[wasm] A simple allocator datastructure for off-the heap

We'll use this allocator in a follow-up CL to:
- allocate speculative sizes of memory for a module that's being
compiled (e.g. 2*size of wasm code).
- each module will own such a sub-pool, and then use it to allocate
contiguous chunks of memory for code.

The underlying assumptions for the chosen allocation strategy is that:
- the allocation granularity for pools is 1 page, so that no one page
is owned by more than one wasm module
- typical pool sizes (given module sizes) are multiple pages.
- modules and module instances are typically few and long lived. Typically,
we expect one module and one instance. 

This means we shouldn't expect fragmentations that lead to code being
non-allocatable, or prohibitively many ranges.

The data structure just manages ranges of addresses. Virtual memory management
will be separate, as part of the responsibility of a "WasmHeap"
that will be introduced in the future. So will concurrency control.

Bug: 
Change-Id: Id99f46d10c25553b013054d994760f3c2a737c39
Reviewed-on: https://chromium-review.googlesource.com/669296
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48053}

BUILD.gn

@@ -2059,6 +2059,8 @@ v8_source_set("v8_base") {
     "src/wasm/wasm-debug.cc",
     "src/wasm/wasm-external-refs.cc",
     "src/wasm/wasm-external-refs.h",
+    "src/wasm/wasm-heap.cc",
+    "src/wasm/wasm-heap.h",
     "src/wasm/wasm-interpreter.cc",
     "src/wasm/wasm-interpreter.h",
     "src/wasm/wasm-js.cc",

src/v8.gyp

@@ -1447,6 +1447,8 @@
         'wasm/wasm-debug.cc',
         'wasm/wasm-external-refs.cc',
         'wasm/wasm-external-refs.h',
+        'wasm/wasm-heap.cc',
+        'wasm/wasm-heap.h',
         'wasm/wasm-js.cc',
         'wasm/wasm-js.h',
         'wasm/wasm-limits.h',

src/wasm/wasm-heap.cc

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/wasm/wasm-heap.h"
+
+namespace v8 {
+namespace internal {
+namespace wasm {
+
+DisjointAllocationPool::DisjointAllocationPool(Address start, Address end) {
+  ranges_.push_back({start, end});
+}
+
+void DisjointAllocationPool::Release(DisjointAllocationPool&& other) {
+  auto dest_it = ranges_.begin();
+  auto dest_end = ranges_.end();
+
+  for (auto src_it = other.ranges_.begin(), src_end = other.ranges_.end();
+       src_it != src_end;) {
+    if (dest_it == dest_end) {
+      // everything else coming from src will be inserted
+      // at the back of ranges_ from now on.
+      ranges_.push_back(*src_it);
+      ++src_it;
+      continue;
+    }
+    // Before or adjacent to dest. Insert or merge, and advance
+    // just src.
+    if (dest_it->first >= src_it->second) {
+      if (dest_it->first == src_it->second) {
+        dest_it->first = src_it->first;
+      } else {
+        ranges_.insert(dest_it, {src_it->first, src_it->second});
+      }
+      ++src_it;
+      continue;
+    }
+    // Src is strictly after dest. Skip over this dest.
+    if (dest_it->second < src_it->first) {
+      ++dest_it;
+      continue;
+    }
+    // Src is adjacent from above. Merge and advance
+    // just src, because the next src, if any, is bound to be
+    // strictly above the newly-formed range.
+    DCHECK_EQ(dest_it->second, src_it->first);
+    dest_it->second = src_it->second;
+    ++src_it;
+    // Now that we merged, maybe this new range is adjacent to
+    // the next. Since we assume src to have come from the
+    // same original memory pool, it follows that the next src
+    // must be above or adjacent to the new bubble.
+    auto next_dest = dest_it;
+    ++next_dest;
+    if (next_dest != dest_end && dest_it->second == next_dest->first) {
+      dest_it->second = next_dest->second;
+      ranges_.erase(next_dest);
+    }
+
+    // src_it points now at the next, if any, src
+    DCHECK_IMPLIES(src_it != src_end, src_it->first >= dest_it->second);
+  }
+}
+
+DisjointAllocationPool DisjointAllocationPool::Extract(size_t size,
+                                                       ExtractionMode mode) {
+  DisjointAllocationPool ret;
+  for (auto it = ranges_.begin(), end = ranges_.end(); it != end;) {
+    auto current = it;
+    ++it;
+    DCHECK_LT(current->first, current->second);
+    size_t current_size = reinterpret_cast<size_t>(current->second) -
+                          reinterpret_cast<size_t>(current->first);
+    if (size == current_size) {
+      ret.ranges_.push_back(*current);
+      ranges_.erase(current);
+      return ret;
+    }
+    if (size < current_size) {
+      ret.ranges_.push_back({current->first, current->first + size});
+      current->first += size;
+      DCHECK(current->first < current->second);
+      return ret;
+    }
+    if (mode != kContiguous) {
+      size -= current_size;
+      ret.ranges_.push_back(*current);
+      ranges_.erase(current);
+    }
+  }
+  if (size > 0) {
+    Release(std::move(ret));
+    return {};
+  }
+  return ret;
+}
+
+}  // namespace wasm
+}  // namespace internal
+}  // namespace v8

