Fix memory access local dominance function for live on entry

[sebpop]

Summary

A memory access defined on function entry cannot be locally dominated by another memory access.
The patch was split from D19338 which exposes the problem.

[george.burgess.iv]

Thanks for the patch!

ISTM this function is a bit broken in general if Dominator == Dominatee (since a node technically does dominate itself). I know it's my fault, but can you add a check for that as well, please? :)

Other than that, LGTM with a comment.

llvm/lib/Transforms/Utils/MemorySSA.cpp

629

Can we please have a check for isLiveOnEntryDef(Dominator), too? liveOnEntry has a null BB, so I don't see things ending well if we let that through.

[george.burgess.iv]

llvm/lib/Transforms/Utils/MemorySSA.cpp

629

...Scratch the "liveOnEntry has a null BB, so I don't see things ending well if we let that through" bit. We do set liveOnEntry's BB to a non-null value in getWalker() :)

(This is what I get for not verifying the assertions that I make before I hit "reply" ;). My bad.)

[sebpop]

Updated patch to address the comments from George Burgess IV.

llvm/lib/Transforms/Utils/MemorySSA.cpp

 /// \brief Determine, for two memory accesses in the same block,
 /// whether \p Dominator dominates \p Dominatee.
 /// \returns True if \p Dominator dominates \p Dominatee.
 bool MemorySSA::locallyDominates(const MemoryAccess *Dominator,
                                  const MemoryAccess *Dominatee) const {
 
   assert((Dominator->getBlock() == Dominatee->getBlock()) &&
          "Asking for local domination when accesses are in different blocks!");
+
+  // A node dominates itself.
+  if (Dominatee == Dominator)
+    return true;
+
+  // When Dominatee is defined on function entry, it is not dominated by another
+  // memory access.
+  if (isLiveOnEntryDef(Dominatee))
+    return false;
+
+  // When Dominator is defined on function entry, it dominates the other memory
+  // access.
+  if (isLiveOnEntryDef(Dominator))
+    return true;
+
   // Get the access list for the block
   const AccessListType *AccessList = getBlockAccesses(Dominator->getBlock());
   AccessListType::const_reverse_iterator It(Dominator->getIterator());
 
   // If we hit the beginning of the access list before we hit dominatee, we must
   // dominate it
   return std::none_of(It, AccessList->rend(),
                       [&](const MemoryAccess &MA) { return &MA == Dominatee; });