Add comments, add a vector to keep track of which registers are allocatable

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6014 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LiveVariables.h

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