Index: lib/Transforms/Instrumentation/AddressSanitizer.cpp
===================================================================
--- lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -1929,27 +1929,53 @@
 void FunctionStackPoisoner::poisonRedZones(ArrayRef<uint8_t> ShadowBytes,
                                            IRBuilder<> &IRB, Value *ShadowBase,
                                            bool DoPoison) {
-  size_t n = ShadowBytes.size();
-  size_t i = 0;
-  // We need to (un)poison n bytes of stack shadow. Poison as many as we can
-  // using 64-bit stores (if we are on 64-bit arch), then poison the rest
-  // with 32-bit stores, then with 16-byte stores, then with 8-byte stores.
-  for (size_t LargeStoreSizeInBytes = ASan.LongSize / 8;
-       LargeStoreSizeInBytes != 0; LargeStoreSizeInBytes /= 2) {
-    for (; i + LargeStoreSizeInBytes - 1 < n; i += LargeStoreSizeInBytes) {
-      uint64_t Val = 0;
-      for (size_t j = 0; j < LargeStoreSizeInBytes; j++) {
-        if (F.getParent()->getDataLayout().isLittleEndian())
-          Val |= (uint64_t)ShadowBytes[i + j] << (8 * j);
-        else
-          Val = (Val << 8) | ShadowBytes[i + j];
-      }
-      if (!Val) continue;
-      Value *Ptr = IRB.CreateAdd(ShadowBase, ConstantInt::get(IntptrTy, i));
-      Type *StoreTy = Type::getIntNTy(*C, LargeStoreSizeInBytes * 8);
-      Value *Poison = ConstantInt::get(StoreTy, DoPoison ? Val : 0);
-      IRB.CreateStore(Poison, IRB.CreateIntToPtr(Ptr, StoreTy->getPointerTo()));
+  const size_t End = ShadowBytes.size();
+
+  const size_t LargestStoreSizeInBytes =
+      std::min<size_t>(sizeof(uint64_t), ASan.LongSize / 8);
+
+  const bool IsLittleEndian = F.getParent()->getDataLayout().isLittleEndian();
+
+  // Poison given range in shadow using larges store size with out leading and
+  // trailing zeros. Zeros never change, so they need neither poisoning nor
+  // up-poisoning, but we don't mind if some of them get into a middle of a
+  // store.
+  for (size_t i = 0; i < End;) {
+    if (!ShadowBytes[i]) {
+      ++i;
+      continue;
+    }
+
+    size_t StoreSizeInBytes = LargestStoreSizeInBytes;
+    // Fit store size into the range.
+    while (StoreSizeInBytes > End - i)
+      StoreSizeInBytes /= 2;
+
+    // Minimize store size by trimming trailing zeros.
+    for (size_t j = StoreSizeInBytes - 1; j && !ShadowBytes[i + j]; --j) {
+      while (j <= StoreSizeInBytes / 2)
+        StoreSizeInBytes /= 2;
     }
+
+    uint64_t Val = 0;
+    for (size_t j = 0; j < StoreSizeInBytes; j++) {
+      if (IsLittleEndian)
+        Val |= (uint64_t)ShadowBytes[i + j] << (8 * j);
+      else
+        Val = (Val << 8) | ShadowBytes[i + j];
+    }
+
+    assert(Val); // Impossible because ShadowBytes[i] != 0
+
+    if (!DoPoison)
+      Val = 0;
+
+    Value *Ptr = IRB.CreateAdd(ShadowBase, ConstantInt::get(IntptrTy, i));
+    Value *Poison = IRB.getIntN(StoreSizeInBytes * 8, Val);
+    IRB.CreateStore(Poison,
+                    IRB.CreateIntToPtr(Ptr, Poison->getType()->getPointerTo()));
+
+    i += StoreSizeInBytes;
   }
 }
 
Index: test/Instrumentation/AddressSanitizer/lifetime.ll
===================================================================
--- test/Instrumentation/AddressSanitizer/lifetime.ll
+++ test/Instrumentation/AddressSanitizer/lifetime.ll
@@ -108,10 +108,9 @@
   ; CHECK: __asan_poison_stack_memory
 
   ret void
+  ; CHECK: store i32 0
   ; CHECK: store i64 0
   ; CHECK: store i64 0
-  ; CHECK: store i64 0
-  ; CHECK: store i32 0
   ; CHECK-NEXT: __asan_unpoison_stack_memory
 }
 
Index: test/Instrumentation/AddressSanitizer/stack-poisoning.ll
===================================================================
--- test/Instrumentation/AddressSanitizer/stack-poisoning.ll
+++ test/Instrumentation/AddressSanitizer/stack-poisoning.ll
@@ -16,11 +16,12 @@
 ; CHECK-UAR: label
 ; CHECK-UAR: call i64 @__asan_stack_malloc_4
 ; CHECK-UAR: label
+; Poison red zones.
 ; CHECK-UAR: store i64 -1007680412564983311
 ; CHECK-UAR: store i64 72057598113936114
-; CHECK-UAR: store i64 4076008178
-; CHECK-UAR: store i64 -868082074072645632
-; CHECK-UAR: store i32 -202116109
+; CHECK-UAR: store i32 -218959118
+; CHECK-UAR: store i64 -868082074056920316
+; CHECK-UAR: store i16 -3085
 ; CHECK-UAR: call void @Foo
 ; CHECK-UAR: call void @Foo
 ; CHECK-UAR: call void @Foo
@@ -49,9 +50,9 @@
 ; Else Block: no UAR frame. Only unpoison the redzones.
   ; CHECK-UAR: store i64 0
   ; CHECK-UAR: store i64 0
-  ; CHECK-UAR: store i64 0
-  ; CHECK-UAR: store i64 0
   ; CHECK-UAR: store i32 0
+  ; CHECK-UAR: store i64 0
+  ; CHECK-UAR: store i16 0
   ; CHECK-UAR-NOT: store
   ; CHECK-UAR: label
 ; Done, no more stores.