1 files changed, 20 insertions, 2 deletions
diff --git a/llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp b/llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp
index 44bb872554d..c473c6ccd06 100644
--- a/llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp
+++ b/llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp
@@ -543,6 +543,9 @@ Iter Filler::replaceWithCompactBranch(MachineBasicBlock &MBB, Iter Branch,
 
 // For given opcode returns opcode of corresponding instruction with short
 // delay slot.
+// For the pseudo TAILCALL*_MM instrunctions return the short delay slot
+// form. Unfortunately, TAILCALL<->b16 is denied as b16 has a limited range
+// that is too short to make use of for tail calls.
 static int getEquivalentCallShort(int Opcode) {
   switch (Opcode) {
   case Mips::BGEZAL:
@@ -555,6 +558,10 @@ static int getEquivalentCallShort(int Opcode) {
     return Mips::JALRS_MM;
   case Mips::JALR16_MM:
     return Mips::JALRS16_MM;
+  case Mips::TAILCALL_MM:
+    llvm_unreachable("Attempting to shorten the TAILCALL_MM pseudo!");
+  case Mips::TAILCALLREG_MM:
+    return Mips::JR16_MM;
   default:
     llvm_unreachable("Unexpected call instruction for microMIPS.");
   }
@@ -606,6 +613,12 @@ bool Filler::runOnMachineBasicBlock(MachineBasicBlock &MBB) {
             DSI->isCall()) {
           // If instruction in delay slot is 16b change opcode to
           // corresponding instruction with short delay slot.
+
+          // TODO: Implement an instruction mapping table of 16bit opcodes to
+          // 32bit opcodes so that an instruction can be expanded. This would
+          // save 16 bits as a TAILCALL_MM pseudo requires a fullsized nop.
+          // TODO: Permit b16 when branching backwards to the the same function
+          // if it is in range.
           DSI->setDesc(TII->get(getEquivalentCallShort(DSI->getOpcode())));
         }
         continue;
@@ -692,9 +705,14 @@ bool Filler::searchRange(MachineBasicBlock &MBB, IterTy Begin, IterTy End,
     bool InMicroMipsMode = STI.inMicroMipsMode();
     const MipsInstrInfo *TII = STI.getInstrInfo();
     unsigned Opcode = (*Slot).getOpcode();
-    if (InMicroMipsMode && TII->getInstSizeInBytes(*CurrI) == 2 &&
+    // This is complicated by the tail call optimization. For non-PIC code
+    // there is only a 32bit sized unconditional branch which can be assumed
+    // to be able to reach the target. b16 only has a range of +/- 1 KB.
+    // It's entirely possible that the target function is reachable with b16
+    // but we don't have enough information to make that decision.
+     if (InMicroMipsMode && TII->getInstSizeInBytes(*CurrI) == 2 &&
         (Opcode == Mips::JR || Opcode == Mips::PseudoIndirectBranch ||
-         Opcode == Mips::PseudoReturn))
+         Opcode == Mips::PseudoReturn || Opcode == Mips::TAILCALL))
       continue;
 
     Filler = CurrI;