Fix the Convert to scalar to not insert dead loads in the store case. The

load is needed when we have a small store into a large alloca (at which
point we get a load/insert/store sequence), but when you do a full-sized
store, this load ends up being dead.

This dead load is bad in really large nasty testcases where the load ends
up causing mem2reg to insert large chains of dependent phi nodes which only
ADCE can delete.  Instead of doing this, just don't insert the dead load.

This fixes rdar://6864035

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91917 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/ScalarReplAggregates.cpp b/lib/Transforms/Scalar/ScalarReplAggregates.cpp
index 884345d4a978bb469824993fc874f1798114d0a3..79bb7c547caaff31df6fa3e67dc708bdba6f1fa6 100644 (file)
--- a/lib/Transforms/Scalar/ScalarReplAggregates.cpp
+++ b/lib/Transforms/Scalar/ScalarReplAggregates.cpp
@@ -1373,11 +1373,16 @@ void SROA::ConvertUsesToScalar(Value *Ptr, AllocaInst *NewAI, uint64_t Offset) {
     
     if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
       assert(SI->getOperand(0) != Ptr && "Consistency error!");
-      Value *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
+      Instruction *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
       Value *New = ConvertScalar_InsertValue(SI->getOperand(0), Old, Offset,
                                              Builder);
       Builder.CreateStore(New, NewAI);
       SI->eraseFromParent();
+      
+      // If the load we just inserted is now dead, then the inserted store
+      // overwrote the entire thing.
+      if (Old->use_empty())
+        Old->eraseFromParent();
       continue;
     }
     
@@ -1397,11 +1402,16 @@ void SROA::ConvertUsesToScalar(Value *Ptr, AllocaInst *NewAI, uint64_t Offset) {
           for (unsigned i = 1; i != NumBytes; ++i)
             APVal |= APVal << 8;
         
-        Value *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
+        Instruction *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
         Value *New = ConvertScalar_InsertValue(
                                     ConstantInt::get(User->getContext(), APVal),
                                                Old, Offset, Builder);
         Builder.CreateStore(New, NewAI);
+        
+        // If the load we just inserted is now dead, then the memset overwrote
+        // the entire thing.
+        if (Old->use_empty())
+          Old->eraseFromParent();        
       }
       MSI->eraseFromParent();
       continue;