diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.h b/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.h
--- a/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.h
@@ -25,7 +25,7 @@
  public:
   enum class Compression : u8 {
     None = 0,
-    Test,
+    Delta,
   };
 
   constexpr StackStore() = default;
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.cpp
--- a/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.cpp
@@ -10,6 +10,8 @@
 
 #include "sanitizer_atomic.h"
 #include "sanitizer_common.h"
+#include "sanitizer_internal_defs.h"
+#include "sanitizer_leb128.h"
 #include "sanitizer_stacktrace.h"
 
 namespace __sanitizer {
@@ -128,6 +130,37 @@
   return Create();
 }
 
+static u8 *CompressDelta(const uptr *from, const uptr *from_end, u8 *to,
+                         u8 *to_end) {
+  uptr prev = 0;
+  for (; from < from_end; ++from) {
+    sptr diff = *from - prev;
+    to = EncodeSLEB128(diff, to, to_end);
+    prev += diff;
+  }
+  return to;
+}
+
+static uptr *UncompressDelta(const u8 *from, const u8 *from_end, uptr *to,
+                             uptr *to_end) {
+  uptr prev = 0;
+  for (; to < to_end; ++to) {
+    sptr diff;
+    from = DecodeSLEB128<sptr>(from, from_end, &diff);
+    prev += diff;
+    *to = prev;
+  }
+  return to;
+}
+
+namespace {
+struct PackedHeader {
+  uptr size;
+  StackStore::Compression type;
+  u8 data[];
+};
+}  // namespace
+
 uptr *StackStore::BlockInfo::GetOrUnpack() {
   SpinMutexLock l(&mtx_);
   switch (state) {
@@ -140,10 +173,34 @@
       break;
   }
 
-  uptr *ptr = Get();
+  u8 *ptr = reinterpret_cast<u8 *>(Get());
   CHECK_NE(nullptr, ptr);
-  // Fake unpacking.
-  for (uptr i = 0; i < kBlockSizeFrames; ++i) ptr[i] = ~ptr[i];
+  const PackedHeader *header = reinterpret_cast<const PackedHeader *>(ptr);
+  CHECK_LE(header->size, kBlockSizeBytes);
+  CHECK_GE(header->size, sizeof(PackedHeader));
+
+  uptr packed_size_aligned = RoundUpTo(header->size, GetPageSizeCached());
+
+  uptr *unpacked = reinterpret_cast<uptr *>(
+      MmapNoReserveOrDie(kBlockSizeBytes, "StackStoreUnpack"));
+
+  uptr *unpacked_end;
+  switch (header->type) {
+    case Compression::Delta:
+      unpacked_end = UncompressDelta(header->data, ptr + header->size, unpacked,
+                                     unpacked + kBlockSizeFrames);
+      break;
+    default:
+      UNREACHABLE("Unexpected type");
+      break;
+  }
+
+  CHECK_EQ(kBlockSizeFrames, unpacked_end - unpacked);
+
+  MprotectReadOnly(reinterpret_cast<uptr>(unpacked), kBlockSizeBytes);
+  atomic_store(&data_, reinterpret_cast<uptr>(unpacked), memory_order_release);
+  UnmapOrDie(ptr, packed_size_aligned);
+
   state = State::Unpacked;
   return Get();
 }
@@ -165,17 +222,56 @@
   if (!ptr || !Stored(0))
     return 0;
 
