In Thumb1, materialize a move between low registers as a `movs`, if CPSR isn't live.

Summary: Previously, it had always been materialized as a push/pop sequence.

Reviewers: labrinea, jroelofs

Reviewed By: jroelofs

Subscribers: llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D30648

llvm-svn: 297134

1 files changed, 11 insertions, 5 deletions

diff --git a/llvm/lib/Target/ARM/Thumb1InstrInfo.cpp b/llvm/lib/Target/ARM/Thumb1InstrInfo.cpp
index f1d6fa68d3a..27bff4d75ac 100644
--- a/llvm/lib/Target/ARM/Thumb1InstrInfo.cpp
+++ b/llvm/lib/Target/ARM/Thumb1InstrInfo.cpp
@@ -54,11 +54,17 @@ void Thumb1InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
         .addReg(SrcReg, getKillRegState(KillSrc))
         .add(predOps(ARMCC::AL));
   else {
-    // FIXME: The performance consequences of this are going to be atrocious.
-    // Some things to try that should be better:
-    //   * 'mov hi, $src; mov $dst, hi', with hi as either r10 or r11
-    //   * 'movs $dst, $src' if cpsr isn't live
-    // See: http://lists.llvm.org/pipermail/llvm-dev/2014-August/075998.html
+    // FIXME: Can also use 'mov hi, $src; mov $dst, hi',
+    // with hi as either r10 or r11.
+
+    const TargetRegisterInfo *RegInfo = st.getRegisterInfo();
+    if (MBB.computeRegisterLiveness(RegInfo, ARM::CPSR, I)
+        == MachineBasicBlock::LQR_Dead) {
+      BuildMI(MBB, I, DL, get(ARM::tMOVSr), DestReg)
+          .addReg(SrcReg, getKillRegState(KillSrc))
+          ->addRegisterDead(ARM::CPSR, RegInfo);
+      return;
+    }
 
     // 'MOV lo, lo' is unpredictable on < v6, so use the stack to do it
     BuildMI(MBB, I, DL, get(ARM::tPUSH))