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+			======================
+			Live Variable Analysis
+			======================
+
+1. Introduction
+===============
+
+Purpose: This file contains implementation information about live variable
+	 analysis. 
+Author : Ruchira Sasanka
+Date   : Dec 8, 2001
+
+
+
+2. Entry Point
+==============
+The class MethodLiveVarInfo (declared in MethodLiveVarInfo.h and implemented in
+MethodLiveVarInfo.cpp) is the main entry point of live variable analysis. This
+class performs live variable analysis of one method. This class implements
+iterative live variable analysis.
+
+
+3. Usage
+========
+This class must be used like:	
+
+       MethodLiveVarInfo MLVI( Mehtod *);  // initializes data structures
+       MLVI.analyze();                     // do the actual live variable anal
+
+
+After the live variable analysis is complete, the user can call methods to get
+
+(1) InSet/OutSet of any BasicBlock:
+
+     (a) MethodLiveVarInfo::getOutSetOfBB
+     (b) MethodLiveVarInfo::getInSetOfBB
+
+(2) Any LiveVarSet before/after a machine instruction in method: 
+
+     (a) MethodLiveVarInfo::getLiveVarSetBeforeMInst
+     (b) MethodLiveVarInfo::getLiveVarSetBeforeMInst
+
+ See MethodLiveVarInfo.h for interface methods.
+
+
+ Note:
+ ----
+ The live var set before an instruction can be obtained in 2 ways:
+
+ 1. Use the method getLiveVarSetAfterInst(Instruction *) to get the LV Info 
+    just after an instruction. (also exists getLiveVarSetBeforeInst(..))
+
+    This function calculates the LV info for a BB only once and caches that 
+    info. If the cache does not contain the LV info of the instruction, it 
+    calculates the LV info for the whole BB and caches them.
+
+    Getting liveVar info this way uses more memory since, LV info should be 
+    cached. However, if you need LV info of nearly all the instructions of a
+    BB, this is the best and simplest interface.
+
+
+ 2. Use the OutSet and applyTranferFuncForInst(const Instruction *const Inst) 
+    declared in LiveVarSet and  traverse the instructions of a basic block in 
+    reverse (using const_reverse_iterator in the BB class). 
+
+    This is the most memory efficient method if you need LV info for 
+    only several instructions in a BasicBlock. An example is given below:
+
+
+    LiveVarSet LVSet;  // this will be the set used to traverse through each BB
+
+    // Initialize LVSet so that it is the same as OutSet of the BB
+    LVSet.setUnion( LVI->getOutSetOfBB( *BBI ) );  
+ 
+    BasicBlock::InstListType::const_reverse_iterator 
+      InstIterator = InstListInBB.rbegin(); // get the rev iter for inst in BB
+
+      // iterate over all the instructions in BB in reverse
+    for( ; InstIterator != InstListInBB.rend(); InstIterator++) {  
+
+      //...... all  code here which uses LVSet ........
+
+      LVSet.applyTranferFuncForInst(*InstIterator);
+
+      // Now LVSet contains live vars ABOVE the current instruction
+    }
+
+    See buildInterferenceGraph() in ../llvm/lib/CodeGen/RegAlloc/ for the 
+    above example.
+
+
+4. Input and Preconditions
+==========================
+
+Live variable analysis is done using machine instructions. The constructor
+to the class takes a pointer to a method, and machine instructions must be
+already available for this method before calling the constructor.
+
+
+
+5. Assumptions
+==============
+1. Instruction selection is complete (i.e., machine instructions are 
+   generated) for the method before the live variable analysis
+
+2. There may be dummy phi machine instructions in the machine code. The code
+   works with and without dummy phi instructions (i.e., this code can be
+   called before or after phi elimination). Currently, it is called without
+   phi instructions.
+
+3. Only the basic blocks that can be reached by the post-order iterator will
+   be analyzed (i.e., basic blocks for dead code will not be analyzed). The 
+   live variable sets returned for such basic blocks is not defined.
+
+
+
+6. Data Structures, Classes and Main Objects
+============================================
+
+LiveVarSet: This class implements a basic live variable set. It contains a
+set of Value objects.
+
+BBLiveVar: This class is used to keep live variable information like the inset,
+outset, etc. about a basic block. We create one object of this type for each
+basic block in the original method in  MethodLiveVarInfo::constructBBs(). This
+class is declared in BBLiveVar.h
+
+
+BBToBBLiveVarMapType: This class is used to create a map between the original
+basic blocks in the method and new BBLiveVar objects created for each original
+basic block. There is only one object of this type called  BB2BBLVMap in 
+class MethodLiveVarInfo. 
+
+MInstToLiveVarSetMapType: This class is used to create a map between a machine
+instruction and a LiveVarSet before/after that machine instruction. There are
+two objects of this type:  MInst2LVSetBI and  MInst2LVSetAI, declared in 
+class is declared in BBLiveVar.h. Both these objects are used as caches. When a
+user of class MethodLiveVarInfo requests a LiveVarSet before/after a machine'
+instruction, these two caches are used to find the those live variable sets
+quickly. See MethodLiveVarInfo::calcLiveVarSetsForBB for implementation.
+
+
+7. Algorithms
+=============
+
+7.1 LiveVaraible Analysis Algorithm
+------------------------------------
+
+Live variable analysis is done using iterative analysis. The main steps are:
+
+1. Construct BBLiveVar objects for each BasicBlock in the method. 
+2. While OutSets change
+     do one pass over the entire CFG
+
+
+The above algorithm is implemented in:
+
+ 	MethodLiveVarInfo::analyze()      	
+	MethodLiveVarInfo::doSingleBackwardPass() 
+
+
+
+7.2 Algorithm for obtaining LiveVarSets before/after a machine instruction 
+--------------------------------------------------------------------------
+
+ LiveVarSets before/after a machine instruction can be obtained using:
+
+     (a) MethodLiveVarInfo::getLiveVarSetBeforeMInst
+     (b) MethodLiveVarInfo::getLiveVarSetBeforeMInst
+
+ The basic algorithm is:
+
+1. When a LiveVarSet before/after a machine instruction is requested, look
+   in the  MInst2LVSetBI/MInst2LVSetAI to see whether a LiveVarSet is
+   calculated and already entered in the corresponding cache. 
+
+   If yes, 
+   	just return the LiveVarSet from the cache
+   Else
+        call MethodLiveVarInfo::calcLiveVarSetsForBB to calculate live variable
+	sets for ALL machine instructions in a BasicBlock and to enter all 
+	those calculated LiveVarSets in caches ( MInst2LVSetBI/MInst2LVSetAI)
+
+
+
+
+
+