Fix for unbounded memory growth problem reproduced with GCC 4.4.1

on 32 bit x86 Fedora. This happens because the lazy list variable
``data'' in the match_files function is optimized to a register,
but a stale value of that variable persists in the backing storage.

4 files changed, 35 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 96c99930..9871e1a4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,19 @@
 2010-01-21  Kaz Kylheku  <kkylheku@gmail.com>
 
+	Fix for unbounded memory growth problem reproduced with GCC 4.4.1
+	on 32 bit x86 Fedora. This happens because the lazy list variable
+	``data'' in the match_files function is optimized to a register,
+	but a stale value of that variable persists in the backing storage.
+
+	* gc.h (gc_hint): New macro.
+	(gc_hint_func): Declared.
+
+	* gc.c (gc_hint_func): New function.
+
+	* match.c (match_files): Use gc_hint on the data lazy list.
+
+2010-01-21  Kaz Kylheku  <kkylheku@gmail.com>
+
 	* match.c (match_files): Reduce scope, and bogus use of, dataline
 	variable.
 
diff --git a/gc.c b/gc.c
index 2a659e2b..284ca711 100644
--- a/gc.c
+++ b/gc.c
@@ -486,3 +486,20 @@ void unmark(void)
     }
   }
 }
+
+/*
+ * This function does nothing.
+ * gc_hint(x) just takes the address of local variable x
+ * and passes it to this function. This prevents the compiler
+ * from caching the value across function calls.
+ * This is needed for situations where
+ * - a compiler caches a variable in a register, but not entirely (the variable
+ *   has a backing memory location); and 
+ * - that location contains a stale old value of the variable, which cannot be
+ *   garbage-collected as a result; and
+ * - this causes a problem, like unbounded memory growth.
+ */
+void gc_hint_func(val *val)
+{
+  (void) val;
+}
diff --git a/gc.h b/gc.h
index 760f7f1f..d2fa038d 100644
--- a/gc.h
+++ b/gc.h
@@ -34,3 +34,5 @@ int gc_state(int);
 void gc_mark(val);
 int gc_is_reachable(val);
 void unmark(void);
+void gc_hint_func(val *);
+#define gc_hint(var) gc_hint_func(&var)
diff --git a/match.c b/match.c
index 04554cff..85e65cf0 100644
--- a/match.c
+++ b/match.c
@@ -874,6 +874,8 @@ static val match_files(val spec, val files,
   val data = nil;
   cnum data_lineno = 0;
 
+  gc_hint(data);
+
   if (listp(first_file_parsed)) {
     data = first_file_parsed;
     data_lineno = c_num(data_linenum);