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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.
-
-The preconditions are:
-
-1. Instruction selection is complete (i.e., machine instructions are 
-   generated) for the method before the live variable analysis
-
-
-
-5. Assumptions
-==============
-1. 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.
-
-2. 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)
-
-
-8. Future work
-==============
-If it is necessary to do live variable analysis using LLVM instructions rather
-than using machine instructions, it is easy to modify the existing code to
-do so. Current implementation use isDef() to find any MachineOperand is a
-definition or a use. We just need to change all the places that check whether
-a particular Value is a definition/use with MachineInstr. Instead, we
-would check whether an LLVM value is a def/use using LLVM instructions. All
-the underlying data structures will remain the same. However, iterators that
-go over machine instructions must be changed to the corresponding iterators
-that go over the LLVM instructions. The logic to support Phi's in LLVM
-instructions is already there. In fact, live variable analysis was first
-done using LLVM instructions and later changed to use machine instructions.
-Hence, it is quite straightforward to revert it to LLVM instructions if
-necessary.
-
-
-
-
-
-
-