Adjust initial size in StringMap constructor to guarantee no grow()

Summary:
StringMap ctor accepts an initialize size, but expect it to be
rounded to the next power of 2. The ctor can handle that directly
instead of expecting clients to round it. Also, since the map will
resize itself when 75% full, take this into account an initialize
a larger initial size to avoid any growth.

Reviewers: dblaikie

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18344

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264385 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/StringMap.cpp

@@ -17,12 +17,26 @@
 #include <cassert>
 using namespace llvm;
 
+/// Returns the number of buckets to allocate to ensure that the DenseMap can
+/// accommodate \p NumEntries without need to grow().
+static unsigned getMinBucketToReserveForEntries(unsigned NumEntries) {
+  // Ensure that "NumEntries * 4 < NumBuckets * 3"
+  if (NumEntries == 0)
+    return 0;
+  // +1 is required because of the strict equality.
+  // For example if NumEntries is 48, we need to return 401.
+  return NextPowerOf2(NumEntries * 4 / 3 + 1);
+}
+
 StringMapImpl::StringMapImpl(unsigned InitSize, unsigned itemSize) {
   ItemSize = itemSize;
   
   // If a size is specified, initialize the table with that many buckets.
   if (InitSize) {
-    init(InitSize);
+    // The table will grow when the number of entries reach 3/4 of the number of
+    // buckets. To guarantee that "InitSize" number of entries can be inserted
+    // in the table without growing, we allocate just what is needed here.
+    init(getMinBucketToReserveForEntries(InitSize));
     return;
   }