src/wasm/wasm-heap.h

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_WASM_HEAP_H_
+#define V8_WASM_HEAP_H_
+
+#include <list>
+
+#include "src/base/macros.h"
+#include "src/vector.h"
+
+namespace v8 {
+namespace internal {
+namespace wasm {
+
+// Sorted, disjoint and non-overlapping memory ranges. A range is of the
+// form [start, end). So there's no [start, end), [end, other_end),
+// because that should have been reduced to [start, other_end).
+using AddressRange = std::pair<Address, Address>;
+class V8_EXPORT_PRIVATE DisjointAllocationPool final {
+ public:
+  enum ExtractionMode : bool { kAny = false, kContiguous = true };
+  DisjointAllocationPool() {}
+
+  explicit DisjointAllocationPool(Address, Address);
+
+  DisjointAllocationPool(DisjointAllocationPool&& other) = default;
+  DisjointAllocationPool& operator=(DisjointAllocationPool&& other) = default;
+
+  // Merge the ranges of the parameter into this object. Ordering is
+  // preserved. The assumption is that the passed parameter is
+  // not intersecting this object - for example, it was obtained
+  // from a previous Allocate{Pool}.
+  void Release(DisjointAllocationPool&&);
+
+  // Allocate a contiguous range of size {size}. Return an empty pool on
+  // failure.
+  DisjointAllocationPool Allocate(size_t size) {
+    return Extract(size, kContiguous);
+  }
+
+  // Allocate a sub-pool of size {size}. Return an empty pool on failure.
+  DisjointAllocationPool AllocatePool(size_t size) {
+    return Extract(size, kAny);
+  }
+
+  bool IsEmpty() const { return ranges_.empty(); }
+  const std::list<AddressRange>& ranges() const { return ranges_; }
+
+ private:
+  // Extract out a total of {size}. By default, the return may
+  // be more than one range. If kContiguous is passed, the return
+  // will be one range. If the operation fails, this object is
+  // unchanged, and the return {IsEmpty()}
+  DisjointAllocationPool Extract(size_t size, ExtractionMode mode);
+
+  std::list<AddressRange> ranges_;
+
+  DISALLOW_COPY_AND_ASSIGN(DisjointAllocationPool)
+};
+
+}  // namespace wasm
+}  // namespace internal
+}  // namespace v8
+#endif

test/unittests/BUILD.gn

@@ -157,6 +157,7 @@ v8_executable("unittests") {
     "wasm/loop-assignment-analysis-unittest.cc",
     "wasm/module-decoder-unittest.cc",
     "wasm/streaming-decoder-unittest.cc",
+    "wasm/wasm-heap-unittest.cc",
     "wasm/wasm-macro-gen-unittest.cc",
     "wasm/wasm-module-builder-unittest.cc",
     "wasm/wasm-opcodes-unittest.cc",

test/unittests/unittests.gyp

@@ -154,6 +154,7 @@
       'wasm/control-transfer-unittest.cc',
       'wasm/decoder-unittest.cc',
       'wasm/function-body-decoder-unittest.cc',
+      'wasm/wasm-heap-unittest.cc',
       'wasm/leb-helper-unittest.cc',
       'wasm/loop-assignment-analysis-unittest.cc',
       'wasm/module-decoder-unittest.cc',

