Reimplement memory merging routines

This commit simplifies the codebase by factoring out memory merging and
intersection checking routines into separate functions. This allows us
to check for corner cases more easily.

spike_main/spike.cc

@@ -123,31 +123,49 @@ bool sort_mem_region(const mem_cfg_t &a, const mem_cfg_t &b)
     return (a.base < b.base);
 }
 
-void merge_overlapping_memory_regions(std::vector<mem_cfg_t> &mems)
+static bool check_mem_overlap(const mem_cfg_t& L, const mem_cfg_t& R)
 {
-  // check the user specified memory regions and merge the overlapping or
-  // eliminate the containing parts
-  assert(!mems.empty());
+  const reg_t L_end   = L.base + L.size - 1;
+  const reg_t R_end   = R.base + R.size - 1;
+
+  return std::max(L.base, R.base) <= std::min(L_end, R_end);
+}
+
+static mem_cfg_t merge_mem_regions(const mem_cfg_t& L, const mem_cfg_t& R)
+{
+  // one can merge only intersecting regions
+  assert(check_mem_overlap(L, R));
+
+  const reg_t merged_base = std::min(L.base, R.base);
+  const reg_t merged_end_incl = std::max(L.base + L.size - 1, R.base + R.size - 1);
+  const reg_t merged_size = merged_end_incl - merged_base + 1;
+
+  return mem_cfg_t(merged_base, merged_size);
+}
+
+// check the user specified memory regions and merge the overlapping or
+// eliminate the containing parts
+static std::vector<mem_cfg_t>
+merge_overlapping_memory_regions(std::vector<mem_cfg_t> mems)
+{
+  if (mems.empty())
+    return {};
 
   std::sort(mems.begin(), mems.end(), sort_mem_region);
-  for (auto it = mems.begin() + 1; it != mems.end(); ) {
-    reg_t start = prev(it)->base;
-    reg_t end = prev(it)->base + prev(it)->size;
-    reg_t start2 = it->base;
-    reg_t end2 = it->base + it->size;
-
-    //contains -> remove
-    if (start2 >= start && end2 <= end) {
-      it = mems.erase(it);
-    //partial overlapped -> extend
-    } else if (start2 >= start && start2 < end) {
-      prev(it)->size = std::max(end, end2) - start;
-      it = mems.erase(it);
-    // no overlapping -> keep it
-    } else {
-      it++;
+
+  std::vector<mem_cfg_t> merged_mem;
+  merged_mem.push_back(mems.front());
+
+  for (auto mem_it = std::next(mems.begin()); mem_it != mems.end(); ++mem_it) {
+    const auto& mem_int = *mem_it;
+    if (!check_mem_overlap(merged_mem.back(), mem_int)) {
+      merged_mem.push_back(mem_int);
+      continue;
     }
+    merged_mem.back() = merge_mem_regions(merged_mem.back(), mem_int);
   }
+
+  return merged_mem;
 }
 
 static std::vector<mem_cfg_t> parse_mem_layout(const char* arg)
@@ -217,9 +235,10 @@ static std::vector<mem_cfg_t> parse_mem_layout(const char* arg)
     arg = p + 1;
   }
 
-  merge_overlapping_memory_regions(res);
+  auto merged_mem = merge_overlapping_memory_regions(res);
 
-  return res;
+  assert(!merged_mem.empty());
+  return merged_mem;
 }
 
 static std::vector<std::pair<reg_t, mem_t*>> make_mems(const std::vector<mem_cfg_t> &layout)