DenseMap: make .resize() do the intuitive thing

[escha]

Summary

In some places, like InstCombine, we resize a DenseMap to fit the elements we intend to put in it, then insert those elements (to avoid continual reallocations as it grows). But .resize(foo) doesn't actually do what people think; it resizes to foo buckets (which is really an implementation detail the user of DenseMap probably shouldn't care about), not the space required to fit foo elements. DenseMap grows if 3/4 of its buckets are full, so this actually causes one forced reallocation every time instead of avoiding a reallocation.

This patch makes .resize(foo) do the intuitive thing: it grows to the size necessary to fit foo elements without new allocations.

[escha]

Added a test to make sure that resizing is enough to insert N elements without reallocation for a variety of sizes.

I also looked quickly over the uses and none of the uses seem to be already-compensating for this.

include/llvm/ADT/DenseMap.h

   }
   unsigned size() const { return getNumEntries(); }
 
-  /// Grow the densemap so that it has at least Size buckets. Does not shrink
+  /// Grow the densemap so that it can contain at least Size items before
+  /// resizing again. This means somewhat more than Size buckets because
+  /// densemap resizes upon reaching 3/4 full.
   void resize(size_type Size) {
+    // Size *= (4/3), rounding up.
+    Size = (Size * 4 + 2) / 3;
     incrementEpoch();
     if (Size > getNumBuckets())
       grow(Size);

unittests/ADT/DenseMapTest.cpp

   EXPECT_TRUE(cit == cit2);
 }
 
+// Make sure resize actually gives us enough buckets to insert N items
+// without increasing allocation size.
+TEST(DenseMapCustomTest, ResizeTest) {
+  for (unsigned Size = 16; Size < 32; ++Size) {
+    DenseMap<unsigned, unsigned> Map;
+    Map.resize(Size);
+    unsigned MemorySize = Map.getMemorySize();
+    for (unsigned i = 0; i < Size; ++i)
+      Map[i] = i;
+    EXPECT_TRUE(Map.getMemorySize() == MemorySize);
+  }
+}
+
 // Make sure DenseMap works with StringRef keys.
 TEST(DenseMapCustomTest, StringRefTest) {
   DenseMap<StringRef, int> M;