Index: lib/Transforms/Instrumentation/AddressSanitizer.cpp
===================================================================
--- lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -450,19 +450,20 @@
     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();
-  }
-
   /// If it is an interesting memory access, return the PointerOperand
   /// and set IsWrite/Alignment. Otherwise return nullptr.
   Value *isInterestingMemoryAccess(Instruction *I, bool *IsWrite,
@@ -717,7 +718,7 @@
     }
 
     StackAlignment = std::max(StackAlignment, AI.getAlignment());
-    if (ASan.isDynamicAlloca(AI))
+    if (!AI.isStaticAlloca())
       DynamicAllocaVec.push_back(&AI);
     else
       AllocaVec.push_back(&AI);
@@ -899,7 +900,7 @@
   bool IsInteresting =
       (AI.getAllocatedType()->isSized() &&
        // alloca() may be called with 0 size, ignore it.
-       getAllocaSizeInBytes(&AI) > 0 &&
+       ((!AI.isStaticAlloca()) || getAllocaSizeInBytes(&AI) > 0) &&
        // We are only interested in allocas not promotable to registers.
        // Promotable allocas are common under -O0.
        (!ClSkipPromotableAllocas || !isAllocaPromotable(&AI)) &&
@@ -1992,7 +1993,7 @@
     assert(APC.InsBefore);
     assert(APC.AI);
     assert(ASan.isInterestingAlloca(*APC.AI));
-    bool IsDynamicAlloca = ASan.isDynamicAlloca(*APC.AI);
+    bool IsDynamicAlloca = !(*APC.AI).isStaticAlloca();
     if (!ClInstrumentAllocas && IsDynamicAlloca)
       continue;
 
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 +1,44 @@
+// Regression test for https://github.com/google/sanitizers/issues/691
+
+// RUN: %clangxx_asan -O0 %s -o %t -fstack-protector
+// RUN: %run %t 1 2>&1 | FileCheck %s
+// RUN: %run %t 2 2>&1 | FileCheck %s
+
+#include <alloca.h>
+#include <stdio.h>
+#include <string.h>
+
+void f1_alloca() {
+  char *dynamic_buffer = (char *)alloca(200);
+  fprintf(stderr, "dynamic_buffer = %p\n", dynamic_buffer);
+  memset(dynamic_buffer, 'y', 200);
+  return;
+}
+
+static const int kDynamicArraySize = 200;
+
+void f1_vla() {
+  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[]) {
+  if (!strcmp(argv[1], "1")) {
+    f1_alloca();
+  } else if (!strcmp(argv[1], "2")) {
+    f1_vla();
+  }
+  f2();
+  fprintf(stderr, "Done.\n");
+  return 0;
+}
+
+// CHECK-NOT: ERROR: AddressSanitizer
+// CHECK: Done.