-  // Fake packing.
-  for (uptr i = 0; i < kBlockSizeFrames; ++i) ptr[i] = ~ptr[i];
+  u8 *packed = reinterpret_cast<u8 *>(
+      MmapNoReserveOrDie(kBlockSizeBytes, "StackStorePack"));
+  PackedHeader *header = reinterpret_cast<PackedHeader *>(packed);
+  u8 *alloc_end = packed + kBlockSizeBytes;
+
+  u8 *packed_end = nullptr;
+  switch (type) {
+    case Compression::Delta:
+      packed_end =
+          CompressDelta(ptr, ptr + kBlockSizeFrames, header->data, alloc_end);
+      break;
+    default:
+      UNREACHABLE("Unexpected type");
+      break;
+  }
+
+  header->type = type;
+  header->size = packed_end - packed;
+
+  VPrintf(1, "Packed block of %zu KiB to %zu KiB\n", kBlockSizeBytes >> 10,
+          header->size >> 10);
+
+  if (kBlockSizeBytes - header->size < kBlockSizeBytes / 8) {
+    VPrintf(1, "Undo and keep block unpacked\n");
+    MprotectReadOnly(reinterpret_cast<uptr>(ptr), kBlockSizeBytes);
+    UnmapOrDie(packed, kBlockSizeBytes);
+    state = State::Unpacked;
+    return 0;
+  }
+
+  uptr packed_size_aligned = RoundUpTo(header->size, GetPageSizeCached());
+  UnmapOrDie(packed + packed_size_aligned,
+             kBlockSizeBytes - packed_size_aligned);
+  MprotectReadOnly(reinterpret_cast<uptr>(packed), packed_size_aligned);
+
+  atomic_store(&data_, reinterpret_cast<uptr>(packed), memory_order_release);
+  UnmapOrDie(ptr, kBlockSizeBytes);
+
   state = State::Packed;
-  return kBlockSizeBytes - kBlockSizeBytes / 10;
+  return kBlockSizeBytes - packed_size_aligned;
 }
 
 uptr StackStore::BlockInfo::Allocated() const {
   SpinMutexLock l(&mtx_);
   switch (state) {
-    case State::Packed:
-      return kBlockSizeBytes / 10;
+    case State::Packed: {
+      const PackedHeader *ptr = reinterpret_cast<const PackedHeader *>(Get());
+      CHECK_NE(nullptr, ptr);
+      return RoundUpTo(ptr->size, GetPageSizeCached());
+    }
     case State::Unpacked:
     case State::Storing:
       return kBlockSizeBytes;
@@ -184,7 +280,7 @@
 
 void StackStore::BlockInfo::TestOnlyUnmap() {
   if (uptr *ptr = Get())
-    UnmapOrDie(ptr, StackStore::kBlockSizeBytes);
+    UnmapOrDie(ptr, kBlockSizeBytes);
 }
 
 bool StackStore::BlockInfo::Stored(uptr n) {
diff --git a/compiler-rt/lib/sanitizer_common/tests/sanitizer_stack_store_test.cpp b/compiler-rt/lib/sanitizer_common/tests/sanitizer_stack_store_test.cpp
--- a/compiler-rt/lib/sanitizer_common/tests/sanitizer_stack_store_test.cpp
+++ b/compiler-rt/lib/sanitizer_common/tests/sanitizer_stack_store_test.cpp
@@ -135,7 +135,8 @@
 INSTANTIATE_TEST_SUITE_P(
     PackUnpacks, StackStorePackTest,
     ::testing::ValuesIn({
-        StackStorePackTest::ParamType(StackStore::Compression::Test, 4),
+        StackStorePackTest::ParamType(StackStore::Compression::Delta,
+                                      FIRST_32_SECOND_64(2, 6)),
     }));
 
 TEST_P(StackStorePackTest, PackUnpack) {
@@ -175,4 +176,23 @@
   EXPECT_EQ(0u, CountPackedBlocks());
 }
 
+TEST_P(StackStorePackTest, Failed) {
+  MurMur2Hash64Builder h(0);
+  StackStore::Compression type = GetParam().first;
+  std::vector<uptr> frames(200);
+  for (uptr i = 0; i < kBlockSizeFrames * 4 / frames.size(); ++i) {
+    for (uptr& f : frames) {
+      h.add(1);
+      // Make it difficult to pack.
+      f = h.get();
+    }
+    uptr pack = 0;
+    store_.Store(StackTrace(frames.data(), frames.size()), &pack);
+    if (pack)
+      EXPECT_EQ(0u, store_.Pack(type));
+  }
+
+  EXPECT_EQ(0u, CountPackedBlocks());
+}
+
 }  // namespace __sanitizer