From 5aa6d99982a9d6e2736f1eedb7fcdf6b9d05b6a0 Mon Sep 17 00:00:00 2001
From: Guillaume Melquiond
Date: Mon, 24 Aug 2020 15:11:16 +0200
Subject: Modify bytecode representation of closures to please OCaml's GC (fix
 #12636).

The second field of a closure can no longer be the value of the first free
variable (or another closure of a mutually recursive block) but must be an
offset to the first free variable.

This commit makes the bytecode compiler and interpreter agnostic to the
actual representation of closures. To do so, the execution environment
(variable coq_env) no longer points to the currently executed closure but
to the last one. This has the following consequences:

- OFFSETCLOSURE(n) now always loads the n-th closure of a recursive block
  (counted from last to first);
- ENVACC(n) now always loads the value of the n-th free variable.

These two changes make the bytecode compiler simpler, since it no
longer has to track the relative position of closures and free variables.
The last change makes the interpreter a bit slower, since it has to adjust
coq_env when executing GRABREC. Hopefully, cache locality will make the
overhead negligible.
---
 kernel/vmemitcodes.ml | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

(limited to 'kernel/vmemitcodes.ml')

diff --git a/kernel/vmemitcodes.ml b/kernel/vmemitcodes.ml
index 2dfc9a2941..f913cb906c 100644
--- a/kernel/vmemitcodes.ml
+++ b/kernel/vmemitcodes.ml
@@ -270,12 +270,12 @@ let emit_instr env = function
   | Kacc n ->
       if n < 8 then out env(opACC0 + n) else (out env opACC; out_int env n)
   | Kenvacc n ->
-      if n >= 1 && n <= 4
-      then out env(opENVACC1 + n - 1)
+      if n >= 0 && n <= 3
+      then out env(opENVACC0 + n)
       else (out env opENVACC; out_int env n)
   | Koffsetclosure ofs ->
-      if Int.equal ofs (-2) || Int.equal ofs 0 || Int.equal ofs 2
-      then out env (opOFFSETCLOSURE0 + ofs / 2)
+      if Int.equal ofs 0 || Int.equal ofs 1
+      then out env (opOFFSETCLOSURE0 + ofs)
       else (out env opOFFSETCLOSURE; out_int env ofs)
   | Kpush ->
       out env opPUSH
@@ -385,13 +385,13 @@ let rec emit env insns remaining = match insns with
       if n < 8 then out env(opPUSHACC0 + n) else (out env opPUSHACC; out_int env n);
       emit env c remaining
   | Kpush :: Kenvacc n :: c ->
-      if n >= 1 && n <= 4
-      then out env(opPUSHENVACC1 + n - 1)
+      if n >= 0 && n <= 3
+      then out env(opPUSHENVACC0 + n)
       else (out env opPUSHENVACC; out_int env n);
       emit env c remaining
   | Kpush :: Koffsetclosure ofs :: c ->
-      if Int.equal ofs (-2) || Int.equal ofs 0 || Int.equal ofs 2
-      then out env(opPUSHOFFSETCLOSURE0 + ofs / 2)
+      if Int.equal ofs 0 || Int.equal ofs 1
+      then out env(opPUSHOFFSETCLOSURE0 + ofs)
       else (out env opPUSHOFFSETCLOSURE; out_int env ofs);
       emit env c remaining
   | Kpush :: Kgetglobal id :: c ->
-- 
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