TweetFollow Us on Twitter

Sep 96 Factory Floor
Volume Number:12
Issue Number:9
Column Tag:From The Factory Floor

Andreas Hommel, Compiler Architect

By Dave Mark

This month’s interview is with Andreas Hommel, one of the original minds behind Metrowerks’ compiler architecture. (See “A Little CodeWarrior History”, MacTech Magazine 12.7 [July 1996] 61-64, where John McEnerney recalls being shown Metrowerks’ newly acquired C compiler which “a guy named Andreas Hommel in Hamburg had been writing as a hobby”.) You’ll meet a pretty interesting person and, at the same time, learn a thing or two about the compilation process.

Dave: How did you hook up with Metrowerks?

Andreas: I got interested in compiler construction while I was still in University. I was writing computer games, and most C compilers didn’t really produce very good code. Also, I liked ANSI C a lot, but back then, Mac compilers didn’t really conform to this standard. So at one point, I decided that it would be fun to write my own compiler in my spare time. Two or three years later, I had my own little IDE and an ANSI C compiler with some C++ extensions.

I was about to finish my CS degree, and I had a good desktop publishing job offer in Hamburg, but I really liked working on my compiler project, so I decided to give it a try. I sent out a bunch of demo disks to some Macintosh compiler-related companies. A few months later, Greg Galanos called me and we started talking about technical details of the compiler and how we could come together. After a couple of Transatlantic phone calls, Greg invited me to come to Montreal to meet with him and Jean Belanger. They spoke about the incredible opportunities for a compiler company, given Apple’s pending transition to the PowerPC chip. We also talked a lot about all the technical aspects of the compiler and how it could be changed to support another code generator and a Pascal front-end. A week later, we had signed a contract.

The next 6 months were pretty busy. I still had some work to do for my old job, I had to finish and defend my thesis, and I had to start moving the compiler towards C++ for Metrowerks.

Dave: For folks who’ve never written a compiler, can you describe the compilation/link process?

Andreas: The compiler transforms each individual source file (or “translation unit”, to be technically correct) into an object file. The functions, procedures, and variables in a source file are transformed into code and data in the object file. The code in an object file is not executable because it usually contains references to code or data in other object files; these references have yet to be resolved by the linker. The unresolved references are also stored in the object file. The CodeWarrior IDE actually hides much of this process, because it stores all the object files in the project file, so you don’t see them on your hard drive (if you use the CW MPW tools, you will actually generate individual object files). The object file also stores symbolic information that is used by the Source Debugger to map source code to executable code and to find variables and their types. If you are interested in this, and you know a little bit of Assembly Language, you can use the Disassemble command (in the Project menu), which will generate a pretty complete object file dump for a particular source file.

The linker then takes all the object files (a library file is basically just an object file), resolves the external code and data references, and generates an executable file from that. On the Macintosh, the linker also merges your application with the resource data and generates a SYM file that is used by the debugger.

Dave: What about the actual compilation process?

Andreas: The compiler itself can be grouped into several phases. A user typically doesn’t notice these individual phases, and in fact most compilers do not strictly execute one phase after the other, but this logical grouping really makes it a lot easier to implement a compiler.

The first phase is the Lexical Analyzer or Scanner. This part of the compiler “looks” at your source code and splits it into individual tokens. A token is a small lexical element. For example, in C, operators such as '+', '--', '->', and keywords such as for and while are individual tokens. Identifiers, numbers and strings are also considered as tokens with attributes. For example, "123" is a numerical token with the attribute/value 123. A lexical analyzer for C and C++ is quite complicated, because it usually also implements the preprocessor, so it has to take care of source file inclusion (#include), macro expansions (#define) and conditional compilation (#if). All this is hidden from the remaining parts of the compiler, and the CodeWarrior lexical analyzer transforms a source file into a uniform stream of tokens.

The next phase is the Syntax Analyzer or Parser. Most computer languages (including C, C++, Pascal and Java) have a grammar which is a set of rules that describes which token sequences form a legal program. The parser makes sure that the stream of tokens conforms to these rules. For example, the rule for a while statement is:

<iteration-statement>: while ( <expression> ) <statement>

<iteration-statement> is the name of this grammar rule. while, '(' and ')' are tokens; <expression> and <statement> are called non-terminal tokens, which means they have to be replaced by other tokens or rules. The parser transforms the stream of individual tokens into another data structure (usually a syntax tree) that is used by the remaining phases.

The next phase is the Semantic Analyzer. The parser makes sure that a program conforms to the grammar, but it doesn’t catch any other types of error. For example, “1=2;” is a syntactically correct C assignment expression statement that is semantically incorrect, because you cannot assign 2 to 1. So this phase makes sure that types in a program match, that all identifiers (or variables) have been defined, and that operand types in expressions match.

