+++ /dev/null
- ======================
- 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.
-
-
-
-
-
-
-
+++ /dev/null
- ======================
- 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.
-
-
-
-
-
-
-
projects / oota-llvm.git / commitdiff