We found it is much easier to handle exceptions without risking stack leaks using a 3-address register machine. Code generation is also simplified in a register machine compared to a stack machine (at least in the case of a VM designed specifically to run SQL statement).
The VM used by SQLite is different from the VM used by things like Javascript or Python in that SQL is not a general-purpose programming language. Most of the opcodes in the SQLite VM are heavy-weight concepts such as "insert a key/value pair into a b-tree" or "decode the N-th column of a table row". These opcodes are implemented with hundreds or thousands of lines of C code. The overhead of instruction dispatch is insignificant in comparison. And so there isn't much of a performance advantage one way or the other between a stack machine and a register machine in SQLite. The choice of a register machine is for maintainability, reliability, and testability.
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Thanks for the insight. Do you know the reasons why they switched from using stack to registers in their VM implementation?
We found it is much easier to handle exceptions without risking stack leaks using a 3-address register machine. Code generation is also simplified in a register machine compared to a stack machine (at least in the case of a VM designed specifically to run SQL statement).
The VM used by SQLite is different from the VM used by things like Javascript or Python in that SQL is not a general-purpose programming language. Most of the opcodes in the SQLite VM are heavy-weight concepts such as "insert a key/value pair into a b-tree" or "decode the N-th column of a table row". These opcodes are implemented with hundreds or thousands of lines of C code. The overhead of instruction dispatch is insignificant in comparison. And so there isn't much of a performance advantage one way or the other between a stack machine and a register machine in SQLite. The choice of a register machine is for maintainability, reliability, and testability.
I do not know, but if I had to guess, it is for performance.
I am no expert in this field, but AFAIK http://static.usenix.org/events/vee05/full_papers/p153-yunhe... (discussed on http://news.ycombinator.com/item?id=704576) has not really been refuted.
I base that on the fact that both Dalvik (http://davidehringer.com/software/android/The_Dalvik_Virtual...) and Lua 5.0 (http://www.lua.org/doc/jucs05.pdf) use register-based VMs.
The Parrot VM is also register based: http://en.wikipedia.org/wiki/Parrot_virtual_machine