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Minimum information needed for writing a code generator
If a compiler writer is faced with writing a back-end for an undocumented processor, what is the minimum amount of information that needs to be reverse engineered?
It is possible to implement a universal computer using a cpu that has a single instruction, subtract-and-branch-if-lessthan-or-equal-to-zero. This is all very well, but processors based on using a single instruction are a bit thin on the ground and the processor to hand is likely to support a larger number of simpler instructions.
A subtract-and-branch-if-lessthan-or-equal-to-zero instruction could be implemented on a register based machine using the appropriate sequence of load-from-memory, subtract-two-registers, store-register-to-memory and jump-if-subtract-lessthan-or-equal instructions. Information about other instructions, such as add and multiply, would be useful for code optimization. (The Turing machine model of computation is sufficiently far removed from how most programs and computers operate that it is not considered further.)
Are we done? In theory yes, in practice no. A couple pf practical problems have been glossed over; how do source literals (e.g., "Hello World") initially get written to storage, where does the storage used by the program come from and what is the file format of an executable?
Literals that are not created using an instruction (most processors have instructions for loading an integer constant into a register) are written to a part of the executable file that is read into storage by the loader on program startup. All well and good if we know enough about the format of an executable file to be able to correct generate this information and can get the operating system to put in the desired storage location. Otherwise we have to figure out some other solution.
If we know two storage locations containing values that differ by one a sequence of instructions could subtract one value from the other to eventually obtain any desired value. This bootstrap process would speed up as a wider range of know value/location pairs was built up.
How do we go about obtaining a chunk of storage? An executable file usually contains information on the initial amount of storage needed by a program when it is loaded. Calls to the heap manager are another way of obtaining storage. Again we need to know where to write the appropriate information in the executable file.
What minimum amount of storage might be expected to be available? A program executing within a stack/heap based memory model has a default amount of storage allocated for the stack (a minimum of 16k I believe under Mac OS X or iPhone OS). A program could treat the stack as its storage. Ideally what we need is the ability to access storage via an offset from the stack pointer, at worse we would have to adjust the stack pointer to the appropriate offset, pop the value into a register and then reset the stack pointer; storing would involve a push.
Having performed some calculation we probably want to communicate one or more values to the outside world. A call to a library routine, such as printf, needs information on the parameter passing conventions (e.g., which parameters get stored in which registers or have storage allocated for them {a function returning a structure type usually has the necessary storage allocated by the calling function with the address passed as an extra parameter}) and the location of any return value. If ABI information is not available a bit of lateral thinking might be needed to come up with an alternative output method.
I have not said anything about making use of signals and exception handling. These are hard enough to get right when documentation is available. The Turing machine theory folk usually skip over these real-world issues and I will join them for the time being.
Secret instruction sets about to make a come-back
Now it has happened Apple’s launch of a new processor, the A4, seems such an obvious thing to do. As the manufacturer of propriety products Apple wants to have complete control over the software that customers run on those products. Using a processor whose instruction set and electrical signals are not publicly available goes a very long way to ensuring that somebody else’s BIOS/OS/drivers/etc do not replace Apple’s or that the distributed software is not ‘usefully’ patched (Apple have yet to reveal their intentions on publishing instruction set details, this is an off-the-cuff prediction on my part).
Why are Apple funding the development of the LLVM C/C++ compiler? Because it enables them to write a back-end for the A4 without revealing the instruction set (using gcc for this purpose would require that the source be distributed, revealing the instruction set). So much for my prediction that Apple will stop funding LLVM.
The landscape of compute intensive cpus used to be populated with a wide variety of offerings from many manufacturers. The very high price of riding the crest of Moore’s law is one of the reasons that Intel’s x86 acquired such a huge chunk of this market; one by one processor companies decided not to make the investment needed to keep their products competitive. Now that the applicability of Moore’s law is drawing to an end the treadmill of continued processor performance upgrades is starting to fading away. Apple are not looking at a processor upgrade cycle, that they need to keep up with to be competitive, that stretches very far into the future.
Isn’t keeping an instruction set secret commercial suicide? The way to sell hardware is to make sure that lots of software runs on it and software developers want instruction set details (well ok, a small number do, but I’m sure lots of others get a warm fuzzy feeling knowing that the information is available should they need it). This is very much a last century view. The world is awash with an enormous variety of software, including lot of it very useful open source, and there is a lot more information available about the set of applications many customers want to use most of the time. Other than existing practice there is little reason why a manufacturer of a proprietary product, that is not a processor, needs to release instruction set details.
In the early 1980s I was at the technical launch of the Inmos Transputer and was told that the instruction set would not be published because the company did not want to be tied down to backwards compatibility. Perish the thought that customers would write assembler because the Inmos supplied compilers produced poor quality code or that a third party compiler would good enough to gain a significant market share. In marketing ability Inmos was the polar opposite of Apple. For a while HP were not very forthcoming on the instruction set behind their PA-RISC processor.
Will the A4 instruction set remain secret? On the whole, perhaps it can. The software based approaches to reverse engineering an instruction set require access to a compiler that generates it. For instance, changing a plus to a minus in one expression and looking for the small change in the generated executable; figuring out which status flags are set under which conditions is harder, but given time it can be done. Why would Apple release executable compilers if it wants to control the software that runs on the A4?
If the instruction set were cracked where would a developer go to obtain a compiler targeting the A4?
