[ARM] Refactor the prologue/epilogue emission to be more robust.

This is the first step toward supporting shrink-wrapping for this target.

The changes could be summarized by these items:
- Expand the tail-call return as part of the expand pseudo pass.
- Get rid of the assumptions that the epilogue is the exit block:
  * Do not assume which registers are free in the epilogue. (This indirectly
    improve the lowering of the code for the segmented stacks, see the test
    cases.)
  * Take into account that the basic block can be empty.

Related to <rdar://problem/20821730>

llvm-svn: 242714

1 files changed, 49 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp b/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
index 4438f50758d..f090e5abd95 100644
--- a/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
+++ b/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
@@ -747,6 +747,55 @@ bool ARMExpandPseudo::ExpandMI(MachineBasicBlock &MBB,
   switch (Opcode) {
     default:
       return false;
+
+    case ARM::TCRETURNdi:
+    case ARM::TCRETURNri: {
+      MachineBasicBlock::iterator MBBI = MBB.getLastNonDebugInstr();
+      assert(MBBI->isReturn() &&
+             "Can only insert epilog into returning blocks");
+      unsigned RetOpcode = MBBI->getOpcode();
+      DebugLoc dl = MBBI->getDebugLoc();
+      const ARMBaseInstrInfo &TII = *static_cast<const ARMBaseInstrInfo *>(
+          MBB.getParent()->getSubtarget().getInstrInfo());
+
+      // Tail call return: adjust the stack pointer and jump to callee.
+      MBBI = MBB.getLastNonDebugInstr();
+      MachineOperand &JumpTarget = MBBI->getOperand(0);
+
+      // Jump to label or value in register.
+      if (RetOpcode == ARM::TCRETURNdi) {
+        unsigned TCOpcode =
+            STI->isThumb()
+                ? (STI->isTargetMachO() ? ARM::tTAILJMPd : ARM::tTAILJMPdND)
+                : ARM::TAILJMPd;
+        MachineInstrBuilder MIB = BuildMI(MBB, MBBI, dl, TII.get(TCOpcode));
+        if (JumpTarget.isGlobal())
+          MIB.addGlobalAddress(JumpTarget.getGlobal(), JumpTarget.getOffset(),
+                               JumpTarget.getTargetFlags());
+        else {
+          assert(JumpTarget.isSymbol());
+          MIB.addExternalSymbol(JumpTarget.getSymbolName(),
+                                JumpTarget.getTargetFlags());
+        }
+
+        // Add the default predicate in Thumb mode.
+        if (STI->isThumb())
+          MIB.addImm(ARMCC::AL).addReg(0);
+      } else if (RetOpcode == ARM::TCRETURNri) {
+        BuildMI(MBB, MBBI, dl,
+                TII.get(STI->isThumb() ? ARM::tTAILJMPr : ARM::TAILJMPr))
+            .addReg(JumpTarget.getReg(), RegState::Kill);
+      }
+
+      MachineInstr *NewMI = std::prev(MBBI);
+      for (unsigned i = 1, e = MBBI->getNumOperands(); i != e; ++i)
+        NewMI->addOperand(MBBI->getOperand(i));
+
+      // Delete the pseudo instruction TCRETURN.
+      MBB.erase(MBBI);
+      MBBI = NewMI;
+      return true;
+    }
     case ARM::VMOVScc:
     case ARM::VMOVDcc: {
       unsigned newOpc = Opcode == ARM::VMOVScc ? ARM::VMOVS : ARM::VMOVD;