From bb4edc8a6fa612d67bd67af5918baf269acf1e06 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 | 50 +++++++++++++++++++++++++++++++------
 1 file changed, 42 insertions(+), 8 deletions(-)

diff --git a/erts/emulator/beam/erl_gc.c b/erts/emulator/beam/erl_gc.c
index c821ff9f3a42..ce57697db103 100644
--- a/erts/emulator/beam/erl_gc.c
+++ b/erts/emulator/beam/erl_gc.c
@@ -525,12 +525,27 @@ delay_garbage_collection(Process *p, int need, int fcalls)
     ssz = orig_hend - orig_stop;
     hsz = ssz + need + ERTS_DELAY_GC_EXTRA_FREE + S_RESERVED;
 
-    hfrag = new_message_buffer(hsz);
+    hfrag = new_message_buffer(hsz + 1);
 
     copy_erlang_stack(p, &hfrag->mem[0], hsz);
 
     p->heap = p->htop = &hfrag->mem[0];
-    p->hend = hend = &hfrag->mem[hsz];
+    hend = &hfrag->mem[hsz];
+
+    /* Save the original high water mark at the end of the current
+     * heap to make it possible to do a minor GC later. */
+    if (p->abandoned_heap) {
+        *hend = (Eterm) (p->hend[0]);
+    } else {
+        *hend = (Eterm) p->high_water;
+    }
+
+    p->hend = hend;
+
+    /* Keep the high water mark pointing into the current heap to ensure
+     * that the test for the safe range in the update_record_in_place (JIT)
+     * stays honest. */
+    p->high_water = p->heap;
 
     if (p->abandoned_heap) {
 	/*
@@ -559,11 +574,6 @@ delay_garbage_collection(Process *p, int need, int fcalls)
 	}
 	p->abandoned_heap = orig_heap;
         erts_adjust_memory_break(p, orig_htop - p->high_water);
-
-        /* 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. */
-        p->high_water = (Eterm *) (Uint) -1;
     }
 
 #ifdef CHECK_FOR_HOLES
@@ -1383,12 +1393,36 @@ minor_collection(Process* p, ErlHeapFragment *live_hf_end,
 		 Uint ygen_usage, Uint *recl)
 {
     Eterm *mature = p->abandoned_heap ? p->abandoned_heap : p->heap;
-    Uint mature_size = p->high_water - mature;
+    Eterm *high_water;
+    Uint mature_size;
     Uint size_before = ygen_usage;
 #ifdef DEBUG
     Uint debug_tmp = 0;
 #endif
 
+    if (p->abandoned_heap) {
+        /* See delay_garbage_collection(). */
+        high_water = (Eterm *)(p->hend[0]);
+    } else {
+        high_water = p->high_water;
+    }
+
+#ifdef DEBUG
+    if (p->abandoned_heap) {
+        ASSERT(p->abandoned_heap <= high_water);
+        ASSERT(high_water - p->abandoned_heap <= size_before);
+
+        /* The high water pointer must be aligned to a word boundary. */
+#ifdef ARCH_64
+        ASSERT((((Uint) high_water) & 0x07) == 0);
+#else
+        ASSERT((((Uint) high_water) & 0x03) == 0);
+#endif
+    }
+#endif
+
+    mature_size = high_water - mature;
+
     need += S_RESERVED;
 
     /*