Index: llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -634,22 +634,24 @@
     if (destSize < srcSize)
       return false;
   } else if (Argument *A = dyn_cast<Argument>(cpyDest)) {
-    // If the destination is an sret parameter then only accesses that are
-    // outside of the returned struct type can trap.
-    if (!A->hasStructRetAttr())
-      return false;
+    if (A->getDereferenceableBytes() < srcSize) {
+      // If the destination is an sret parameter then only accesses that are
+      // outside of the returned struct type can trap.
+      if (!A->hasStructRetAttr())
+        return false;
 
-    Type *StructTy = cast<PointerType>(A->getType())->getElementType();
-    if (!StructTy->isSized()) {
-      // The call may never return and hence the copy-instruction may never
-      // be executed, and therefore it's not safe to say "the destination
-      // has at least <cpyLen> bytes, as implied by the copy-instruction",
-      return false;
-    }
+      Type *StructTy = cast<PointerType>(A->getType())->getElementType();
+      if (!StructTy->isSized()) {
+        // The call may never return and hence the copy-instruction may never
+        // be executed, and therefore it's not safe to say "the destination
+        // has at least <cpyLen> bytes, as implied by the copy-instruction",
+        return false;
+      }
 
-    uint64_t destSize = DL->getTypeAllocSize(StructTy);
-    if (destSize < srcSize)
-      return false;
+      uint64_t destSize = DL->getTypeAllocSize(StructTy);
+      if (destSize < srcSize)
+        return false;
+    }
   } else {
     return false;
   }
Index: llvm/trunk/test/Transforms/MemCpyOpt/callslot_deref.ll
===================================================================
--- llvm/trunk/test/Transforms/MemCpyOpt/callslot_deref.ll
+++ llvm/trunk/test/Transforms/MemCpyOpt/callslot_deref.ll
@@ -0,0 +1,29 @@
+; RUN: opt < %s -S -basicaa -memcpyopt | FileCheck %s
+target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly, i64, i32, i1) unnamed_addr nounwind
+declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i32, i1) nounwind
+
+; all bytes of %dst that are touch by the memset are dereferenceable
+define void @must_remove_memcpy(i8* noalias nocapture dereferenceable(4096) %dst) {
+; CHECK-LABEL: @must_remove_memcpy(
+; CHECK: call void @llvm.memset.p0i8.i64
+; CHECK-NOT: call void @llvm.memcpy.p0i8.p0i8.i64
+  %src = alloca [4096 x i8], align 1
+  %p = getelementptr inbounds [4096 x i8]* %src, i64 0, i64 0
+  call void @llvm.memset.p0i8.i64(i8* %p, i8 0, i64 4096, i32 1, i1 false)
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %dst, i8* %p, i64 4096, i32 1, i1 false) #2
+  ret void
+}
+
+; memset touch more bytes than those guaranteed to be dereferenceable
+define void @must_not_remove_memcpy(i8* noalias nocapture dereferenceable(1024) %dst) {
+; CHECK-LABEL: @must_not_remove_memcpy(
+; CHECK: call void @llvm.memset.p0i8.i64
+; CHECK: call void @llvm.memcpy.p0i8.p0i8.i64
+  %src = alloca [4096 x i8], align 1
+  %p = getelementptr inbounds [4096 x i8]* %src, i64 0, i64 0
+  call void @llvm.memset.p0i8.i64(i8* %p, i8 0, i64 4096, i32 1, i1 false)
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %dst, i8* %p, i64 4096, i32 1, i1 false) #2
+  ret void
+}