Revert r348645 - "[MemCpyOpt] memset->memcpy forwarding with undef tail"

This revision caused trucated memsets for structs with padding. See:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181210/610520.html

llvm-svn: 349002

1 files changed, 16 insertions, 30 deletions

diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index fa44cd9e7df..4e82e2bd42c 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1144,21 +1144,6 @@ bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
   return true;
 }
 
-/// Determine whether the instruction has undefined content for the given Size,
-/// either because it was freshly alloca'd or started its lifetime.
-static bool hasUndefContents(Instruction *I, ConstantInt *Size) {
-  if (isa<AllocaInst>(I))
-    return true;
-
-  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
-    if (II->getIntrinsicID() == Intrinsic::lifetime_start)
-      if (ConstantInt *LTSize = dyn_cast<ConstantInt>(II->getArgOperand(0)))
-        if (LTSize->getZExtValue() >= Size->getZExtValue())
-          return true;
-
-  return false;
-}
-
 /// Transform memcpy to memset when its source was just memset.
 /// In other words, turn:
 /// \code
@@ -1182,23 +1167,12 @@ bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
   if (!AA.isMustAlias(MemSet->getRawDest(), MemCpy->getRawSource()))
     return false;
 
-  // A known memset size is required.
+  ConstantInt *CopySize = cast<ConstantInt>(MemCpy->getLength());
   ConstantInt *MemSetSize = dyn_cast<ConstantInt>(MemSet->getLength());
-  if (!MemSetSize)
-    return false;
-
   // Make sure the memcpy doesn't read any more than what the memset wrote.
   // Don't worry about sizes larger than i64.
-  ConstantInt *CopySize = cast<ConstantInt>(MemCpy->getLength());
-  if (CopySize->getZExtValue() > MemSetSize->getZExtValue()) {
-    // If the memcpy is larger than the memset, but the memory was undef prior
-    // to the memset, we can just ignore the tail.
-    MemDepResult DepInfo = MD->getDependency(MemSet);
-    if (DepInfo.isDef() && hasUndefContents(DepInfo.getInst(), CopySize))
-      CopySize = MemSetSize;
-    else
-      return false;
-  }
+  if (!MemSetSize || CopySize->getZExtValue() > MemSetSize->getZExtValue())
+    return false;
 
   IRBuilder<> Builder(MemCpy);
   Builder.CreateMemSet(MemCpy->getRawDest(), MemSet->getOperand(1),
@@ -1278,7 +1252,19 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M) {
     if (MemCpyInst *MDep = dyn_cast<MemCpyInst>(SrcDepInfo.getInst()))
       return processMemCpyMemCpyDependence(M, MDep);
   } else if (SrcDepInfo.isDef()) {
-    if (hasUndefContents(SrcDepInfo.getInst(), CopySize)) {
+    Instruction *I = SrcDepInfo.getInst();
+    bool hasUndefContents = false;
+
+    if (isa<AllocaInst>(I)) {
+      hasUndefContents = true;
+    } else if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
+      if (II->getIntrinsicID() == Intrinsic::lifetime_start)
+        if (ConstantInt *LTSize = dyn_cast<ConstantInt>(II->getArgOperand(0)))
+          if (LTSize->getZExtValue() >= CopySize->getZExtValue())
+            hasUndefContents = true;
+    }
+
+    if (hasUndefContents) {
       MD->removeInstruction(M);
       M->eraseFromParent();
       ++NumMemCpyInstr;