1 files changed, 7 insertions, 6 deletions
diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 1a24ae3dba15..f9ecbc043261 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -801,14 +801,15 @@ ModRefInfo BasicAAResult::getModRefInfo(ImmutableCallSite CS,
 
   const Value *Object = GetUnderlyingObject(Loc.Ptr, DL);
 
-  // If this is a tail call and Loc.Ptr points to a stack location, we know that
-  // the tail call cannot access or modify the local stack.
-  // We cannot exclude byval arguments here; these belong to the caller of
-  // the current function not to the current function, and a tail callee
-  // may reference them.
+  // Calls marked 'tail' cannot read or write allocas from the current frame
+  // because the current frame might be destroyed by the time they run. However,
+  // a tail call may use an alloca with byval. Calling with byval copies the
+  // contents of the alloca into argument registers or stack slots, so there is
+  // no lifetime issue.
   if (isa<AllocaInst>(Object))
     if (const CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
-      if (CI->isTailCall())
+      if (CI->isTailCall() &&
+          !CI->getAttributes().hasAttrSomewhere(Attribute::ByVal))
         return ModRefInfo::NoModRef;
 
   // If the pointer is to a locally allocated object that does not escape,