When processing frame index virtual registers, consider all available registers

(if there are any) and use the one which remains available for the longest
rather than just using the first one. This should help enable better re-use
of the loaded frame index values. rdar://7318760



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107847 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 12 insertions, 2 deletions

diff --git a/lib/CodeGen/PrologEpilogInserter.cpp b/lib/CodeGen/PrologEpilogInserter.cpp
index d6ee03477d..d1112d3c14 100644
--- a/lib/CodeGen/PrologEpilogInserter.cpp
+++ b/lib/CodeGen/PrologEpilogInserter.cpp
@@ -885,10 +885,20 @@ void PEI::scavengeFrameVirtualRegs(MachineFunction &Fn) {
             // Scavenge a new scratch register
             CurrentVirtReg = Reg;
             const TargetRegisterClass *RC = Fn.getRegInfo().getRegClass(Reg);
-            CurrentScratchReg = RS->FindUnusedReg(RC);
-            if (CurrentScratchReg == 0)
+            const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
+            BitVector Candidates(TRI->getNumRegs());
+            RS->getRegsAvailable(RC, Candidates);
+
+            // If there are any registers available, use the one that's
+            // unused for the longest after this instruction. That increases
+            // the ability to reuse the value.
+            if (Candidates.any()) {
+              MachineBasicBlock::iterator UMI;
+              CurrentScratchReg = RS->findSurvivorReg(I, Candidates, 25, UMI);
+            } else {
               // No register is "free". Scavenge a register.
               CurrentScratchReg = RS->scavengeRegister(RC, I, SPAdj);
+            }
 
             PrevValue = Value;
           }