As described earlier (see Stack Layout), Guile’s calling convention is that arguments are passed and values returned on the stack.
For calls, both in tail position and in non-tail position, we require
that the procedure and the arguments already be shuffled into place
before the call instruction. “Into place” for a tail call means that
the procedure should be in slot 0, relative to the
fp, and the
arguments should follow. For a non-tail call, if the procedure is in
fp-relative slot n, the arguments should follow from slot
n+1, and there should be two free slots at n-1 and n-2
in which to save the
Returning values is similar. Multiple-value returns should have values
already shuffled down to start from
fp-relative slot 1 before
return-values. We start from slot 1 instead of slot 0
to make tail calls to
In both calls and returns, the
sp is used to indicate to the
callee or caller the number of arguments or return values, respectively.
After receiving return values, it is the caller’s responsibility to
restore the frame by resetting the
sp to its former value.
Call a procedure. proc is the local corresponding to a procedure. The two values below proc will be overwritten by the saved call frame data. The new frame will have space for nlocals locals: one for the procedure, and the rest for the arguments which should already have been pushed on.
When the call returns, execution proceeds with the next instruction.
There may be any number of values on the return stack; the precise
number can be had by subtracting the address of proc from the
Call a procedure in the same compilation unit.
This instruction is just like
call, except that instead of
dereferencing proc to find the call target, the call target is
known to be at label, a signed 32-bit offset in 32-bit units from
ip. Since proc is not dereferenced, it may be
some other representation of the closure.
Tail-call a procedure. Requires that the procedure and all of the arguments have already been shuffled into position. Will reset the frame to nlocals.
Tail-call a known procedure. As
call is to
tail-call is to
Tail-call a procedure. The procedure should already be set to slot 0.
The rest of the args are taken from the frame, starting at from,
shuffled down to start at slot 0. This is part of the implementation of
Receive a single return value from a call whose procedure was in proc, asserting that the call actually returned at least one value. Afterwards, resets the frame to nlocals locals.
Receive a return of multiple values from a call whose procedure was in
proc. If fewer than nvalues values were returned, signal an
error. Unless allow-extra? is true, require that the number of
return values equals nvalues exactly. After
has run, the values can be copied down via
mov, or used in place.
Return a number of values from a call frame. This opcode corresponds to
an application of
values in tail position. As with tail calls,
we expect that the values have already been shuffled down to a
contiguous array starting at slot 1. If nlocals is nonzero, reset
the frame to hold that number of locals. Note that a frame reset to 1
local returns 0 values.
Capture the current continuation, and tail-apply the procedure in local
slot 1 to it. This instruction is part of the implementation of
call/cc, and is not generated by the compiler.