[memprof] Speed up caller-callee pair extraction

We know that the MemProf profile has a lot of duplicate call stacks.
Extracting caller-callee pairs from a call stack we've seen before is
a wasteful effort.

This patch makes the extraction more efficient by first coming up with
a work list of linear call stack IDs -- the set of starting positions
in the radix tree array -- and then extract caller-callee pairs from
each call stack in the work list.

We implement the work list as a bit vector because we expect the work
list to be dense in the range [0, RadixTreeSize).  Also, we want the
set insertion to be cheap.

Without this patch, it takes 25 seconds to extract caller-callee pairs
from a large MemProf profile.  This patch shortenes that down to 4
seconds.

Author: kazutakahirata

llvm/include/llvm/ProfileData/InstrProfReader.h

@@ -683,6 +683,8 @@ class IndexedMemProfReader {
   const unsigned char *FrameBase = nullptr;
   /// The starting address of the call stack array.
   const unsigned char *CallStackBase = nullptr;
+  // The number of elements in the radix tree array.
+  unsigned RadixTreeSize = 0;
 
   Error deserializeV012(const unsigned char *Start, const unsigned char *Ptr,
                         uint64_t FirstWord);

llvm/lib/ProfileData/InstrProfReader.cpp

@@ -1303,6 +1303,10 @@ Error IndexedMemProfReader::deserializeV3(const unsigned char *Start,
   FrameBase = Ptr;
   CallStackBase = Start + CallStackPayloadOffset;
 
+  // Compute the number of elements in the radix tree array.
+  RadixTreeSize = (RecordPayloadOffset - CallStackPayloadOffset) /
+                  sizeof(memprof::LinearFrameId);
+
   // Now initialize the table reader with a pointer into data buffer.
   MemProfRecordTable.reset(MemProfRecordHashTable::Create(
       /*Buckets=*/Start + RecordTableOffset,
@@ -1674,11 +1678,22 @@ IndexedMemProfReader::getMemProfCallerCalleePairs() const {
   memprof::LinearFrameIdConverter FrameIdConv(FrameBase);
   memprof::CallerCalleePairExtractor Extractor(CallStackBase, FrameIdConv);
 
+  // The set of linear call stack IDs that we need to traverse from.  We expect
+  // the set to be dense, so we use a BitVector.
+  BitVector Worklist(RadixTreeSize);
+
+  // Collect the set of linear call stack IDs.  Since we expect a lot of
+  // duplicates, we first collect them in the form of a bit vector before
+  // processing them.
   for (const memprof::IndexedMemProfRecord &IndexedRecord :
        MemProfRecordTable->data())
     for (const memprof::IndexedAllocationInfo &IndexedAI :
          IndexedRecord.AllocSites)
-      Extractor(IndexedAI.CSId);
+      Worklist.set(IndexedAI.CSId);
+
+  // Collect caller-callee pairs for each linear call stack ID in Worklist.
+  for (unsigned CS : Worklist.set_bits())
+    Extractor(CS);
 
   DenseMap<uint64_t, SmallVector<memprof::CallEdgeTy, 0>> Pairs =
       std::move(Extractor.CallerCalleePairs);