keywords (except preprocessor directives) and does no syntax checking. The whole file is scanned at once and the tokens are placed in a TokenList class.

Every source line of tokens then goes to the Parser. It will recognize the keywords, check the syntax and pass the information like mnemonic and registers to the code generator.

The code generation is done with the Synthesizer class. It will put the information from the parsed instruction into bytes for the machine code.

The rules for the code generation are stored in the InstructionSet class. The parser uses this class to select the matching instruction(s), and the synthesizer uses it to know how to encode the instruction.

The bytes from the synthesizer are stored into an Encoding class. It also stores information about labels and references to labels to resolve jump addresses.

All encodings are stored in the Loader class. After all instructions have been assembled, this class will resolve all the references and write the machine-code bytes into a buffer. Externally declared data will be resolved by the assembler's Linker class. The loader also searches for the code entry point. When the assembler is destructed, the assembled code is also destroyed, except when using acquire(). The linker data is also cleared.

When a syntax error occurs, the assembler throws an Error class. This class simply holds a string with the error description. This message will be printed to the console by using the DebugOutput::printf method. You can easily use your custom error output system by deriving from the DebugOutput class. Besides syntax errors, the assembler might also throw internal errors. This is an alternative to assert(), so it should not happen. If you get an internal error, or worse, an unhandled exception, please contact the author.

Run-time intrinsics are generated by the InstructionSet, and stored in Intrinsics.hpp . You need to uncomment the generateIntrinsics() line in the InstructionSet constructor when you've made changes to the instruction set and need new intrinsics. Do not attempt to modify the Intrinsics.hpp file manually. The arguments for the intrinsics are defined in Operand.hpp.

Automatic register allocation is controlled by the CodeGenerator class. For every physical register it keeps a reference to the memory location it is associated with. When spilling is needed it writes the least frequently used register back to memory.

7. License Conditions

All source files fall under the LGPL (License.txt) and are Copyright (C) 2002-2003 Nicolas Capens:

If you extend the possibilities of the classes in these files, please send your changes to the copyright holder(s). Do not change this file or License.txt, but use a change log. If you only derive from a class to write your own specific implementation, you don't have to release the source code of your whole project, just give credit where due. This can be done by mentioning my name in your credits list and/or providing a link to the original SoftWire source code (e.g.http://softwire.sourceforge.net).

Don't hesitate to contact me and show what you've created with SoftWire!

8. Contributions & Credits

- Jude Venn: helped porting SoftWire to UNIX/Linux by providing alternative functions for the Microsoft specific code (File.hpp, String.hpp, CharType.hpp). He also wrote Makefile.gcc to easily compile SoftWire with GCC.
- 'Carrot': reported and fixed missing FMULP and other variants and JNEA misspelling.
- Walt Woods: reported and fixed memory leak in InstructionSet. Reported bug with "push imm" instruction not pushing dwords for small integers. Fixed stack overflow with TokenLink destructor.
- Edwin Young: reported 16-bit register bug in Factorial.asm.
- 'tawai': reported bug with single-argument IMUL instruction.
- Rene Dudfield: reported 3DNow! instruction encoding bugs. Helped SoftWire 3.0.0 to compiler under Linux.
- Oscar Fuentes: reported missing addressing mode for run-time intrinsics and helped SoftWire 3 to compile with GCC and Intel compilers. Suggested methods to change ownership of machine code.
- Florian Bösch: Reported bugs in run-time intrinsics with references to labels.
- Blake Pelton: Reported small memory leaks caused by strdup() use.
- 'IanM': Contributed changes to assemble from source string.
- Dario Pelella: Fixed "push imm" bug with specifiers. Reported FPU instructions bug with '+r' encoding.

If you feel like you should also have been mentioned on this list (or be removed or have something changed), please do not hesitate to contact me to correct this mistake.

Why are contributions, bug fixes and copyrights not indicated in the code? I do not like this because in my opinion source files should be kept as readable as possible. I think it is very annoying that you first have to scroll past a huge block of comments that don't have anything to do with the code itself. Source files are for code. Licence and readme files are for the things not directly related to the code but to the library as a whole. If you cannot agree with this point of view and have some strong arguments, please contact me to discuss it.

9. Bugs & Feature Requests

SoftWire is a work-in-progress, so every kind of feedback is welcome, good or bad. I'm also always willing to help you out if you don't get something working. If you're a C++ guru and you would have designed some parts differently, I'm all ears. Contact me via e-mail at nicolas@capens.net.

10. Acknowledgements

Special thanks to:

- The creators of NASM, for their detailed overview of the x86 instruction set
- www.sandpile.org, for its great resources about IA-32 encoding rules
- Everyone at www.flipcode.com who has helped me directly or indirectly to make this project possible: http://www.flipcode.com/cgi-bin/msg.cgi?showThread=COTD-SoftWire&forum=cotd&id=-1
- Jude Venn, for helping me get started with UNIX and GCC.
- Anyone who reported bugs or suggested useful features.

Kind regards,

Nicolas Capens

Copyright (C) 2002-2003 Nicolas Capens - nicolas@capens.net