[llvm-commits] [llvm] r119865 - /llvm/trunk/lib/Transforms/Scalar/GVN.cpp

[Owen Anderson]

Author: resistor
Date: Fri Nov 19 16:48:40 2010
New Revision: 119865

URL: http://llvm.org/viewvc/llvm-project?rev=119865&view=rev
Log:
Document the new GVN number table structure.

Modified:
    llvm/trunk/lib/Transforms/Scalar/GVN.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/GVN.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/GVN.cpp?rev=119865&r1=119864&r2=119865&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/GVN.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/GVN.cpp Fri Nov 19 16:48:40 2010
@@ -677,8 +677,13 @@
 
     ValueTable VN;
     
+    /// NumberTable - A mapping from value numers to lists of Value*'s that
+    /// have that value number.  Use lookupNumber to query it.
     DenseMap<uint32_t, std::pair<Value*, void*> > NumberTable;
     BumpPtrAllocator TableAllocator;
+    
+    /// insert_table - Push a new Value to the NumberTable onto the list for
+    /// its value number.
     void insert_table(uint32_t N, Value *V) {
       std::pair<Value*, void*>& Curr = NumberTable[N];
       if (!Curr.first) {
@@ -693,6 +698,8 @@
       Curr.second = Node;
     }
     
+    /// erase_table - Scan the list of values corresponding to a given value
+    /// number, and remove the given value if encountered.
     void erase_table(uint32_t N, Value *V) {
       std::pair<Value*, void*>* Prev = 0;
       std::pair<Value*, void*>* Curr = &NumberTable[N];
@@ -1886,6 +1893,11 @@
   return false;
 }
 
+// lookupNumber - In order to find a leader for a given value number at a 
+// specific basic block, we first obtain the list of all Values for that number,
+// and then scan the list to find one whose block dominates the block in 
+// question.  This is fast because dominator tree queries consist of only
+// a few comparisons of DFS numbers.
 Value *GVN::lookupNumber(BasicBlock *BB, uint32_t num) {
   std::pair<Value*, void*> Vals = NumberTable[num];
   if (!Vals.first) return 0;