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A 1948 viewpoint on developer vs. computer time
For a long time now developer time has been a lot more expensive than computer time. The idea that developers should organize what they do, so as to maximize the efficiency of computer time rather than their own time, is considered to be an echo from a bygone age.
Until recently, I thought the transition from this bygone age, when computer time was considered more important than developer time, started in the late 1960s. Don’t ask me why I thought this, put it down to personal bias.
I was recently reading A Survey of Eniac Operations and Problems: 1946-1952, published in 1952, and what did I find:
“Early in 1948, R. F. Clippinger and some of his associates, in the course of coding the solution of …, were forced to adopt a different method of using the Eniac in order to fit their problem on the machine. …. The experience with this method (first discussed in reference 1), led J. von Neumann to suggest the use of a serial code for control of the Eniac. Such a code was devised and employed with the Eniac beginning in March 1948. Operation of the Eniac with this code was several times slower than either the original method of direct programming or the code for parallel operation. However, the resulting simplification of coding techniques and other advantages far outweighed this disadvantage.”
In other words, in 1948, the people using one of the few computers in the world, which clocked at 100KHz, considered developer time to be more important than computer time.
Register vs. stack based VMs
Traditionally the virtual machine architecture of choice has been the stack machine; benefits include simplicity of VM implementation, ease of writing a compiler back-end (most VMs are originally designed to host a single language) and code density (i.e., executables for stack architectures are invariably smaller than executables for register architectures).
For a stack architecture to be an effective solution, two conditions need to be met:
- The generated code has to ensure that the top of stack is kept in sync with where the next instruction expects it to be. For instance, on its return a function cannot leave stuff lying around on the stack like it can leave values in registers (whose contents can simply be overwritten).
- Instruction execution needs to be generally free of state, so an add-two-integers instruction should not have to consult some state variable to find out the size of integers being added. When the value of such state variables have to be saved and restored around function calls, they effectively become VM registers.
Cobol is one language where it makes more sense to use a register based VM. I wrote one and designed two machine code generators for the MicroFocus Cobol VM and always find it difficult to explain to people what a very different kind of beast it is compared to the VMs usually encountered.
Parrot, the VM designed as the target for compiled PERL, is register based. A choice driven, I suspect, by the difficulty of ensuring a consistent top-of-stack and perhaps the dynamic typing of the language.
On register based cpus with 64k of storage, the code density benefits of a stack based VM are usually sufficient to cancel out the storage overhead of the VM interpreter and support a more feature rich application (provided speed of execution is not crucial).
If storage capacity is not a significant issue and a VM has to be used, what are the runtime performance differences between a register and stack based VM? Answering this question requires compiling and executing the same set of applications for the two kinds of VM. Something that until 2001 nobody had done, or at least not published the results.
A comparison of the Java (stack based) VM with a register VM (The Case for Virtual Register Machines) found that while the stack based code was more compact, fewer instructions needed to be executed on the register based VM.
Most VM instructions are very simple and take relatively little time to execute. When hosted on a pipelined processor the main execution time overhead of a VM is the instruction dispatch (Optimizing Indirect Branch Prediction Accuracy in Virtual Machine Interpreters) and reducing the number of VM instructions executed, even if they are larger and more complicated, can produce a worthwhile performance improvement.
Google has chosen a register based VM for its Android platform. While licensing issues may have been a consideration, there are a number of technical advantages to this decision:
- A register VM is likely to have an intrinsic performance advantage over a stack VM when hosted on a pipelined processor.
- Byte code verification is likely to be faster on a register VM (i.e., faster startup times) because stack height integrity checks will be greatly simplified.
- A register VM will be more forgiving of incorrect code (in the VM, generated by the compiler, code corrupted during program transmission or storage attacked by malware) than a stack VM.
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