Introduce a configure flag --enable-system-heap

If this flag is set UG takes it memory through malloc/free instead of using
the custom heap data structures in low/heaps.c.

Using the system heap has several advantages:
 - there is no artificial upper bound to the amount of memory UG can use
 - valgrind may see more bugs

The system heap may also be faster, but it may also be slower.
No way to know without measuring.

This patch is incomplete: not all memory that UG currently tells the
internal heap to free is freed on the system heap, too.  Hence
running UG with --enable-system-heap now will result in various
memory leaks.

Since this is an experimental feature it is switched off by default

[[Imported from SVN: r8472]]

configure.ac

@@ -151,6 +151,9 @@ fi
 # check for some DDD parameters
 DDD_PARAMETERS
 
+# check whether we are using the UG heap or the operating system heap
+UG_ENABLE_SYSTEM_HEAP
+
 # the DUNE-test for MPI-package and parameters
 UG_MPI
 if test "x$parallel" = "xyes" -a "x$with_mpi" = "xno"; then

low/heaps.c

@@ -279,6 +279,9 @@ HEAP *NS_PREFIX NewHeap (enum HeapType type, MEM size, void *buffer)
 
 void *NS_PREFIX GetMem (HEAP *theHeap, MEM n, HeapAllocMode mode)
 {
+#if UG_USE_SYSTEM_HEAP
+  return malloc(n);
+#else
   BLOCK *theBlock,*newBlock;
   long newsize,allocated;
 
@@ -376,6 +379,7 @@ void *NS_PREFIX GetMem (HEAP *theHeap, MEM n, HeapAllocMode mode)
   }
 
   return(NULL);
+#endif
 }
 
 void *NS_PREFIX GetMemUsingKey (HEAP *theHeap, MEM n, HeapAllocMode mode, INT key)
@@ -448,6 +452,9 @@ void *NS_PREFIX GetMemUsingKey (HEAP *theHeap, MEM n, HeapAllocMode mode, INT ke
 
 void NS_PREFIX DisposeMem (HEAP *theHeap, void *buffer)
 {
+#if US_USE_SYSTEM_HEAP
+  free(buffer);
+#else
   BLOCK *newBlock,*theBlock,*nextBlock;
   MEM b,n,p;
 
@@ -579,6 +586,7 @@ void NS_PREFIX DisposeMem (HEAP *theHeap, void *buffer)
   /* here must be something wrong */
   assert(FALSE);
   return;
+#endif
 }
 
 /****************************************************************************/

m4/system_heap.m4

+dnl This macro introduces a configure flag --enable-system-heap.
+dnl If set UG takes it memory through malloc/free instead of using
+dnl the custom heap data structures in low/heaps.c.
+dnl
+dnl Using the system heap has several advantages:
+dnl - there is no artificial upper bound to the amount of memory UG can use
+dnl - valgrind may see more bugs
+dnl
+dnl The system heap may also be faster, but it may also be slower.
+dnl No way to know without measuring.
+dnl
+dnl Since this is an experimental feature it is switched off by default
+
+AC_DEFUN([UG_ENABLE_SYSTEM_HEAP],[
+  AC_ARG_ENABLE(system-heap,
+    AS_HELP_STRING([--enable-system-heap],[If this is set, the operating system heap is used instead of UG's own heap data structure.]))
+
+  AS_IF([test "x$enable_system_heap" = "xyes"],
+    AC_DEFINE(UG_USE_SYSTEM_HEAP, 1, [If this is set, the operating system heap is used instead of UG's own heap data structure.]))
+])