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Superinstructions

Note: don't invest too much work in (static) superinstructions; a future version of Vmgen will support dynamic superinstructions (see Ian Piumarta and Fabio Riccardi, Optimizing Direct Threaded Code by Selective Inlining, PLDI'98), and static superinstructions have much less benefit in that context (preliminary results indicate only a factor 1.1 speedup).

Here is an example of a superinstruction definition:

lit_sub = lit sub

lit_sub is the name of the superinstruction, and lit and sub are its components. This superinstruction performs the same action as the sequence lit and sub. It is generated automatically by the VM code generation functions whenever that sequence occurs, so if you want to use this superinstruction, you just need to add this definition (and even that can be partially automatized, see VM profiler).

Vmgen requires that the component instructions are simple instructions defined before superinstructions using the components. Currently, Vmgen also requires that all the subsequences at the start of a superinstruction (prefixes) must be defined as superinstruction before the superinstruction. I.e., if you want to define a superinstruction

foo4 = load add sub mul

you first have to define load, add, sub and mul, plus

foo2 = load add
foo3 = load add sub

Here, sumof4 is the longest prefix of sumof5, and sumof3 is the longest prefix of sumof4.

Note that Vmgen assumes that only the code it generates accesses stack pointers, the instruction pointer, and various stack items, and it performs optimizations based on this assumption. Therefore, VM instructions where your C code changes the instruction pointer should only be used as last component; a VM instruction where your C code accesses a stack pointer should not be used as component at all. Vmgen does not check these restrictions, they just result in bugs in your interpreter.

The Vmgen flag include-skipped-insts influences superinstruction code generation. Currently there is no support in the peephole optimizer for both variations, so leave this flag alone for now.