Now we have a legal program and all we have to do is generate code from it. One could generate code directly from a syntax tree, but usually a compiler generates an intermediate code representation. For example, CodeWarrior uses a tree-based intermediate code (IR tree) that is very close to a syntax tree but has a lot of additional information about types. In fact, the CodeWarrior compiler folds the syntactic, semantic and intermediate code generation together, so basically the parser also checks the semantics and it generates an IR tree. This really speeds up the whole process. This IR tree is really the key to our compiler technology. All code generation is based on this tree. So this makes our front-ends (C/C++/Pascal) and back-ends (68K, MIPS, PPC, x86) interchangeable.

This IR tree is then passed to the individual back-ends where it is used to generate the actual 68K, PPC, MIPS, or x86 code. The back-ends are all pretty different, but they all transform the IR tree into machine instruction sequences, allocate memory and registers for variables, and generate an object file. All back-ends also do some machine-level optimizations (peephole optimizations) that replace instructions with better ones or remove redundant instructions.

We have an IR-level optimizer that removes redundant parts from this tree and does some other basic things to optimize branches. We also have a high-level IR optimizer that is currently used in the x86 compiler, but this optimizer will eventually also be used in the MIPS or 68K compilers. The PPC back-end is a little special because most of its optimization is done in the back-end. This makes sense, because the PPC’s RISC instructions are very simple, so you can do a lot of high-level optimizations (such as loop optimizations and common subexpressions) with more fine control on the actual machine level and get better results. This would be very hard to do for a CISC processor like the 68K with all its complex instructions and addressing modes.

Dave: With that in mind, what did your original development environment look like, from a technical perspective?

Andreas: It had pretty much all the basic functionality you need to write programs in C. It had a project window, a multi-window text editor, find and replace, and some simple Preference dialogs. It even had some nice little features such as function popups and multiple access paths, but a lot of major features such as multi-language support, plug-in compiler support, collapsible project views, syntax coloring, split-pane editing, multiple-pane Preference dialogs, a tool bar, and tool-server support, have been added to it since then.

Dave: How would you compare your original compiler architecture with the current CodeWarrior architecture?

Andreas: The original compiler and linker didn’t support a plug-in interface, and everything had to be linked into the IDE. I always had multiple back-end support in mind, and I was always using the IR tree. However, when John McEnerney started writing the PPC back-end, I had to clean up the front-end/back-end interface, and we had to change a few other minor things. There were also some changes in the 68K back-end, to support some Pascal-specific features such as sets and nested local variables.

There have been a lot of changes in the front-end. The original compiler had some basic support for C++ classes and function overloading, so a lot of stuff had to be added since then. I also had to change quite a few things to support multiple platforms (Mac OS, x86, MIPS). Recently, I had to make some additions to the IR to support zero runtime overhead exception handling. This also required changes in the back-ends.

Dave: What special work do you have to do in the front-end to enable multiple platform support? For example, what did you have to do to CodeWarrior to make sure it would support x86, MIPS, and 680x0 code generation?

Andreas: There are a number of areas where a front-end needs to be aware of back-end requirements. For example a C struct’s member layout is done in the front-end, so you have to be aware of the data-member alignment of the target architecture. Another problem has to do with integral and floating-point type differences. For example, a long double is an 80-bit type on 68K but a 64-bit type on the PPC. In the same vein, the x86 uses a different byte ordering (little-endian) than the 68K (big-endian). Most of the functions that deal with these issues have been isolated into target-specific front-end files.

There are also some language-specific issues. For example, both Apple and Microsoft have their own C and C++ language extensions, and the front-end needs to support all of them. One of the biggest problems is C++. There are many very powerful features in C++ like multiple inheritance, polymorphism, exception handling and runtime type identification. The ANSI C++ Standard defines how all these features work, but it does not define how they should be implemented, so every compiler vendor has their own implementation. For example, there are many ways to implement virtual function calls or to allocate base classes in a derived class hierarchy. We always had our own implementation for this, but now, with the x86 compiler, we also have to conform to Microsoft’s standard class layout. We’re still targeting full compatibility, and this requires more work in the front-end as we go forward.

Dave: Beginning with CW8, CodeWarrior offered support for zero runtime overhead. First, what is zero runtime overhead? Second, what advantages does it offer?

Andreas: C++ exception handling requires that all local class variables that have been constructed between a try and a throw be destroyed before an exception is handled in a catch block. This process is called stack unwinding. Our first exception implementation was based on a pseudo-setjmp/longjmp, and a linked list of all active local stack objects needing destruction. This was relatively easy to implement in the front-end, and it required no changes in any of the back-ends. So we were able to support exceptions in all our back-ends at the same time.

