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/MipsAsmPrinter.cpp

@@ -34,6 +34,8 @@
 #include "llvm/Instructions.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCAsmInfo.h"
+#include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/TargetRegistry.h"
@@ -116,6 +118,16 @@ void MipsAsmPrinter::EmitInstruction(const MachineInstr *MI) {
     }
   }
 
+  if (Opc == Mips::SETGP01) {
+    MCInstLowering.LowerSETGP01(MI, MCInsts);
+
+    for (SmallVector<MCInst, 4>::iterator I = MCInsts.begin();
+         I != MCInsts.end(); ++I)
+      OutStreamer.EmitInstruction(*I);
+
+    return;
+  }
+
   OutStreamer.EmitInstruction(TmpInst0);
 }