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Though most applicable objects in Guile are procedures implemented in
bytecode, not all are. There are primitives, continuations, and other
procedure-like objects that have their own calling convention. Instead
of adding special cases to the call instruction, Guile wraps
these other applicable objects in VM trampoline procedures, then
provides special support for these objects in bytecode.
Trampoline procedures are typically generated by Guile at runtime, for
example in response to a call to scm_c_make_gsubr. As such, a
compiler probably shouldn’t emit code with these instructions. However,
it’s still interesting to know how these things work, so we document
these trampoline instructions here.
Call a subr, passing all locals in this frame as arguments, and storing the results on the stack, ready to be returned.
Call a foreign function. Fetch the cif and foreign pointer from cif-idx and ptr-idx closure slots of the callee. Arguments are taken from the stack, and results placed on the stack, ready to be returned.
Load a builtin stub by index into dst.