When clearing a SmallPtrSet, if the set had a huge capacity, but the

contents of the set were small, deallocate and shrink the set.  This
avoids having us to memset as much data, significantly speeding up
some pathological cases.  For example, this speeds up the verifier
from 0.3899s to 0.0763 (5.1x) on the testcase from PR1432 in a 
release build.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40837 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/SmallPtrSet.cpp

@@ -18,6 +18,24 @@
 
 using namespace llvm;
 
+void SmallPtrSetImpl::shrink_and_clear() {
+  assert(!isSmall() && "Can't shrink a small set!");
+  free(CurArray);
+
+  // Reduce the number of buckets.
+  CurArraySize = NumElements > 16 ? 1 << (Log2_32_Ceil(NumElements) + 1) : 32;
+  NumElements = NumTombstones = 0;
+
+  // Install the new array.  Clear all the buckets to empty.
+  CurArray = (const void**)malloc(sizeof(void*) * (CurArraySize+1));
+  assert(CurArray && "Failed to allocate memory?");
+  memset(CurArray, -1, CurArraySize*sizeof(void*));
+  
+  // The end pointer, always valid, is set to a valid element to help the
+  // iterator.
+  CurArray[CurArraySize] = 0;
+}
+
 bool SmallPtrSetImpl::insert(const void * Ptr) {
   if (isSmall()) {
     // Check to see if it is already in the set.