[Thumb] Fix assembler error 'cannot honor width suffix pop {lr}'

Summary:
* avoid generating POP {LR} in Thumb1 epilogues
* combine MOV LR, Rx + BX LR -> BX Rx in a peephole optimization pass
* combine POP {LR} + B + BX LR -> POP {PC} on v5T+

Test cases by Ana Pazos

Differential Revision: http://reviews.llvm.org/D15707

llvm-svn: 256523

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp b/llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
index f16ce69..725b838 100644
--- a/llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
+++ b/llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp
@@ -151,6 +151,7 @@ namespace {
     bool MergeBaseUpdateLSDouble(MachineInstr &MI) const;
     bool LoadStoreMultipleOpti(MachineBasicBlock &MBB);
     bool MergeReturnIntoLDM(MachineBasicBlock &MBB);
+    bool CombineMovBx(MachineBasicBlock &MBB);
   };
   char ARMLoadStoreOpt::ID = 0;
 }
@@ -1825,6 +1826,30 @@ bool ARMLoadStoreOpt::MergeReturnIntoLDM(MachineBasicBlock &MBB) {
   return false;
 }
 
+bool ARMLoadStoreOpt::CombineMovBx(MachineBasicBlock &MBB) {
+  MachineBasicBlock::iterator MBBI = MBB.getFirstTerminator();
+  if (MBBI == MBB.begin() || MBBI == MBB.end() ||
+      MBBI->getOpcode() != ARM::tBX_RET)
+    return false;
+
+  MachineBasicBlock::iterator Prev = MBBI;
+  --Prev;
+  if (Prev->getOpcode() != ARM::tMOVr || !Prev->definesRegister(ARM::LR))
+    return false;
+
+  for (auto Use : Prev->uses())
+    if (Use.isKill()) {
+      AddDefaultPred(BuildMI(MBB, MBBI, MBBI->getDebugLoc(), TII->get(ARM::tBX))
+          .addReg(Use.getReg(), RegState::Kill))
+          .copyImplicitOps(&*MBBI);
+      MBB.erase(MBBI);
+      MBB.erase(Prev);
+      return true;
+    }
+
+  llvm_unreachable("tMOVr doesn't kill a reg before tBX_RET?");
+}
+
 bool ARMLoadStoreOpt::runOnMachineFunction(MachineFunction &Fn) {
   MF = &Fn;
   STI = &static_cast<const ARMSubtarget &>(Fn.getSubtarget());
@@ -1844,6 +1869,8 @@ bool ARMLoadStoreOpt::runOnMachineFunction(MachineFunction &Fn) {
     Modified |= LoadStoreMultipleOpti(MBB);
     if (STI->hasV5TOps())
       Modified |= MergeReturnIntoLDM(MBB);
+    if (isThumb1)
+      Modified |= CombineMovBx(MBB);
   }
 
   Allocator.DestroyAll();