Optimize address range coalescing.

DWARFDebugAranges::Sort() calls std::stable_sort() over a set of address ranges
and then proceeds to collapse neighboring ranges together.

One problem with the current implementation is that it does an incomplete job.
When a pair of ranges are merged the next pair considered does not include the
just-merged range.  IOW, three consecutive ranges are never collapsed into one.

Another problem is that for each range merged we are calling
std::vector::erase() which "shifts" all remaining elements of the vector by one
position on every merge.  The end result (in the worst case) is a quadratic
algorithm -- not good when the input vector is large.

The following patch merges all consecutive ranges and removes the quadratic
behavior.  The implementation uses an auxiliary vector of indices in order to
remember all ranges that can be dropped, then performs the coalescing of ranges
in a single pass.

Patch from Stephen Wilson with some minor modification by me.

llvm-svn: 129595

lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugAranges.cpp

@@ -15,6 +15,7 @@
 #include <algorithm>
 
 #include "lldb/Core/Stream.h"
+#include "lldb/Core/Timer.h"
 
 #include "SymbolFileDWARF.h"
 #include "DWARFDebugInfo.h"
@@ -291,35 +292,47 @@ DWARFDebugAranges::AppendRange (dw_offset_t offset, dw_addr_t low_pc, dw_addr_t
 void
 DWARFDebugAranges::Sort()
 {    
+    std::vector<size_t> indices;
+    size_t end;
+    Timer scoped_timer(__PRETTY_FUNCTION__, "%s this = %p",
+                       __PRETTY_FUNCTION__, this);
+
     // Sort our address range entries
     std::stable_sort (m_aranges.begin(), m_aranges.end(), RangeLessThan);
 
-    // Merge any entries that have the same offset and same start/end address
-    RangeColl::iterator pos = m_aranges.begin();
-    RangeColl::iterator end = m_aranges.end();
-    while (pos != end)
+    // Merge all neighbouring ranges into a single range and remember the
+    // indices of all ranges merged.
+    end = m_aranges.size();
+    for (size_t merge, cursor = 1; cursor < end; ++cursor)
     {
-        RangeColl::iterator next_pos = pos + 1;
-        if (next_pos != end && 
-            pos->offset == next_pos->offset && 
-            pos->hi_pc == next_pos->lo_pc)
+        merge = cursor - 1;
+        Range &r1 = m_aranges[merge];
+        Range &r2 = m_aranges[cursor];
+
+        if (r1.hi_pc == r2.lo_pc && r1.offset == r2.offset)
         {
-            // We have found an entry whose end address it he same as the
-            // next entry's start address and the offsets are the same so 
-            // we can merge these two entries.
-            pos->hi_pc = next_pos->hi_pc;
-            // Erase the next entry that wasn't needed
-            pos = m_aranges.erase (next_pos);
-            // Now recompute the end of the collection
-            end = m_aranges.end();
-        }
-        else
-        {
-            // Two entries have either different offsets or there are gaps
-            // in the address range, move along, nothing to see here.
-            pos = next_pos;
+            r2.lo_pc = r1.lo_pc;
+            indices.push_back(merge);
         }
     }
+
+    if (indices.empty())
+        return;
+
+    // Remove the merged ranges by shifting down all the keepers...
+    std::set<size_t> purged(indices.begin(), indices.end());
+    size_t new_size = m_aranges.size() - indices.size();
+    for (size_t src = 0, dst = 0; dst < new_size; ++dst)
+    {
+        while (purged.count(src) > 0)
+            ++src;
+        if (src == dst)
+            continue;
+        m_aranges[dst] = m_aranges[src];
+    }
+
+    // ...and drop the extra elements.
+    m_aranges.resize(new_size);
 }
 
 //----------------------------------------------------------------------