Add debug output to MipsDelaySlotFiller pass

Summary:
I was tracking down a code-generation bug in this pass and found that the
added output was useful. It is also helpful to find out why a delay slot
could not be filled even though there is clearly a valid instruction (which
appears to mostly be caused by CFI instructions).

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: merge_guards_bot, sdardis, hiraditya, jrtc27, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70940

Author: arichardson

llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp

@@ -612,12 +612,18 @@ bool MipsDelaySlotFiller::runOnMachineBasicBlock(MachineBasicBlock &MBB) {
       if (MipsCompactBranchPolicy.getValue() != CB_Always ||
            !TII->getEquivalentCompactForm(I)) {
         if (searchBackward(MBB, *I)) {
+          LLVM_DEBUG(dbgs() << DEBUG_TYPE ": found instruction for delay slot"
+                                          " in backwards search.\n");
           Filled = true;
         } else if (I->isTerminator()) {
           if (searchSuccBBs(MBB, I)) {
             Filled = true;
+            LLVM_DEBUG(dbgs() << DEBUG_TYPE ": found instruction for delay slot"
+                                            " in successor BB search.\n");
           }
         } else if (searchForward(MBB, I)) {
+          LLVM_DEBUG(dbgs() << DEBUG_TYPE ": found instruction for delay slot"
+                                          " in forwards search.\n");
           Filled = true;
         }
       }
@@ -662,6 +668,8 @@ bool MipsDelaySlotFiller::runOnMachineBasicBlock(MachineBasicBlock &MBB) {
     }
 
     // Bundle the NOP to the instruction with the delay slot.
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE << ": could not fill delay slot for ";
+               I->dump());
     BuildMI(MBB, std::next(I), I->getDebugLoc(), TII->get(Mips::NOP));
     MIBundleBuilder(MBB, I, std::next(I, 2));
     ++FilledSlots;
@@ -679,13 +687,19 @@ bool MipsDelaySlotFiller::searchRange(MachineBasicBlock &MBB, IterTy Begin,
   for (IterTy I = Begin; I != End;) {
     IterTy CurrI = I;
     ++I;
-
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE ": checking instruction: "; CurrI->dump());
     // skip debug value
-    if (CurrI->isDebugInstr())
+    if (CurrI->isDebugInstr()) {
+      LLVM_DEBUG(dbgs() << DEBUG_TYPE ": ignoring debug instruction: ";
+                 CurrI->dump());
       continue;
+    }
 
-    if (terminateSearch(*CurrI))
+    if (terminateSearch(*CurrI)) {
+      LLVM_DEBUG(dbgs() << DEBUG_TYPE ": should terminate search: ";
+                 CurrI->dump());
       break;
+    }
 
     assert((!CurrI->isCall() && !CurrI->isReturn() && !CurrI->isBranch()) &&
            "Cannot put calls, returns or branches in delay slot.");
@@ -731,6 +745,9 @@ bool MipsDelaySlotFiller::searchRange(MachineBasicBlock &MBB, IterTy Begin,
        continue;
 
     Filler = CurrI;
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE ": found instruction for delay slot: ";
+               CurrI->dump());
+
     return true;
   }
 
@@ -751,8 +768,11 @@ bool MipsDelaySlotFiller::searchBackward(MachineBasicBlock &MBB,
 
   MachineBasicBlock::iterator SlotI = Slot;
   if (!searchRange(MBB, ++SlotI.getReverse(), MBB.rend(), RegDU, MemDU, Slot,
-                   Filler))
+                   Filler)) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE ": could not find instruction for delay "
+                                    "slot using backwards search.\n");
     return false;
+  }
 
   MBB.splice(std::next(SlotI), &MBB, Filler.getReverse());
   MIBundleBuilder(MBB, SlotI, std::next(SlotI, 2));
@@ -772,8 +792,11 @@ bool MipsDelaySlotFiller::searchForward(MachineBasicBlock &MBB,
 
   RegDU.setCallerSaved(*Slot);
 
-  if (!searchRange(MBB, std::next(Slot), MBB.end(), RegDU, NM, Slot, Filler))
+  if (!searchRange(MBB, std::next(Slot), MBB.end(), RegDU, NM, Slot, Filler)) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE ": could not find instruction for delay "
+                                    "slot using forwards search.\n");
     return false;
+  }
 
   MBB.splice(std::next(Slot), &MBB, Filler);
   MIBundleBuilder(MBB, Slot, std::next(Slot, 2));