From e447fb152bd2edd686aabf1e1a287a7d0b595731 Mon Sep 17 00:00:00 2001
From: Arseniy Alekseyev <aalekseyev@janestreet.com>
Date: Thu, 23 Jan 2020 16:49:05 +0000
Subject: [PATCH 1/2] Make `Weak.blit` and `Ephemeron.blit_key` faster

---
 Changes             |  6 ++++++
 runtime/caml/weak.h | 34 ++++++++++++++++++++++++----------
 runtime/weak.c      |  4 ++--
 3 files changed, 32 insertions(+), 12 deletions(-)

diff --git a/Changes b/Changes
index 7356759a7eb1..25ab964eaeaf 100644
--- a/Changes
+++ b/Changes
@@ -34,6 +34,12 @@ Working version
 - #9233: Restore the bytecode stack after an allocation.
   (Stephen Dolan, review by Gabriel Scherer and Jacques-Henri Jourdan)
 
+- #9259: Made `Ephemeron.blit_key` and `Weak.blit` faster. They are now
+  linear in the size of the range being copied instead of depending on the
+  total sizes of the ephemerons or weak arrays involved.
+  (Arseniy Alekseyev, design advice by Leo White, review by François Bobot
+  and Damien Doligez)
+
 ### Code generation and optimizations:
 
 - #8637, #8805: Record debug info for each allocation
diff --git a/runtime/caml/weak.h b/runtime/caml/weak.h
index a6259764390e..ee884f6314ae 100644
--- a/runtime/caml/weak.h
+++ b/runtime/caml/weak.h
@@ -161,16 +161,18 @@ extern value caml_ephe_none;
 
 /* In the header, in order to let major_gc.c
    and weak.c see the body of the function */
-static inline void caml_ephe_clean (value v){
+static inline void caml_ephe_clean_partial (value v,
+                                            mlsize_t offset_start,
+                                            mlsize_t offset_end) {
   value child;
   int release_data = 0;
-  mlsize_t size, i;
-  header_t hd;
+  mlsize_t i;
   CAMLassert(caml_gc_phase == Phase_clean);
+  CAMLassert(2 <= offset_start
+             && offset_start <= offset_end
+             && offset_end <= Wosize_hd (Hd_val(v)));
 
-  hd = Hd_val (v);
-  size = Wosize_hd (hd);
-  for (i = 2; i < size; i++){
+  for (i = offset_start; i < offset_end; i++){
     child = Field (v, i);
   ephemeron_again:
     if (child != caml_ephe_none
@@ -198,16 +200,28 @@ static inline void caml_ephe_clean (value v){
 
   child = Field (v, 1);
   if(child != caml_ephe_none){
-      if (release_data){
+    if (release_data){
         Field (v, 1) = caml_ephe_none;
       } else {
-        /* The mark phase must have marked it */
-        CAMLassert( !(Is_block (child) && Is_in_heap (child)
-                  && Is_white_val (child)) );
+        /* If we scanned all the keys and the data field remains filled,
+           then the mark phase must have marked it */
+        CAMLassert( !(offset_start == 2 && offset_end == Wosize_hd (Hd_val(v)) &&
+                      Is_block (child) && Is_in_heap (child)
+                      && Is_white_val (child)));
       }
   }
 }
 
+static inline void caml_ephe_clean (value v) {
+  mlsize_t size;
+  header_t hd;
+  hd = Hd_val (v);
+  size = Wosize_hd (hd);
+
+  caml_ephe_clean_partial(v, 2, size);
+}
+
+
 #endif /* CAML_INTERNALS */
 
 #ifdef __cplusplus
diff --git a/runtime/weak.c b/runtime/weak.c
index 9fda2166d231..492d38780705 100644
--- a/runtime/weak.c
+++ b/runtime/weak.c
@@ -530,8 +530,8 @@ CAMLexport void caml_ephemeron_blit_key(value ars, mlsize_t offset_s,
   offset_d += CAML_EPHE_FIRST_KEY;
 
   if (caml_gc_phase == Phase_clean){
-    caml_ephe_clean(ars);
-    caml_ephe_clean(ard);
+    caml_ephe_clean_partial(ars, offset_s, offset_s + length);
+    caml_ephe_clean_partial(ard, offset_d, offset_d + length);
   }
   if (offset_d < offset_s){
     for (i = 0; i < length; i++){

From 5903aa0f70628e1695a3ca47432c3ec66be92837 Mon Sep 17 00:00:00 2001
From: Arseniy Alekseyev <aalekseyev@janestreet.com>
Date: Fri, 24 Jan 2020 12:15:09 +0000
Subject: [PATCH 2/2] Small optimization: don't clean the keys about to be
 overwritten

---
 runtime/caml/weak.h | 4 ++--
 runtime/weak.c      | 6 +++++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/runtime/caml/weak.h b/runtime/caml/weak.h
index ee884f6314ae..007a17140c09 100644
--- a/runtime/caml/weak.h
+++ b/runtime/caml/weak.h
@@ -205,8 +205,8 @@ static inline void caml_ephe_clean_partial (value v,
       } else {
         /* If we scanned all the keys and the data field remains filled,
            then the mark phase must have marked it */
-        CAMLassert( !(offset_start == 2 && offset_end == Wosize_hd (Hd_val(v)) &&
-                      Is_block (child) && Is_in_heap (child)
+        CAMLassert( !(offset_start == 2 && offset_end == Wosize_hd (Hd_val(v))
+                      && Is_block (child) && Is_in_heap (child)
                       && Is_white_val (child)));
       }
   }
diff --git a/runtime/weak.c b/runtime/weak.c
index 492d38780705..062d0227e658 100644
--- a/runtime/weak.c
+++ b/runtime/weak.c
@@ -531,7 +531,11 @@ CAMLexport void caml_ephemeron_blit_key(value ars, mlsize_t offset_s,
 
   if (caml_gc_phase == Phase_clean){
     caml_ephe_clean_partial(ars, offset_s, offset_s + length);
-    caml_ephe_clean_partial(ard, offset_d, offset_d + length);
+    /* We don't need to clean the keys that are about to be overwritten,
+       except where cleaning them could result in releasing the data,
+       which can't happen if data is already released. */
+    if (Field (ard, CAML_EPHE_DATA_OFFSET) != caml_ephe_none)
+      caml_ephe_clean_partial(ard, offset_d, offset_d + length);
   }
   if (offset_d < offset_s){
     for (i = 0; i < length; i++){