Given the FSF‘s history with Apple I would not be surprised if there was a fatwa against support for a proprietary instruction set in gcc. I suspect Apple would frown very heavily on such support ever being in the standard llvm distribution. I could see Apple’s lawyers sending letters to anybody making available a compiler that generated A4 code.
In the past manufacturers have tried to keep processor instruction sets secret and found that commercial reality required them to make this information freely available. Perhaps in the long term, like Moore’s law, the publication of detailed instruction set information may just be a passing phase.
Assessing my predictions for 2009
I have been rather remiss in revisiting the predictions I made for 2009 to see how they fared. Only two out of the six predictions were sufficiently precise to enable an assessment one year later; the other four talking more about trends. What of these two predictions?
The LLVM project will die. Ok, the project is still going and at the end of December the compiler could build itself but the build is not yet in a state to self host (i.e., llvm compiler creates an executable of itself from its own source and this executable can build an executable from source that is identical to itself {modulo date/time stamps etc}). Self hosting is a major event and on some of the projects I have worked on it has been a contractual payment milestone.
Is llvm competition for gcc? While there might not be much commercial competition as such (would Apple be providing funding to gcc if it were not involved in llvm?), I’m sure that developers working on both projects want their respective compiler to be the better one. According to some llvm benchmarks they compile code twice as fast as gcc. If this performance difference holds up across lots of source how might the gcc folk respond? Is gcc compiler time performance an issue that developers care about or is quality of generated code more important? For me the latter is more important and I have not been able to find a reliable, recent, performance comparison. I understand that almost all gcc funding is targeted at code generation related development, so if compile time is an issue gcc may be in a hole.
I still don’t see the logic behind Apple funding this project and continue to think that this funding will be withdrawn sometime, perhaps I was just a year off. I do wish the llvm developers well and look forward to them celebrating a self hosted compiler.
Static analysis will go mainstream. Ok, I was overly optimistic here. There do not seem to have been any casualties in 2009 among the commercial vendors selling static analysis tools and the growth of PHP has resulted in a number of companies selling tools that scan for source security vulnerabilities (.e.g, SQL injection attacks).
I was hoping that the availability of Treehydra would spark the development of static analysis plugins for gcc. Perhaps the neatness of the idea blinded me to what I already knew; many developers dislike the warnings generated by the -Wall option and therefore might be expected to dislike any related kind of warning. One important usability issue is that Treehydra operates on the abstract syntax tree representation used internally by gcc, GIMPLE. Learning about this representation requires a big investment before a plugin developer becomes productive.
One tool that did experience a lot of growth in 2009 was Coccinelle, at least judged by the traffic on its mailing list. The Coccinelle developers continue to improve it and are responsive the questions. Don’t be put off by the low version number (currently 0.2), it is much better than most tools with larger version numbers.
Why does Intel sell compilers?
Intel is a commercial company and the obvious answer to the question of why it sells compilers is: to make money. Does a company that makes billions of dollars in profits really have any interest in making a few million, I’m guessing, from selling compilers? Of course not, Intel’s interest in compilers is as a means of helping them sell more hardware.
How can a compiler be used to increase computer hardware sales? One possibility is improved performance and another is customer perception of improved performance. My company’s first product was a code optimizer and I was always surprised by the number of customers who bought the product without ever performing any before/after timing benchmarks; I learned that engineers are seduced by the image of performance and only a few are ever forced to get involved in measuring it (having been backed into a corner because what they currently have is not good enough).
Intel are not the only company selling x86 chips, AMD and VIA have their own Intel compatible x86 chips. Intel compatible? Doesn’t that mean that programs compiled using the Intel compiler will execute just as quickly on the equivalent chip sold by competitors? Technically the answer is no, but the performance differences are likely to be small in most cases. However, I’m sure there are many developers who have been seduced by Intel’s marketing machine into believing that they need to purchase x86 chips from Intel to make sure they receive this ‘worthwhile’ benefit.
Where do manufacturer performance differences, for the same sequence of instructions, come from? They are caused by the often substantial internal architectural difference between the processors sold by different manufacturers, also Intel and its competitors are continually introducing new processor architectures and processors from the same company will have differences performance characteristics. It is possible for an optimizer to make use of information on different processor characteristics to tune the machine code generated for a particular high-level language construct, with the developer selecting the desired optimization target processor via compiler switches.
Optimizing code for one particular processor architecture is a specialist market. But let’s not forget all those customers who will be seduced by the image of performance and ignore details such as their programs being executed on a wide variety of architectures.
The quality of a compiler’s runtime library can have a significant impact on a program’s performance. The x86 instruction set has grown over time and large performance gains can be had by a library function that adapts to the instructions available on the processor it is currently executing on. The CPUID instruction provides all of the necessary information.
As well as providing information on the kind of processor and its architectural features the CPUID instruction can return information about the claimed manufacturer of the chip (some manufacturers provide a mechanism that allows users to change the character sequence returned by this instruction).
The behavior of some of the functions in Intel’s runtime library depends on the
character sequence returned by the CPUID instruction, producing better performance for the sequence “GenuineIntel”. The US Federal Trade Commission have filed a complaint alleging that this is anti-competitive (more details) and requested that this manufacturer dependency be removed.
I think that removing this manufacturer dependency will have little impact on sales. Any Intel compiler user who is not targeting Intel chips and who is has a real interest in performance can patch the runtime library, the Supercomputer crowd will want to talk to the kind of sophisticated processor/compiler engineers that Intel makes available and for everybody else it is about the perception of performance. In fact Intel ought to agree to a ‘manufacturer free’ runtime library pronto before too many developers have their delusions shattered.
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