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added a section on how to modify live variable code to use LLVM instructions 

instead of machine instructions.

llvm-svn: 1451
This commit is contained in:
Ruchira Sasanka 2001-12-13 21:25:46 +00:00
parent 6650d18892
commit 1a98b28bde
1 changed files with 25 additions and 5 deletions
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30
llvm/lib/Analysis/LiveVar/README
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@ -96,19 +96,21 @@ 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. 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
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.
3. Only the basic blocks that can be reached by the post-order iterator will
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.
@ -182,6 +184,24 @@ The above algorithm is implemented in:
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.