[mssa] Fix case when there is no definition in a block prior to an inserted use.

Summary:
Check that the first access before one being tested is valid.
Before this patch, if there was no definition prior to the Use being tested,
the first time Iter was deferenced, it hit the sentinel.

Reviewers: dberlin, gbiv

Subscribers: sanjoy, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D33950

llvm-svn: 304926

lib/Analysis/MemorySSAUpdater.cpp

@@ -124,17 +124,12 @@ MemoryAccess *MemorySSAUpdater::getPreviousDefInBlock(MemoryAccess *MA) {
         return &*Iter;
     } else {
       // Otherwise, have to walk the all access iterator.
-      auto Iter = MA->getReverseIterator();
-      ++Iter;
-      while (&*Iter != &*Defs->begin()) {
-        if (!isa<MemoryUse>(*Iter))
-          return &*Iter;
-        --Iter;
-      }
-      // At this point it must be pointing at firstdef
-      assert(&*Iter == &*Defs->begin() &&
-             "Should have hit first def walking backwards");
-      return &*Iter;
+      auto End = MSSA->getWritableBlockAccesses(MA->getBlock())->rend();
+      for (auto &U : make_range(++MA->getReverseIterator(), End))
+        if (!isa<MemoryUse>(U))
+          return cast<MemoryAccess>(&U);
+      // Note that if MA comes before Defs->begin(), we won't hit a def.
+      return nullptr;
     }
   }
   return nullptr;

unittests/Analysis/MemorySSA.cpp

@@ -244,6 +244,52 @@ TEST_F(MemorySSATest, CreateALoadUpdater) {
   MSSA.verifyMemorySSA();
 }
 
+TEST_F(MemorySSATest, SinkLoad) {
+  F = Function::Create(
+      FunctionType::get(B.getVoidTy(), {B.getInt8PtrTy()}, false),
+      GlobalValue::ExternalLinkage, "F", &M);
+  BasicBlock *Entry(BasicBlock::Create(C, "", F));
+  BasicBlock *Left(BasicBlock::Create(C, "", F));
+  BasicBlock *Right(BasicBlock::Create(C, "", F));
+  BasicBlock *Merge(BasicBlock::Create(C, "", F));
+  B.SetInsertPoint(Entry);
+  B.CreateCondBr(B.getTrue(), Left, Right);
+  B.SetInsertPoint(Left, Left->begin());
+  Argument *PointerArg = &*F->arg_begin();
+  B.SetInsertPoint(Left);
+  B.CreateBr(Merge);
+  B.SetInsertPoint(Right);
+  B.CreateBr(Merge);
+
+  // Load in left block
+  B.SetInsertPoint(Left, Left->begin());
+  LoadInst *LoadInst1 = B.CreateLoad(PointerArg);
+  // Store in merge block
+  B.SetInsertPoint(Merge, Merge->begin());
+  B.CreateStore(B.getInt8(16), PointerArg);
+
+  setupAnalyses();
+  MemorySSA &MSSA = *Analyses->MSSA;
+  MemorySSAUpdater Updater(&MSSA);
+
+  // Mimic sinking of a load:
+  // - clone load
+  // - insert in "exit" block
+  // - insert in mssa
+  // - remove from original block
+
+  LoadInst *LoadInstClone = cast<LoadInst>(LoadInst1->clone());
+  Merge->getInstList().insert(Merge->begin(), LoadInstClone);
+  MemoryAccess * NewLoadAccess =
+      Updater.createMemoryAccessInBB(LoadInstClone, nullptr,
+                                     LoadInstClone->getParent(),
+                                     MemorySSA::Beginning);
+  Updater.insertUse(cast<MemoryUse>(NewLoadAccess));
+  MSSA.verifyMemorySSA();
+  Updater.removeMemoryAccess(MSSA.getMemoryAccess(LoadInst1));
+  MSSA.verifyMemorySSA();
+}
+
 TEST_F(MemorySSATest, MoveAStore) {
   // We create a diamond where there is a in the entry, a store on one side, and
   // a load at the end.  After building MemorySSA, we test updating by moving