Index: llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ llvm/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"
@@ -43,6 +45,7 @@
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Endian.h"
+#include "llvm/Support/ScopedPrinter.h"
 #include "llvm/Support/SwapByteOrder.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Instrumentation.h"
@@ -192,6 +195,11 @@
 static cl::opt<bool> ClUseAfterReturn("asan-use-after-return",
                                       cl::desc("Check stack-use-after-return"),
                                       cl::Hidden, cl::init(true));
+static cl::opt<bool> ClRedzoneByvalArgs("asan-redzone-byval-args",
+                                        cl::desc("Create redzones for byval "
+                                                 "arguments (extra copy "
+                                                 "required)"), cl::Hidden,
+                                        cl::init(true));
 static cl::opt<bool> ClUseAfterScope("asan-use-after-scope",
                                      cl::desc("Check stack-use-after-scope"),
                                      cl::Hidden, cl::init(false));
@@ -747,6 +755,9 @@
 
   bool runOnFunction() {
     if (!ClStack) return false;
+
+    if (ClRedzoneByvalArgs) copyArgsPassedByValToAllocas();
+
     // Collect alloca, ret, lifetime instructions etc.
     for (BasicBlock *BB : depth_first(&F.getEntryBlock())) visit(*BB);
 
@@ -763,6 +774,11 @@
     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 +2544,28 @@
   llvm_unreachable("impossible LocalStackSize");
 }
 
+void FunctionStackPoisoner::copyArgsPassedByValToAllocas() {
+  BasicBlock &FirstBB = *F.begin();
+  IRBuilder<> IRB(&FirstBB, FirstBB.getFirstInsertionPt());
+  const DataLayout &DL = F.getParent()->getDataLayout();
+  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 std::string &Name = Arg.hasName() ? Arg.getName().str() :
+          "Arg" + llvm::to_string(Arg.getArgNo());
+      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);
+    }
+  }
+}
+
 PHINode *FunctionStackPoisoner::createPHI(IRBuilder<> &IRB, Value *Cond,
                                           Value *ValueIfTrue,
                                           Instruction *ThenTerm,
Index: llvm/test/Instrumentation/AddressSanitizer/stack-poisoning-byval-args.ll
===================================================================
--- /dev/null
+++ llvm/test/Instrumentation/AddressSanitizer/stack-poisoning-byval-args.ll
@@ -0,0 +1,48 @@
+; This check verifies that arguments passed by value get redzones.
+; RUN: opt < %s -asan -asan-realign-stack=32 -S | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.A = type { [8 x i32] }
+
+declare i32 @bar(%struct.A*)
+
+; Test behavior for named argument with explicit alignment.  The memcpy and
+; alloca alignments should match the explicit alignment of 64.
+define void @foo(%struct.A* byval align 64 %a) sanitize_address {
+entry:
+; CHECK-LABEL: foo
+; CHECK: call i64 @__asan_stack_malloc
+; CHECK: alloca i8, i64 {{.*}} align 64
+; 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]],{{[^,]+}}, i32 64
+; CHECK: call i32 @bar(%struct.A* [[copyPtr]])
+; CHECK: ret void
+
+  %call = call i32 @bar(%struct.A* %a)
+  ret void
+}
+
+; Test behavior for unnamed argument without explicit alignment.  In this case,
+; the first argument is referenced by the identifier %0 and the ABI requires a
+; minimum alignment of 4 bytes since struct.A contains i32s which have 4-byte
+; alignment.  However, the alloca alignment will be 32 since that is the value
+; passed via the -asan-realign-stack option, which is greater than 4.
+define void @baz(%struct.A* byval) sanitize_address {
+entry:
+; CHECK-LABEL: baz
+; CHECK: call i64 @__asan_stack_malloc
+; CHECK: alloca i8, i64 {{.*}} align 32
+; CHECK: [[copyPtr:%[^ \t]+]] = inttoptr i64 %{{[^ \t]+}} to %struct.A*
+; CHECK: [[copyBytePtr:%[^ \t]+]] = bitcast %struct.A* [[copyPtr]]
+; CHECK: [[aBytePtr:%[^ \t]+]] = bitcast %struct.A* %0
+; CHECK: call void @llvm.memcpy{{[^%]+}}[[copyBytePtr]]{{[^%]+}}[[aBytePtr]],{{[^,]+}}, i32 4
+; CHECK: call i32 @bar(%struct.A* [[copyPtr]])
+; CHECK: ret void
+
+  %call = call i32 @bar(%struct.A* %0)
+  ret void
+}