[WebAssembly] iterate stack in DebugFixup from the top.

[aardappel]

Summary

This is more efficient in the typical case.

[aheejin]

Thank you!

llvm/lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp

for (auto MII = MBB.begin(); MII != MBB.end(); ++MII) {
         // Also check if not a $noreg: likely a DBG_VALUE we just inserted.
         if (MO.isReg() && MO.getReg().isValid() &&
             MFI.isVRegStackified(MO.getReg())) {
           // Found a DBG_VALUE with a stackified register we will
           // change into a stack operand.
           // Search for register rather than assume it is on top (which it
           // typically is if it appears right after the def), since
           // DBG_VALUE's may shift under some circumstances.
-          size_t Depth = 0;
-          for (auto &Elem : Stack) {
+          for (auto &Elem : reverse(Stack)) {
             if (MO.getReg() == Elem.Reg) {
+              auto Depth = static_cast<unsigned>(&Elem - &Stack[0]);
               LLVM_DEBUG(dbgs() << "Debug Value VReg " << MO.getReg()
                                 << " -> Stack Relative " << Depth << "\n");
               MO.ChangeToTargetIndex(WebAssembly::TI_OPERAND_STACK, Depth);
               // Save the DBG_VALUE instruction that defined this stackified
               // variable since later we need it to construct another one on
               // pop.
               Elem.DebugValue = &MI;
               break;
             }
-            Depth++;
           }
           // If the Reg was not found, we have a DBG_VALUE outside of its
           // def-use range, and we leave it unmodified as reg, which means
           // it will be culled later.
         }
       } else {
         // Track stack depth.
         for (MachineOperand &MO : reverse(MI.explicit_uses())) {