However, this implementation requires some runtime overhead. For example, each local class object that needs destruction has to be registered when it is constructed and unregistered when it is destroyed. Also, the setjmp call that was required for every try block was very expensive on the PPC, because it had to save all processor registers in a local buffer, and this implementation had to modify a global variable, so it did not work very well with threads.

Our zero runtime overhead implementation is no longer using setjmp or a linked list. Instead, the stack unwinding process is done by examining the processor’s stack and using a pretty complex exception table that tells the exception handler how to locate local variables and how to destroy them. So there is no runtime overhead required for saving registers or registering local objects. Throwing an exception is actually a little bit slower because the stack unwinder is a lot more complicated, but usually exceptions are really exceptional so your application runs much faster. Another advantage is that this exception model doesn’t have to modify any global variables, so it is really thread-safe.

The only disadvantage is that the stack unwinder can only unwind functions that do have an entry in the exception table. The current Mac OS doesn’t have any exception tables, so you can no longer throw an exception in an OS callback function and catch it in your main program, which was possible (but not recommended) in the previous implementation.

Dave: ANSI C and ANSI C++ are two different languages, yet there is a single front-end to handle both. How does this work?

Andreas: ANSI C++ was derived from ANSI C, and most of its new features are really add-ons. Almost all ANSI C features are also supported in C++, so I was able to support both languages from the same front-end by disabling C++ features depending on the state of a global variable. There are some syntactic and semantic differences, but I was able to code around those with some “if-else” statements. This really makes adding features or fixing bugs a lot easier, because everything will only have to be done once. What’s also really interesting is that our compiler architecture allows us to treat both C and C++ with the same front-end, making the transition from C to C++ much easier because it’s the same compiler. Just flip a switch and write your code.

Dave: When you first started on your compiler, work on a C++ standard had just begun (with the publication of the Annotated C++ Reference Manual, or ARM). How has this process evolved?

Andreas: The ARM was the only real useful C++ language reference when I started. This book was used as the base document for the ANSI C++ standard. It has a lot of gray areas, and the template chapter is really vague, but it has some useful sections that explain how certain C++ features like virtual functions can be implemented. Unfortunately, those sections have been removed from the standard.

The current ANSI C++ draft is the size of a phone book. Many features (namespaces, RTTI, bool/true/false, a complete C++ library) have been added to the original definition, and there are three or four revisions every year. It is very hard to keep track of all these changes, and I think it will take another two years until there will be a final ANSI C++ standard.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

calibre 2.17 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
OmniGraffle Pro 6.1.2 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniGraffle 6.1.2 - Create diagrams, flo...
OmniGraffle helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use Graffle to... Read more
RoboForm 2.0.2 - Password manager; syncs...
RoboForm is a password manager that offers one-click login, mobile syncing, easy form filling, and reliable security. Password Manager. RoboForm remembers your passwords so you don't have to! Just... Read more
Apple MainStage 3.1 - Live performance t...
Love the sound you got on your recording? MainStage 3 makes it easy to bring all the same instruments and effects to the stage. Everything from the Sound Library and Smart Controls you're familiar... Read more
Freeway Pro 7.0.2 - Drag-and-drop Web de...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With its user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more
A Better Finder Rename 9.44 - File, phot...
A Better Finder Rename is the most complete renaming solution available on the market today. That's why, since 1996, tens of thousands of hobbyists, professionals and businesses depend on A Better... Read more
Stacks 2.6.9 - New way to create pages i...
Stacks is a new way to create pages in RapidWeaver. It's a plugin designed to combine drag-and-drop simplicity with the power of fluid layout. Features: Fluid Layout: Stacks lets you build pages... Read more
Sid Meier's Civilization: Beyond Ea...
Sid Meier's Civilization: Beyond Earth is a new science-fiction-themed entry into the award-winning Civilization series. Set in the future, global events have destabilized the world leading to a... Read more
Logic Pro X 10.1 - Music creation and au...
Apple Logic Pro X is the most advanced version of Logic ever. Sophisticated new tools for professional songwriting, editing, and mixing are built around a modern interface that's designed to get... Read more

