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.
...

llvm-svn: 151402

1 files changed, 8 insertions, 2 deletions

diff --git a/llvm/lib/Target/Mips/MipsRegisterInfo.cpp b/llvm/lib/Target/Mips/MipsRegisterInfo.cpp
index f55ef4e581f..aa921bc8d1e 100644
--- a/llvm/lib/Target/Mips/MipsRegisterInfo.cpp
+++ b/llvm/lib/Target/Mips/MipsRegisterInfo.cpp
@@ -101,12 +101,12 @@ BitVector MipsRegisterInfo::
 getReservedRegs(const MachineFunction &MF) const {
   static const unsigned ReservedCPURegs[] = {
     Mips::ZERO, Mips::AT, Mips::K0, Mips::K1, 
-    Mips::GP, Mips::SP, Mips::FP, Mips::RA
+    Mips::SP, Mips::FP, Mips::RA
   };
 
   static const unsigned ReservedCPU64Regs[] = {
     Mips::ZERO_64, Mips::AT_64, Mips::K0_64, Mips::K1_64, 
-    Mips::GP_64, Mips::SP_64, Mips::FP_64, Mips::RA_64
+    Mips::SP_64, Mips::FP_64, Mips::RA_64
   };
 
   BitVector Reserved(getNumRegs());
@@ -135,6 +135,12 @@ getReservedRegs(const MachineFunction &MF) const {
       Reserved.set(*Reg);
   }
   
+  // If GP is dedicated as a global base register, reserve it. 
+  if (MF.getInfo<MipsFunctionInfo>()->globalBaseRegFixed()) {
+    Reserved.set(Mips::GP);
+    Reserved.set(Mips::GP_64);
+  }
+
   return Reserved;
 }