Index: lib/Transforms/Instrumentation/AddressSanitizer.cpp
===================================================================
--- lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -22,9 +22,11 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Triple.h"
+#include "llvm/ADT/Twine.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/Argument.h"
 #include "llvm/IR/CallSite.h"
 #include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DataLayout.h"
@@ -747,6 +749,9 @@
 
   bool runOnFunction() {
     if (!ClStack) return false;
+
+    copyArgsPassedByValToAllocas();
+
     // Collect alloca, ret, lifetime instructions etc.
     for (BasicBlock *BB : depth_first(&F.getEntryBlock())) visit(*BB);
 
@@ -763,6 +768,11 @@
     return true;
   }
 
+  // Arguments marked with the "byval" attribute are implicitly copied without
+  // using an alloca instruction.  To produce redzones for those arguments, we
+  // copy them a second time into memory allocated with an alloca instruction.
+  void copyArgsPassedByValToAllocas();
+
   // Finds all Alloca instructions and puts
   // poisoned red zones around all of them.
   // Then unpoison everything back before the function returns.
@@ -2528,6 +2538,32 @@
   llvm_unreachable("impossible LocalStackSize");
 }
 
+void FunctionStackPoisoner::copyArgsPassedByValToAllocas() {
+  BasicBlock &FirstBB = *F.begin();
+  IRBuilder<> IRB(&FirstBB, FirstBB.getFirstInsertionPt());
+  const DataLayout &DL = F.getParent()->getDataLayout();
+  unsigned ArgNum = 0;
+  for (Argument &Arg : F.args()) {
+    if (Arg.hasByValAttr()) {
+      Type *Ty = Arg.getType()->getPointerElementType();
+      unsigned Align = Arg.getParamAlignment();
+      if (Align == 0) {
+        Align = DL.getABITypeAlignment(Ty);
+      }
+
+      const StringRef &Name = Arg.getName().empty() ?
+          StringRef("Arg" + std::to_string(ArgNum)) : Arg.getName();
+      AllocaInst *AI = IRB.CreateAlloca(Ty, nullptr, Twine(Name) + ".byval");
+      AI->setAlignment(Align);
+      Arg.replaceAllUsesWith(AI);
+
+      uint64_t AllocSize = DL.getTypeAllocSize(Ty);
+      IRB.CreateMemCpy(AI, &Arg, AllocSize, Align);
+    }
+    ++ArgNum;
+  }
+}
+
 PHINode *FunctionStackPoisoner::createPHI(IRBuilder<> &IRB, Value *Cond,
                                           Value *ValueIfTrue,
                                           Instruction *ThenTerm,
Index: test/Instrumentation/AddressSanitizer/stack-poisoning-byval-args.ll
===================================================================
--- /dev/null
+++ test/Instrumentation/AddressSanitizer/stack-poisoning-byval-args.ll
@@ -0,0 +1,22 @@
+; This check verifies that arguments passed by value get redzones.
+; RUN: opt < %s -asan -S | FileCheck %s
+
+%struct.A = type { [8 x i32] }
+
+declare i32 @bar(%struct.A*)
+
+define void @foo(%struct.A* byval align 8 %a) sanitize_address {
+entry:
+; CHECK-LABEL: foo
+; CHECK: call i64 @__asan_stack_malloc
+; CHECK: alloca i8, i64
+; CHECK: [[copyPtr:%[^ \t]+]] = inttoptr i64 %{{[^ \t]+}} to %struct.A*
+; CHECK: [[copyBytePtr:%[^ \t]+]] = bitcast %struct.A* [[copyPtr]]
+; CHECK: [[aBytePtr:%[^ \t]+]] = bitcast %struct.A* %a
+; CHECK: call void @llvm.memcpy{{[^%]+}}[[copyBytePtr]]{{[^%]+}}[[aBytePtr]]
+; CHECK: call i32 @bar(%struct.A* [[copyPtr]])
+; CHECK: ret void
+
+  %call = call i32 @bar(%struct.A* %a)
+  ret void
+}