Speed up returns in JIT

This patch is a bit unfortunate, in the sense that it exposes some of
the JIT guts to the rest of the VM.  Code needs to treat "machine return
addresses" as valid if non-NULL (as before) and also not equal to a
tier-down trampoline.  This is because tier-down at a return needs the
old frame pointer to load the "virtual return address", and the way this
patch works is that it passes the vra in a well-known register.  It's a
custom calling convention for a certain kind of return.

* libguile/jit.h (scm_jit_return_to_interpreter_trampoline): New
  internal global.
* libguile/jit.c: (scm_jit_clear_mcode_return_addresses): Move here,
  from vm.c.  Instead of zeroing return addresses, set them to the
  return-to-interpreter trampoline.
* libguile/vm-engine.c (return-values): Don't enter mcode if the mra is
  scm_jit_return_to_interpreter_trampoline.
* libguile/vm.c (capture_continuation): Treat the tier-down trampoline
  as NULL.

1 files changed, 4 insertions, 1 deletions

diff --git a/libguile/jit.h b/libguile/jit.h
index 365be07e9..455f9c79c 100644
--- a/libguile/jit.h
+++ b/libguile/jit.h
@@ -1,7 +1,7 @@
 #ifndef SCM_JIT_H
 #define SCM_JIT_H
 
-/* Copyright 2018
+/* Copyright 2018-2019
      Free Software Foundation, Inc.
 
    This file is part of Guile.
@@ -62,6 +62,9 @@ SCM_INTERNAL void scm_jit_enter_mcode (scm_thread *thread,
                                        const uint8_t *mcode);
 SCM_INTERNAL void scm_jit_state_free (struct scm_jit_state *j);
 
+SCM_INTERNAL void *scm_jit_return_to_interpreter_trampoline;
+SCM_INTERNAL void scm_jit_clear_mcode_return_addresses (scm_thread *thread);
+
 SCM_INTERNAL void scm_init_jit (void);
 
 #endif  /* SCM_JIT_H */