Choice Provisions is Set to Launch Destr...
Choice Provisions is Set to Launch Destructamundo on iOS This Month Posted by Tre Lawrence on January 23rd, 2015 [ permalink ] Choice Provisions – home stable to | Read more »
King of Thieves – An Interview With Zept...
Ahead of the release of ZeptoLab’s King of Thieves, we were able to ask ZeptoLab’s co-founder, Semyon Voinov, a few questions about the inspiration behind the game and what that means for the Cut the Rope franchise. | Read more »
Handle Review
Handle Review By Jennifer Allen on January 23rd, 2015 Our Rating: :: SPEEDY ORGANIZINGUniversal App - Designed for iPhone and iPad Handle is a very convenient way of juggling your emails, To Do list, and Calendar all through one... | Read more »
The New Disney Inquizitive App Offers a...
The New Disney Inquizitive App Offers a Place for Fans to Take Disney Quizzes Posted by Tre Lawrence on January 23rd, 2015 [ permalink ] | Read more »
Hands-On With Cut the Rope Developer Zep...
Marking quite a departure from ZeptoLab’s past successes, namely the Cut The Rope series, King of Thieves is shaping up to be quite promising. Due for release in February, we were lucky enough to have some time with a preview build to see exactly... | Read more »
Fast Fishing Review
Fast Fishing Review By Jennifer Allen on January 23rd, 2015 Our Rating: :: LIVES UP TO ITS NAMEUniversal App - Designed for iPhone and iPad Fishing is far from relaxing in Fast Fishing, but it is fun.   | Read more »
The LEGO Movie Video Game is Available N...
The LEGO Movie Video Game is Available Now for iOS Posted by Ellis Spice on January 23rd, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Satellina Review
Satellina Review By Jennifer Allen on January 23rd, 2015 Our Rating: :: TWITCHY BUT TACTICALUniversal App - Designed for iPhone and iPad Satellina requires quick thinking and twitchy fingers, and it’s pretty fun.   | Read more »
Tail Drift, the Crazy 360 Degree Flyer,...
Tail Drift, the Crazy 360 Degree Flyer, Has Gone Free-to-Play in a New Update Posted by Jessica Fisher on January 22nd, 2015 [ permalink ] | Read more »
PureSkate 2 Review
PureSkate 2 Review By Tre Lawrence on January 22nd, 2015 Our Rating: :: ALMOST ALL AIRUniversal App - Designed for iPhone and iPad PureSkate 2 lets one’s fingers do the skateboarding.   | Read more »

Price Scanner via MacPrices.net

College Student Deals are back, additional $5...
Take an additional $50 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through April 11, 2015. Anyone with a valid .EDU email address can take advantage... Read more
iPhone 6 and 6 Plus GIve Apple Half Of US Mob...
Chicago-based Consumer Intelligence Research Partners, LLC (CIRP) have released analysis of the results of its research on mobile phone manufacturers for the calendar quarter that ended December 31,... Read more
Save $100 on MacBook Airs with 256GB of stora...
B&H Photo has 256GB MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.4GHz/256GB MacBook Air: $999 $100 off MSRP - 13″ 1.4GHz/256GB MacBook... Read more
21-inch 2.7GHz iMac on sale for $1179, save $...
B&H Photo has the 21″ 2.7GHz iMac on sale for $1179 including free shipping plus NY sales tax only. Their price is $120 off MSRP, and it’s the lowest price available for this model from any... Read more
iPhone Usage Rates by State Correlate With Ed...
Chitika Insights notes that despite iPhones being the largest source of smartphone Internet traffic in North America, their latest study finds a relatively high degree of variation of iPhone usage... Read more
ProGearX Extendable Pole “Pov/Selfie Stick” M...
There’s something inescapably narcissistic about the concept of selfies as they’ve developed as a smartphone-driven social (particularly social media) phenomenon that rubs me the wrong way. However,... Read more
iPad Air 2 on sale for up to $100 off MSRP, 2...
 Best Buy has iPad Air 2s on sale for up to $100 off MSRP on their online store for the next two days. Choose free shipping or free local store pickup (if available). Sale prices available for online... Read more
Roundup of Apple refurbished MacBook Pros and...
The Apple Store has Apple Certified Refurbished 2014 MacBook Pros and MacBook Airs available for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Sale! 13-inch 2.8GHz Retina MacBook Pro for $...
 B&H Photo has the 13″ 2.8GHz Retina MacBook Pro on sale for $1599 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available for this model... Read more
Next OS X/iOS Version Upgrades Should Concent...
On stage at Apple’s World Wide Developers’ Conference in June 2009, Bertrand Serlet, the company’s Senior Vice President of Software Engineering at the time, announced that the forthcoming OS X... Read more

Jobs Board

Business Development Manager - *Apple* Pay...
**Job Summary** Apple Pay is seeking an experienced business development manager to support the identification, recruitment, negotiation and ongoing management of Read more
*Apple* Solutions Consultant (ASC)- Retail S...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Lead Operator, GSOC - Apple (United...
**Job Summary** Apple is seeking an exceptional, customer service oriented and experienced persons to fulfill the role of Apple Lead Operator (ALO) as part of the Read more
Order Support Supervisor- *Apple* Online Sto...
**Job Summary** The Apple Online Store (AOS) Order Administration team is looking for an Order Support Supervisor to manage and lead a team of Specialists through the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.