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

Currently memcpyopt optimizes cases like

    memset(a, byte, N);
    memcpy(b, a, M);

to

    memset(a, byte, N);
    memset(b, byte, M);

if M <= N. Often this allows further simplifications down the line,
which drop the first memset entirely.

This patch extends this optimization for the case where M > N, but we
know that the bytes a[N..M] are undef due to alloca/lifetime.start.

This situation arises relatively often for Rust code, because Rust does
not initialize trailing structure padding and loves to insert redundant
memcpys. This also fixes https://bugs.llvm.org/show_bug.cgi?id=39844.

The previous version of this patch did not perform dependency checking
properly: While the dependency is checked at the position of the memset,
the used size must be that of the memcpy. Previously the size of the
memset was used, which missed modification in the region
MemSetSize..CopySize, resulting in miscompiles. The added tests cover
variations of this issue.

Differential Revision: https://reviews.llvm.org/D55120

llvm-svn: 349078

1 files changed, 34 insertions, 16 deletions

diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index 4e82e2bd42c..8756a1afcdd 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1144,6 +1144,21 @@ 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
@@ -1167,12 +1182,27 @@ bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
   if (!AA.isMustAlias(MemSet->getRawDest(), MemCpy->getRawSource()))
     return false;
 
-  ConstantInt *CopySize = cast<ConstantInt>(MemCpy->getLength());
+  // A known memset size is required.
   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.
-  if (!MemSetSize || CopySize->getZExtValue() > MemSetSize->getZExtValue())
-    return false;
+  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. Technically we're only
+    // interested in the bytes from MemSetSize..CopySize here, but as we can't
+    // easily represent this location, we use the full 0..CopySize range.
+    MemoryLocation MemCpyLoc = MemoryLocation::getForSource(MemCpy);
+    MemDepResult DepInfo = MD->getPointerDependencyFrom(
+        MemCpyLoc, true, MemSet->getIterator(), MemSet->getParent());
+    if (DepInfo.isDef() && hasUndefContents(DepInfo.getInst(), CopySize))
+      CopySize = MemSetSize;
+    else
+      return false;
+  }
 
   IRBuilder<> Builder(MemCpy);
   Builder.CreateMemSet(MemCpy->getRawDest(), MemSet->getOperand(1),
@@ -1252,19 +1282,7 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M) {
     if (MemCpyInst *MDep = dyn_cast<MemCpyInst>(SrcDepInfo.getInst()))
       return processMemCpyMemCpyDependence(M, MDep);
   } else if (SrcDepInfo.isDef()) {
-    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) {
+    if (hasUndefContents(SrcDepInfo.getInst(), CopySize)) {
       MD->removeInstruction(M);
       M->eraseFromParent();
       ++NumMemCpyInstr;