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