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Add comments, add a vector to keep track of which registers are allocatable
llvm-svn: 6014
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@ -1,12 +1,21 @@
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//===-- llvm/CodeGen/LiveVariables.h - Live Variable Analysis ---*- C++ -*-===//
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//
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// This file implements the LiveVariable analysis pass. For each machine
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// instruction in the function, this pass calculates the set of registers that
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// are immediately dead after the instruction (i.e., the instruction calculates
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// the value, but it is never used) and the set of registers that are used by
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// the instruction, but are never used after the instruction (i.e., they are
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// killed).
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//
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// This class computes live variables using are sparse implementation based on
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// the machine code SSA form. This class computes live variable information for
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// each virtual and physical register in a function. It uses the dominance
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// properties of SSA form to efficiently compute live variables for virtual
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// registers, and assumes that physical registers are only live within a single
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// basic block (allowing it to do a single local analysis to resolve physical
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// register lifetimes in each basic block).
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// each virtual and _register allocatable_ physical register in a function. It
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// uses the dominance properties of SSA form to efficiently compute live
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// variables for virtual registers, and assumes that physical registers are only
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// live within a single basic block (allowing it to do a single local analysis
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// to resolve physical register lifetimes in each basic block). If a physical
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// register is not register allocatable, it is not tracked. This is useful for
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// things like the stack pointer and condition codes.
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//
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//===----------------------------------------------------------------------===//
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@ -58,6 +67,11 @@ class LiveVariables : public MachineFunctionPass {
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///
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std::multimap<MachineInstr*, unsigned> RegistersDead;
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/// AllocatablePhysicalRegisters - This vector keeps track of which registers
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/// are actually register allocatable by the target machine. We can not track
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/// liveness for values that are not in this set.
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///
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std::vector<bool> AllocatablePhysicalRegisters;
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private: // Intermediate data structures
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/// BBMap - Maps LLVM basic blocks to their corresponding machine basic block.
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