From e7e458acb7915c239d8c9fc40b102c67dabea2f8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 27 Aug 2024 08:35:27 +0200
Subject: [PATCH] Fix performance regression caused by a bug fix

a1805974f864fe in #8539, a correction for an unsafe in-place update
of a record, had the side effect of causing a major (fullsweep)
garbage collection when GC has been temporarily disabled and then
again enabled. Since many BIFs, such as `term_to_binary/1` and
`iolist_to_binary/1`, may temporarily disable GC, this could lead to
noticeable performance regressions.

This commit ensures that it is again possible to do a minor collection
when GC is being enabled.
---
 erts/emulator/beam/erl_gc.c      | 31 ++++++++++++++++++++++++++++---
 erts/emulator/beam/erl_process.h |  5 +++++
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/erts/emulator/beam/erl_gc.c b/erts/emulator/beam/erl_gc.c
index c821ff9f3a42..fb044e2484cb 100644
--- a/erts/emulator/beam/erl_gc.c
+++ b/erts/emulator/beam/erl_gc.c
@@ -560,10 +560,15 @@ delay_garbage_collection(Process *p, int need, int fcalls)
 	p->abandoned_heap = orig_heap;
         erts_adjust_memory_break(p, orig_htop - p->high_water);
 
+#ifdef BEAMASM
         /* Point at the end of the address range to ensure that
          * test for the safe range in the new heap in the
-         * update_record_in_place instruction fails. */
+         * update_record_in_place instruction fails. Save the previous
+         * value of the high water mark to make it possible to make the
+         * next GC a minor one. */
+        p->abandoned_high_water = p->high_water;
         p->high_water = (Eterm *) (Uint) -1;
+#endif
     }
 
 #ifdef CHECK_FOR_HOLES
@@ -1382,8 +1387,8 @@ minor_collection(Process* p, ErlHeapFragment *live_hf_end,
 		 Uint need, Eterm* objv, int nobj,
 		 Uint ygen_usage, Uint *recl)
 {
-    Eterm *mature = p->abandoned_heap ? p->abandoned_heap : p->heap;
-    Uint mature_size = p->high_water - mature;
+    Eterm *mature;
+    Uint mature_size;
     Uint size_before = ygen_usage;
 #ifdef DEBUG
     Uint debug_tmp = 0;
@@ -1391,6 +1396,26 @@ minor_collection(Process* p, ErlHeapFragment *live_hf_end,
 
     need += S_RESERVED;
 
+#if defined(DEBUG) && defined(BEAMASM)
+    if (p->abandoned_heap) {
+        ASSERT(p->abandoned_heap <= p->abandoned_high_water);
+        ASSERT(p->abandoned_high_water != (Uint) -1);
+    }
+#endif
+
+    if (p->abandoned_heap) {
+#ifdef BEAMASM
+        Eterm *high_water = p->abandoned_high_water;
+#else
+        Eterm *high_water = p->high_water;
+#endif
+        mature = p->abandoned_heap;
+        mature_size = high_water - mature;
+    } else {
+        mature = p->heap;
+        mature_size = p->high_water - mature;
+    }
+
     /*
      * Check if we have gone past the max heap size limit
      */
diff --git a/erts/emulator/beam/erl_process.h b/erts/emulator/beam/erl_process.h
index b3eae36e7089..8e2f558bd389 100644
--- a/erts/emulator/beam/erl_process.h
+++ b/erts/emulator/beam/erl_process.h
@@ -1113,6 +1113,11 @@ struct process {
     Uint16 gen_gcs;             /* Number of (minor) generational GCs. */
     Uint16 max_gen_gcs;         /* Max minor gen GCs before fullsweep. */
     Eterm *high_water;
+
+#ifdef BEAMASM
+    Eterm *abandoned_high_water;
+#endif
+
     Eterm *old_hend;            /* Heap pointers for generational GC. */
     Eterm *old_htop;
     Eterm *old_heap;