mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-10-30 07:22:55 +01:00
209 lines
7.2 KiB
Plaintext
209 lines
7.2 KiB
Plaintext
======================
|
|
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.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|