[MemorySSAUpdater] Avoid creating self-referencing MemoryDefs

Fix for https://bugs.llvm.org/show_bug.cgi?id=38807, which occurred
while compiling SemaTemplateInstantiate.cpp with clang and GVNHoist
enabled. In the following example:

      1=def(entry)
      /        \
2=def(1)       4=def(1)
3=def(2)       5=def(4)

When removing the MemoryDef 2=def(1) from its basic block, and just
before adding it to the end of the parent basic block, we first
replace all its uses with the defining memory access:

3=def(2) -> 3=def(1)

Then we call insertDef for adding 2=def(1) to the parent basic block,
where we replace the uses of 1=def(entry) with 2=def(1). Doing so we
create a self reference:

2=def(1) -> 2=def(2)  (bad)
3=def(1) -> 3=def(2)  (ok)
4=def(1) -> 4=def(2)  (ok)

Differential Revision: https://reviews.llvm.org/D51801

llvm-svn: 341947

1 files changed, 3 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/MemorySSAUpdater.cpp b/llvm/lib/Analysis/MemorySSAUpdater.cpp
index 502f884705c..51a5733a3ef 100644
--- a/llvm/lib/Analysis/MemorySSAUpdater.cpp
+++ b/llvm/lib/Analysis/MemorySSAUpdater.cpp
@@ -266,16 +266,15 @@ void MemorySSAUpdater::insertDef(MemoryDef *MD, bool RenameUses) {
     for (auto UI = DefBefore->use_begin(), UE = DefBefore->use_end();
          UI != UE;) {
       Use &U = *UI++;
-      // Leave the uses alone
-      if (isa<MemoryUse>(U.getUser()))
+      // Leave the MemoryUses alone.
+      // Also make sure we skip ourselves to avoid self references.
+      if (isa<MemoryUse>(U.getUser()) || U.getUser() == MD)
         continue;
       U.set(MD);
     }
   }
 
   // and that def is now our defining access.
-  // We change them in this order otherwise we will appear in the use list
-  // above and reset ourselves.
   MD->setDefiningAccess(DefBefore);
 
   SmallVector<WeakVH, 8> FixupList(InsertedPHIs.begin(), InsertedPHIs.end());