Index: lib/Transforms/Instrumentation/AddressSanitizer.cpp
===================================================================
--- lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -450,17 +450,23 @@
     AU.addRequired<TargetLibraryInfoWrapperPass>();
   }
   uint64_t getAllocaSizeInBytes(AllocaInst *AI) const {
+    uint64_t ArraySize = 1;
+    if (AI->isArrayAllocation()) {
+      ConstantInt *CI = dyn_cast<ConstantInt>(AI->getArraySize());
+      assert(CI && "non-constant array size");
+      ArraySize = CI->getZExtValue();
+    }
     Type *Ty = AI->getAllocatedType();
     uint64_t SizeInBytes =
         AI->getModule()->getDataLayout().getTypeAllocSize(Ty);
-    return SizeInBytes;
+    return SizeInBytes * ArraySize;
   }
   /// Check if we want (and can) handle this alloca.
   bool isInterestingAlloca(AllocaInst &AI);
 
   // Check if we have dynamic alloca.
   bool isDynamicAlloca(AllocaInst &AI) const {
-    return AI.isArrayAllocation() || !AI.isStaticAlloca();
+    return !AI.isStaticAlloca();
   }
 
   /// If it is an interesting memory access, return the PointerOperand
@@ -897,6 +903,7 @@
     return PreviouslySeenAllocaInfo->getSecond();
 
   bool IsInteresting =
+      isDynamicAlloca(AI) ||
       (AI.getAllocatedType()->isSized() &&
        // alloca() may be called with 0 size, ignore it.
        getAllocaSizeInBytes(&AI) > 0 &&
Index: projects/compiler-rt/test/asan/TestCases/alloca_constant_size.cc
===================================================================
--- projects/compiler-rt/test/asan/TestCases/alloca_constant_size.cc
+++ projects/compiler-rt/test/asan/TestCases/alloca_constant_size.cc
@@ -0,0 +1,30 @@
+// Regression test for https://github.com/google/sanitizers/issues/691
+
+// RUN: %clangxx_asan -O0 %s -o %t -fstack-protector
+// RUN: %run %t 2>&1 | FileCheck %s
+
+#include <alloca.h>
+#include <stdio.h>
+#include <string.h>
+
+void f1() {
+  char *dynamic_buffer = (char *)alloca(200);
+  fprintf(stderr, "dynamic_buffer = %p\n", dynamic_buffer);
+  memset(dynamic_buffer, 'y', 200);
+  return;
+}
+
+void f2() {
+  char buf[1024];
+  memset(buf, 'x', 1024);
+}
+
+int main(int argc, const char *argv[]) {
+  f1();
+  f2();
+  fprintf(stderr, "Done.\n");
+  return 0;
+}
+
+// CHECK-NOT: ERROR: AddressSanitizer
+// CHECK: Done.
Index: projects/compiler-rt/test/asan/TestCases/alloca_constant_size2.cc
===================================================================
--- projects/compiler-rt/test/asan/TestCases/alloca_constant_size2.cc
+++ projects/compiler-rt/test/asan/TestCases/alloca_constant_size2.cc
@@ -0,0 +1,32 @@
+// Regression test for https://github.com/google/sanitizers/issues/691
+
+// RUN: %clangxx_asan -O0 %s -o %t -fstack-protector
+// RUN: %run %t 2>&1 | FileCheck %s
+
+#include <alloca.h>
+#include <stdio.h>
+#include <string.h>
+
+static const int kDynamicArraySize = 200;
+
+void f1() {
+  char dynamic_buffer[kDynamicArraySize];
+  fprintf(stderr, "dynamic_buffer = %p\n", dynamic_buffer);
+  memset(dynamic_buffer, 'y', kDynamicArraySize);
+  return;
+}
+
+void f2() {
+  char buf[1024];
+  memset(buf, 'x', 1024);
+}
+
+int main(int argc, const char *argv[]) {
+  f1();
+  f2();
+  fprintf(stderr, "Done.\n");
+  return 0;
+}
+
+// CHECK-NOT: ERROR: AddressSanitizer
+// CHECK: Done.