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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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.