[RLE-DSE]. Add some comments about memory location canonicalization in DSE. NFC

Author: trentxintong

lib/SILPasses/Scalar/DeadStoreElimination.cpp

@@ -242,6 +242,17 @@ bool BBState::isTrackingMemLocation(unsigned bit) {
 
 void BBState::initialize(const BBState &Succ) { WriteSetOut = Succ.WriteSetIn; }
 
+/// Intersect is very frequently performed, so it is important to make it as
+/// cheap as possible.
+/// 
+/// To do so, we canonicalize MemLocations, i.e. traced back to the underlying
+/// object. Therefore, no need to do a O(N^2) comparison to figure out what is
+/// dead along all successors. 
+///
+/// NOTE: Canonicalizing does not solve the problem entirely. i.e. it is still
+/// possible that 2 MemLocations with different bases that happen to be the
+/// same object and field. In such case, we would miss a dead store
+/// opportunity. But this happens less often with canonicalization.
 void BBState::intersect(const BBState &Succ) { WriteSetOut &= Succ.WriteSetIn; }
 
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