[Stack Slot Coloring] Skip pseudo debug instructions in removing dead stores

[sgundapa]

Summary

In the below scenario, we must be able to skip the a DBG_VALUE instruction and
remove the dead store.

%vreg0<def> = L2_loadri_io <fi#15>, 0; mem:LD4[%dataF](align=4)
DBG_VALUE %vreg0, %noreg, !"dataF", <!184>; IntRegs:%vreg0
S2_storeri_io <fi#15>, 0, %vreg0; mem:ST4[%dataF]

In reality, this kind of stores are eliminated before Stack Slot Coloring pass,
possibly in instruction lowering

[sgundapa]

I find it hard to come up with a unit test for this scenario. I would really appreciate if some one from community can help me in this regard.
I tried to play with mir code but in-vain.

[kparzysz]

LGTM with a few minor comments.

lib/CodeGen/StackSlotColoring.cpp

400

I suggest changing it to "... debug pseudo instructions ...". Also, add a period at the end of the sentence.

401

The parentheses around != are not necessary.

403

The indent it too large: should be 2 columns.

[sgundapa]

This patch was merged on 11/14/2016

[sgundapa]

This was not merged yet.

lib/CodeGen/StackSlotColoring.cpp

bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
   bool changed = false;
 
   SmallVector<MachineInstr*, 4> toErase;
 
   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
        I != E; ++I) {
     if (DCELimit != -1 && (int)NumDead >= DCELimit)
       break;
-
     int FirstSS, SecondSS;
     if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
         FirstSS != -1) {
       ++NumDead;
       changed = true;
       toErase.push_back(&*I);
       continue;
     }
 
     MachineBasicBlock::iterator NextMI = std::next(I);
-    if (NextMI == MBB->end()) continue;
+    MachineBasicBlock::iterator ProbableLoadMI = I;
 
     unsigned LoadReg = 0;
     unsigned StoreReg = 0;
     if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS)))
       continue;
+    // Skip the pseudo debugging instructions between a load and store
+    while ((NextMI != E) && NextMI->isDebugValue()) {
+        NextMI++;
+        ++I;
+    }
+    if (NextMI == E) continue;
     if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS)))
       continue;
     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
 
     ++NumDead;
     changed = true;
 
     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
       ++NumDead;
-      toErase.push_back(&*I);
+      toErase.push_back(&*ProbableLoadMI);
     }
 
     toErase.push_back(&*NextMI);
     ++I;
   }
 
   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
        E = toErase.end(); I != E; ++I)