From 53d55f252204f8537ed5014e1896328df5652049 Mon Sep 17 00:00:00 2001
From: Xavier Leroy <xavier.leroy@college-de-france.fr>
Date: Wed, 3 Jun 2020 16:38:02 +0200
Subject: [PATCH] Revised linking of trap frames in bytecode interpreter stack

Rather than storing a pointer to the previous frame in the Trap_link
field of the current frame, store the distance (pointer difference)
between the current frame and the previous frame, tagged as an OCaml
integer.

Using a tagged integer instead of a raw pointer means fever problems
later with strict no-naked-pointer support.

Using a distance rather than an absolute address simplifies
the code that resizes the stack.
---
 runtime/backtrace_byt.c | 2 +-
 runtime/caml/stacks.h   | 2 +-
 runtime/interp.c        | 6 +++---
 runtime/stacks.c        | 3 ---
 4 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/runtime/backtrace_byt.c b/runtime/backtrace_byt.c
index 28fe44c75..2641daedd 100644
--- a/runtime/backtrace_byt.c
+++ b/runtime/backtrace_byt.c
@@ -304,7 +304,7 @@ code_t caml_next_frame_pointer(value ** sp, value ** trsp)
     if (Is_long(*spv)) continue;
     p = (code_t*) spv;
     if(&Trap_pc(*trsp) == p) {
-      *trsp = Trap_link(*trsp);
+      *trsp = *trsp + Long_val(Trap_link_offset(*trsp));
       continue;
     }
 
diff --git a/runtime/caml/stacks.h b/runtime/caml/stacks.h
index 8cbb02a83..d309141f9 100644
--- a/runtime/caml/stacks.h
+++ b/runtime/caml/stacks.h
@@ -33,7 +33,7 @@
 #define caml_trap_barrier (Caml_state_field(trap_barrier))
 
 #define Trap_pc(tp) (((code_t *)(tp))[0])
-#define Trap_link(tp) (((value **)(tp))[1])
+#define Trap_link_offset(tp) (((value *)(tp))[1])
 
 void caml_init_stack (uintnat init_max_size);
 void caml_realloc_stack (asize_t required_size);
diff --git a/runtime/interp.c b/runtime/interp.c
index 146c8346e..e5c110812 100644
--- a/runtime/interp.c
+++ b/runtime/interp.c
@@ -850,7 +850,7 @@ value caml_interprete(code_t prog, asize_t prog_size)
     Instruct(PUSHTRAP):
       sp -= 4;
       Trap_pc(sp) = pc + *pc;
-      Trap_link(sp) = Caml_state->trapsp;
+      Trap_link_offset(sp) = Val_long(Caml_state->trapsp - sp);
       sp[2] = env;
       sp[3] = Val_long(extra_args);
       Caml_state->trapsp = sp;
@@ -865,7 +865,7 @@ value caml_interprete(code_t prog, asize_t prog_size)
         pc--; /* restart the POPTRAP after processing the signal */
         goto process_actions;
       }
-      Caml_state->trapsp = Trap_link(sp);
+      Caml_state->trapsp = sp + Long_val(Trap_link_offset(sp));
       sp += 4;
       Next;
 
@@ -898,7 +898,7 @@ value caml_interprete(code_t prog, asize_t prog_size)
       }
       sp = Caml_state->trapsp;
       pc = Trap_pc(sp);
-      Caml_state->trapsp = Trap_link(sp);
+      Caml_state->trapsp = sp + Long_val(Trap_link_offset(sp));
       env = sp[2];
       extra_args = Long_val(sp[3]);
       sp += 4;
diff --git a/runtime/stacks.c b/runtime/stacks.c
index 2e3be6a0f..a1409b2ab 100644
--- a/runtime/stacks.c
+++ b/runtime/stacks.c
@@ -47,7 +47,6 @@ void caml_realloc_stack(asize_t required_space)
 {
   asize_t size;
   value * new_low, * new_high, * new_sp;
-  value * p;
 
   CAMLassert(Caml_state->extern_sp >= Caml_state->stack_low);
   size = Caml_state->stack_high - Caml_state->stack_low;
@@ -72,8 +71,6 @@ void caml_realloc_stack(asize_t required_space)
   caml_stat_free(Caml_state->stack_low);
   Caml_state->trapsp = (value *) shift(Caml_state->trapsp);
   Caml_state->trap_barrier = (value *) shift(Caml_state->trap_barrier);
-  for (p = Caml_state->trapsp; p < new_high; p = Trap_link(p))
-    Trap_link(p) = (value *) shift(Trap_link(p));
   Caml_state->stack_low = new_low;
   Caml_state->stack_high = new_high;
   Caml_state->stack_threshold =