[MemorySSA] NFC small fixes

[Prazek]

Summary

2 small fixes extracted from
https://reviews.llvm.org/D29064

[davide]

You can probably on and commit a subset of this. I'm in doubt about the first chunk so I asked Danny/George to take a look.

lib/Transforms/Utils/MemorySSA.cpp

1324–1335

hmm, this is probably correct but there's something that doesn't really convinces me about it, I'll let Danny/George to sign-off on this part of the patch.

1453–1461

This is obviously dead code, so I'm fine with this part of the patch.,

1462

Nice comment.

[george.burgess.iv]

LGTM with one nit. Thanks for the patch!

lib/Transforms/Utils/MemorySSA.cpp

1327

Nit: Indent the " to under the !, please

[Prazek]

lib/Transforms/Utils/MemorySSA.cpp

1462

Thanks, I actually moved it from top of the function :)

lib/Transforms/Utils/MemorySSA.cpp

void MemorySSA::OptimizeUses::optimizeUsesInBlock(
     DenseMap<MemoryLocOrCall, MemlocStackInfo> &LocStackInfo) {
 
   /// If no accesses, nothing to do.
   MemorySSA::AccessList *Accesses = MSSA->getWritableBlockAccesses(BB);
   if (Accesses == nullptr)
     return;
 
   // Pop everything that doesn't dominate the current block off the stack,
   // increment the PopEpoch to account for this.
-  while (!VersionStack.empty()) {
+  while (true) {
+    assert(!VersionStack.empty() &&
+        "Version stack should have liveOnEntry sentinel dominating everything");
     BasicBlock *BackBlock = VersionStack.back()->getBlock();
     if (DT->dominates(BackBlock, BB))
       break;
     while (VersionStack.back()->getBlock() == BackBlock)
       VersionStack.pop_back();
     ++PopEpoch;
   }
+
   for (MemoryAccess &MA : *Accesses) {
     auto *MU = dyn_cast<MemoryUse>(&MA);
     if (!MU) {
       VersionStack.push_back(&MA);
       ++StackEpoch;
       continue;
     }
 
for (MemoryAccess &MA : *Accesses) {
     }
     LocInfo.LowerBound = VersionStack.size() - 1;
     LocInfo.LowerBoundBlock = BB;
   }
 }
 
 /// Optimize uses to point to their actual clobbering definitions.
 void MemorySSA::OptimizeUses::optimizeUses() {
-
-  // We perform a non-recursive top-down dominator tree walk
-  struct StackInfo {
-    const DomTreeNode *Node;
-    DomTreeNode::const_iterator Iter;
-  };
-
   SmallVector<MemoryAccess *, 16> VersionStack;
-  SmallVector<StackInfo, 16> DomTreeWorklist;
   DenseMap<MemoryLocOrCall, MemlocStackInfo> LocStackInfo;
   VersionStack.push_back(MSSA->getLiveOnEntryDef());
 
   unsigned long StackEpoch = 1;
   unsigned long PopEpoch = 1;
+  // We perform a non-recursive top-down dominator tree walk.
   for (const auto *DomNode : depth_first(DT->getRootNode()))
     optimizeUsesInBlock(DomNode->getBlock(), StackEpoch, PopEpoch, VersionStack,
                         LocStackInfo);
 }
 
 void MemorySSA::placePHINodes(
     const SmallPtrSetImpl<BasicBlock *> &DefiningBlocks,
     const DenseMap<const BasicBlock *, unsigned int> &BBNumbers) {