[MLton] A few changes proposals for MLton

[Matthew Fluet]

On Thu, 13 Nov 2008, Nicolas Bertolotti wrote:
> *   21-prevent-null-heap-size.patch
>    *   This patch addresses the issue I submitted a few days ago
>      (Segmentation fault on a 6 GB machine with ram-slop 0.85)

--- mlton-r6698.patched/runtime/gc/heap.c	2008-11-03 23:45:47.000000000 +0100
+++ mlton-r6698/runtime/gc/heap.c	2008-11-05 23:53:54.000000000 +0100
@@ -39,6 +39,7 @@
  size_t sizeofHeapDesired (GC_state s, size_t liveSize, size_t currentSize) {
    size_t liveWithMapsSize;
    size_t res;
+  size_t maxSize;
    float withMapsRatio;

    liveSize = align (liveSize, s->sysvals.pageSize);
@@ -82,6 +83,9 @@
      res = s->sysvals.ram;
    } else { /* Required live ratio. */
      res = liveSize * s->controls.ratios.markCompact;
+    /* Avoid overflow */
+    if (res < liveSize)
+      res = SIZE_MAX;
      /* If the current heap is bigger than res, then shrinking always
       * sounds like a good idea.  However, depending on what pages the
       * VM keeps around, growing could be very expensive, if it
@@ -104,6 +108,16 @@
        die ("Out of memory with max heap size %s.",
             uintmaxToCommaString(s->controls.maxHeap));
    }
+
+  /* Avoid overflow. */
+  maxSize = align(SIZE_MAX - s->sysvals.pageSize, s->sysvals.pageSize);
+  maxSize -= sizeofCardMapAndCrossMap(s, maxSize);
+  if (res > maxSize)
+    res = maxSize;
+  if (res < liveSize)
+    die ("Out of memory with system max heap size %s.", 
+	 uintmaxToCommaString(res));
+
    if (DEBUG_RESIZING)
      fprintf (stderr, "%s = sizeofHeapDesired (%s, %s)\n",
               uintmaxToCommaString(res),


Looks simple and effective.  Equivalent, but perhaps easier to understand 
would be to replace

+  maxSize = align(SIZE_MAX - s->sysvals.pageSize, s->sysvals.pageSize);

with

+  maxSize = alignDown(SIZE_MAX, s->sysvals.pageSize);