fix for ocaml#9853

damiendoligez · Feb 12, 2021 · 2925829 · 2925829
1 parent 9f51fac
commit 2925829
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diff --git a/Changes b/Changes
@@ -39,6 +39,9 @@ Working version
 - #10136: Minor clean-ups in runtime/io.c and runtime/caml/io.h
   (Xavier Leroy, review by David Allsopp and Guillaume Munch-Maccagnoni)
 
+- #xxxxx: Fix a segfault in a corner case of compaction (reported in #9853)
+  (Damien Doligez, report by Sadiq Jaffer, review by xxxx)
+
 ### Code generation and optimizations:
 
 - #9876: do not cache the young_limit GC variable in a processor register.

diff --git a/runtime/compact.c b/runtime/compact.c
@@ -122,10 +122,7 @@ static char *compact_allocate (mlsize_t size)
 {
   char *chunk, *adr;
 
-  while (Chunk_size (compact_fl) - Chunk_alloc (compact_fl) <= Bhsize_wosize (3)
-         && Chunk_size (Chunk_next (compact_fl))
-            - Chunk_alloc (Chunk_next (compact_fl))
-            <= Bhsize_wosize (3)){
+  while (Chunk_size(compact_fl) - Chunk_alloc(compact_fl) <= Bhsize_wosize(0)){
     compact_fl = Chunk_next (compact_fl);
   }
   chunk = compact_fl;

diff --git a/testsuite/tests/regression/pr9853/compaction_corner_case.ml b/testsuite/tests/regression/pr9853/compaction_corner_case.ml
@@ -0,0 +1,10 @@
+(* TEST *)
+
+(* Compaction crash when there is only one heap chunk and it is fully used. *)
+let c = ref []
+
+let () =
+  for i = 0 to 25000 do
+    c := 0 :: !c;
+    Gc.compact ()
+  done