VM continuations store FP/SP by offset

* libguile/continuations.c (scm_i_continuation_to_frame):
* libguile/stacks.c (scm_make_stack):
* libguile/vm.c (scm_i_vm_cont_to_frame, scm_i_vm_capture_stack):
  (vm_return_to_continuation_inner)
  (struct vm_reinstate_partial_continuation_data):
  (vm_reinstate_partial_continuation_inner):
  (vm_reinstate_partial_continuation):
* libguile/vm.h (sstruct scm_vm_cont): Simplify VM continuations by
  recording the top FP by offset, not value + reloc.
* libguile/frames.c (frame_offset, scm_i_vm_frame_offset): Remove unused
  functions.
* libguile/frames.h (SCM_VALIDATE_VM_FRAME, scm_i_vm_frame_offset):
  Remove.
* libguile/control.c (reify_partial_continuation): Once we know the
  base_fp, relocate the dynamic stack.
* libguile/dynstack.h:
* libguile/dynstack.c (scm_dynstack_relocate_prompts): New function.
  (scm_dynstack_wind_prompt): Adapt to add new fp offset.

1 files changed, 9 insertions, 2 deletions

diff --git a/libguile/vm.h b/libguile/vm.h
index b26f7f406..a1cac391f 100644
--- a/libguile/vm.h
+++ b/libguile/vm.h
@@ -80,12 +80,19 @@ SCM_INTERNAL void scm_i_vm_free_stack (struct scm_vm *vp);
 #define SCM_F_VM_CONT_REWINDABLE 0x2
 
 struct scm_vm_cont {
-  union scm_vm_stack_element *fp;
+  /* IP of newest frame.  */
   scm_t_uint32 *ra;
+  /* Offset of FP of newest frame, relative to stack top.  */
+  scm_t_ptrdiff fp_offset;
+  /* Besides being the stack size, this is also the offset of the SP of
+     the newest frame.  */
   scm_t_ptrdiff stack_size;
+  /* Stack bottom, which also keeps saved stack alive for GC.  */
   union scm_vm_stack_element *stack_bottom;
-  scm_t_ptrdiff reloc;
+  /* Saved dynamic stack, with prompts relocated to record saved SP/FP
+     offsets from the stack top of this scm_vm_cont.  */
   scm_t_dynstack *dynstack;
+  /* See the continuation is partial and/or rewindable.  */
   scm_t_uint32 flags;
 };