The GCC bug (I hope I have ruled out all possibilities of a bug at my side)

[libucw.git] / lib / sorter / s-multiway.h

diff --git a/lib/sorter/s-multiway.h b/lib/sorter/s-multiway.h
index 097679acf1613772299f13aad369f79c0d16b75e..83e928f2621c6a8ba07adeb244b0b3ad63a5a5e3 100644 (file)
--- a/lib/sorter/s-multiway.h
+++ b/lib/sorter/s-multiway.h
@@ -7,44 +7,52 @@
  *     of the GNU Lesser General Public License.
  */
 
-typedef struct P(mwt_node) {
-  int i;
+/*
+ * We use a binary tree to keep track of the current minimum. The tree is
+ * represented by an array (in the same way as binary heaps usually are),
+ * leaves correspond to input streams and each internal vertex remembers
+ * the leaf in its subtree, which has the lowest key.
+ */
+
+typedef struct P(mwt) {
+  int i;               // Minimum of the subtree
 #ifdef SORT_UNIFY
-  int eq;
+  int eq;              // Did we encounter equality anywhere in the subtree?
 #endif
-} P(mwt_node);
+} P(mwt);
 
-static inline void P(update_tree)(P(key) *keys, P(mwt_node) *tree, uns i)
+static inline void P(update_tree)(P(key) *keys, P(mwt) *tree, uns i)
 {
   while (i /= 2)
     {
-      P(mwt_node) new;
       if (tree[2*i].i < 0)
-       new = tree[2*i+1];
+       tree[i] = tree[2*i+1];
       else if (tree[2*i+1].i < 0)
-       new = tree[2*i];
+       tree[i] = tree[2*i];
       else
        {
          int cmp = P(compare)(&keys[tree[2*i].i], &keys[tree[2*i+1].i]);
-         if (cmp <= 0)
-           new = tree[2*i];
-         else
-           new = tree[2*i+1];
+         tree[i] = (cmp <= 0) ? tree[2*i] : tree[2*i+1];
 #ifdef SORT_UNIFY
          if (!cmp)
-           new.eq = 1;
+           tree[i].eq = 1;
 #endif
        }
-#ifdef SORT_UNIFY_XXX          // FIXME
-      /* If we are not unifying, the keys are usually distinct, so this check is pointless */
-      if (!memcmp(&tree[i], &new, sizeof(new)))
-       break;
-#endif
-      tree[i] = new;
+      /*
+       * It is very tempting to stop as soon as the current node does not
+       * change, but it is wrong, because even if the stream index stored in
+       * the tree is the same, the actual key value can differ.
+       */
     }
+  /*
+   * This function sometimes triggers optimizer bugs in GCC versions up to 4.2.1,
+   * leading to an assumption that tree[1] does not change during this function.
+   * We add an explicit memory barrier as a work-around. Ugh. See GCC Bug #33262.
+   */
+  asm volatile ("" : : : "memory");
 }
 
-static inline void P(set_tree)(P(key) *keys, P(mwt_node) *tree, uns i, int val)
+static inline void P(set_tree)(P(key) *keys, P(mwt) *tree, uns i, int val)
 {
   tree[i].i = val;
   P(update_tree)(keys, tree, i);
@@ -63,14 +71,9 @@ static void P(multiway_merge)(struct sort_context *ctx UNUSED, struct sort_bucke
   struct fastbuf *fout = sbuck_write(out);
   struct fastbuf *fins[num_ins];
   P(key) keys[num_ins];
-  P(mwt_node) tree[2*n2];              // A complete binary tree, leaves are input streams, each internal vertex contains a minimum of its sons
+  P(mwt) tree[2*n2];
   for (uns i=1; i<2*n2; i++)
-    {
-      tree[i].i = -1;
-#ifdef SORT_UNIFY
-      tree[i].eq = 0;
-#endif
-    }
+    tree[i] = (P(mwt)) { .i = -1 };
 
   for (uns i=0; i<num_ins; i++)
     {
@@ -88,7 +91,7 @@ static void P(multiway_merge)(struct sort_context *ctx UNUSED, struct sort_bucke
   while (likely(tree[1].i >= 0))
     {
       int i = tree[1].i;
-      if (!tree[1].eq && 0)    // FIXME: This does not work for some reason
+      if (!tree[1].eq)
        {
          /* The key is unique, so let's go through the fast path */
          P(copy_data)(&keys[i], fins[i], fout);