[MLton-devel] bug fix for MLton 20030312 runtime

[Stephen Weeks]

Hi Alain.  I just found and fixed a bug in the MLton runtime that
could cause a segfault.  The bug was introduced on Aug 7, 2002, and
was in both the 20020923 and 20030312 releases.  The bug would happen
if the heap was allocated at a very low (<10M) address, which actually
happened on a user's Cygwin system.

Here is patch to gc.c that fixes the bug.  I recommend applying the
patch before shipping anything compiled with 20030312.  Let us know if
there is a problem.

--------------------------------------------------------------------------------
diff -u -r1.130 -r1.131
--- gc.c	2 May 2003 23:47:49 -0000	1.130
+++ gc.c	9 May 2003 18:21:45 -0000	1.131
@@ -2635,19 +2635,25 @@
 	/* Allocate a space of the desired size. */
 	if (heapCreate (s, &h2, desired, minSize)) {
 		pointer from;
+		uint remaining;
 		pointer to;
 
 		from = old + size;
 		to = h2.start + size;
+		remaining = size;
 copy:			
-		from -= COPY_CHUNK_SIZE;
-		to -= COPY_CHUNK_SIZE;
-		if (from > old) {
+		assert (remaining == from - old 
+				and from >= old and to >= h2.start);
+		if (remaining < COPY_CHUNK_SIZE) {
+			copy (old, h2.start, remaining);
+		} else {
+			remaining -= COPY_CHUNK_SIZE;
+			from -= COPY_CHUNK_SIZE;
+			to -= COPY_CHUNK_SIZE;
 			copy (from, to, COPY_CHUNK_SIZE);
-			heapShrink (s, h, from - old);
+			heapShrink (s, h, remaining);
 			goto copy;
 		}
-		copy (old, h2.start, from + COPY_CHUNK_SIZE - old);
 		heapRelease (s, h);
 		*h = h2;
 	} else {


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