Add an option to use a virtual register as the global base register instead of

reserving a physical register ($gp or $28) for that purpose.

This will completely eliminate loads that restore the value of $gp after every
function call, if the register allocator assigns a callee-saved register, or
eliminate unnecessary loads if it assigns a temporary register. 

example:

.cpload $25       // set $gp.
...
.cprestore 16     // store $gp to stack slot 16($sp).
...
jalr $25          // function call. clobbers $gp.
lw $gp, 16($sp)   // not emitted if callee-saved reg is chosen.
...
lw $2, 4($gp)
...
jalr $25          // function call.
lw $gp, 16($sp)   // not emitted if $gp is not live after this instruction.
...



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151402 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/Mips/MipsTargetMachine.cpp

@@ -106,7 +106,7 @@ public:
 
   virtual bool addInstSelector();
   virtual bool addPreRegAlloc();
-  virtual bool addPostRegAlloc();
+  virtual bool addPreSched2();
   virtual bool addPreEmitPass();
 };
 } // namespace
@@ -140,7 +140,7 @@ bool MipsPassConfig::addPreRegAlloc() {
   return true;
 }
 
-bool MipsPassConfig::addPostRegAlloc() {
+bool MipsPassConfig::addPreSched2() {
   PM.add(createMipsExpandPseudoPass(getMipsTargetMachine()));
   return true;
 }