https://git.ruby-lang.org/ruby.git/commit/?id=f7cf600c8b

From f7cf600c8ba483d389ea400071aa2aea2e5a57e0 Mon Sep 17 00:00:00 2001
From: Koichi Sasada <ko1@a...>
Date: Sun, 2 Aug 2020 03:23:16 +0900
Subject: fix mark bit operation.

To optimize the sweep phase, there is bit operation to set mark
bits for out-of-range bits in the last bit_t.
However, if there is no out-of-ragnge bits, it set all last bit_t
as mark bits and it braek the assumption (unmarked objects will
be swept).
GC_DEBUG=1 makes sizeof(RVALUE)=64 on my machine and this condition
happens.

It took me one Saturday to debug this.

diff --git a/gc.c b/gc.c
index d2d7b06..03649a2 100644
--- a/gc.c
+++ b/gc.c
@@ -4247,7 +4247,11 @@ gc_page_sweep(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_ https://github.com/ruby/ruby/blob/trunk/gc.c#L4247
 
     /* create guard : fill 1 out-of-range */
     bits[BITMAP_INDEX(p)] |= BITMAP_BIT(p)-1;
-    bits[BITMAP_INDEX(p) + sweep_page->total_slots / BITS_BITLENGTH] |= ~(((bits_t)1 << ((NUM_IN_PAGE(p) + sweep_page->total_slots) % BITS_BITLENGTH)) - 1);
+
+    int out_of_range_bits = (NUM_IN_PAGE(p) + sweep_page->total_slots) % BITS_BITLENGTH;
+    if (out_of_range_bits != 0) { // sizeof(RVALUE) == 64
+        bits[BITMAP_INDEX(p) + sweep_page->total_slots / BITS_BITLENGTH] |= ~(((bits_t)1 << out_of_range_bits) - 1);
+    }
 
     for (i=0; i < HEAP_PAGE_BITMAP_LIMIT; i++) {
 	bitset = ~bits[i];
-- 
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