test/unittests/wasm/wasm-heap-unittest.cc

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "test/unittests/test-utils.h"
+#include "testing/gmock/include/gmock/gmock.h"
+
+#include "src/wasm/wasm-heap.h"
+
+namespace v8 {
+namespace internal {
+namespace wasm {
+
+class DisjointAllocationPoolTest : public testing::Test {
+ public:
+  Address A(size_t n) { return reinterpret_cast<Address>(n); }
+  void CheckLooksLike(const DisjointAllocationPool& mem,
+                      std::vector<std::pair<size_t, size_t>> expectation);
+  DisjointAllocationPool Make(std::vector<std::pair<size_t, size_t>> model);
+};
+
+void DisjointAllocationPoolTest::CheckLooksLike(
+    const DisjointAllocationPool& mem,
+    std::vector<std::pair<size_t, size_t>> expectation) {
+  const auto& ranges = mem.ranges();
+  CHECK_EQ(ranges.size(), expectation.size());
+  auto iter = expectation.begin();
+  for (auto it = ranges.begin(), e = ranges.end(); it != e; ++it, ++iter) {
+    CHECK_EQ(it->first, A(iter->first));
+    CHECK_EQ(it->second, A(iter->second));
+  }
+}
+
+DisjointAllocationPool DisjointAllocationPoolTest::Make(
+    std::vector<std::pair<size_t, size_t>> model) {
+  DisjointAllocationPool ret;
+  for (auto& pair : model) {
+    ret.Release(DisjointAllocationPool(A(pair.first), A(pair.second)));
+  }
+  return ret;
+}
+
+TEST_F(DisjointAllocationPoolTest, Construct) {
+  DisjointAllocationPool a;
+  CHECK(a.IsEmpty());
+  CHECK_EQ(a.ranges().size(), 0);
+  DisjointAllocationPool b = Make({{1, 5}});
+  CHECK(!b.IsEmpty());
+  CHECK_EQ(b.ranges().size(), 1);
+  a.Release(std::move(b));
+  CheckLooksLike(a, {{1, 5}});
+  DisjointAllocationPool c;
+  a.Release(std::move(c));
+  CheckLooksLike(a, {{1, 5}});
+  DisjointAllocationPool e, f;
+  e.Release(std::move(f));
+  CHECK(e.IsEmpty());
+}
+
+TEST_F(DisjointAllocationPoolTest, SimpleExtract) {
+  DisjointAllocationPool a = Make({{1, 5}});
+  DisjointAllocationPool b = a.AllocatePool(2);
+  CheckLooksLike(a, {{3, 5}});
+  CheckLooksLike(b, {{1, 3}});
+  a.Release(std::move(b));
+  CheckLooksLike(a, {{1, 5}});
+  CHECK_EQ(a.ranges().size(), 1);
+  CHECK_EQ(a.ranges().front().first, A(1));
+  CHECK_EQ(a.ranges().front().second, A(5));
+}
+
+TEST_F(DisjointAllocationPoolTest, ExtractAll) {
+  DisjointAllocationPool a(A(1), A(5));
+  DisjointAllocationPool b = a.AllocatePool(4);
+  CheckLooksLike(b, {{1, 5}});
+  CheckLooksLike(a, {});
+  a.Release(std::move(b));
+  CheckLooksLike(a, {{1, 5}});
+}
+
+TEST_F(DisjointAllocationPoolTest, ExtractAccross) {
+  DisjointAllocationPool a = Make({{1, 5}, {10, 20}});
+  DisjointAllocationPool b = a.AllocatePool(5);
+  CheckLooksLike(a, {{11, 20}});
+  CheckLooksLike(b, {{1, 5}, {10, 11}});
+  a.Release(std::move(b));
+  CheckLooksLike(a, {{1, 5}, {10, 20}});
+}
+
+TEST_F(DisjointAllocationPoolTest, ReassembleOutOfOrder) {
+  DisjointAllocationPool a = Make({{1, 5}, {10, 15}});
+  DisjointAllocationPool b = Make({{7, 8}, {20, 22}});
+  a.Release(std::move(b));
+  CheckLooksLike(a, {{1, 5}, {7, 8}, {10, 15}, {20, 22}});
+
+  DisjointAllocationPool c = Make({{1, 5}, {10, 15}});
+  DisjointAllocationPool d = Make({{7, 8}, {20, 22}});
+  d.Release(std::move(c));
+  CheckLooksLike(d, {{1, 5}, {7, 8}, {10, 15}, {20, 22}});
+}
+
+TEST_F(DisjointAllocationPoolTest, FailToExtract) {
+  DisjointAllocationPool a = Make({{1, 5}});
+  DisjointAllocationPool b = a.AllocatePool(5);
+  CheckLooksLike(a, {{1, 5}});
+  CHECK(b.IsEmpty());
+}
+
+TEST_F(DisjointAllocationPoolTest, FailToExtractExact) {
+  DisjointAllocationPool a = Make({{1, 5}, {10, 14}});
+  DisjointAllocationPool b = a.Allocate(5);
+  CheckLooksLike(a, {{1, 5}, {10, 14}});
+  CHECK(b.IsEmpty());
+}
+
+TEST_F(DisjointAllocationPoolTest, ExtractExact) {
+  DisjointAllocationPool a = Make({{1, 5}, {10, 15}});
+  DisjointAllocationPool b = a.Allocate(5);
+  CheckLooksLike(a, {{1, 5}});
+  CheckLooksLike(b, {{10, 15}});
+}
+
+TEST_F(DisjointAllocationPoolTest, Merging) {
+  DisjointAllocationPool a = Make({{10, 15}, {20, 25}});
+  a.Release(Make({{15, 20}}));
+  CheckLooksLike(a, {{10, 25}});
+}
+
+TEST_F(DisjointAllocationPoolTest, MergingMore) {
+  DisjointAllocationPool a = Make({{10, 15}, {20, 25}, {30, 35}});
+  a.Release(Make({{15, 20}, {25, 30}}));
+  CheckLooksLike(a, {{10, 35}});
+}
+
+TEST_F(DisjointAllocationPoolTest, MergingSkip) {
+  DisjointAllocationPool a = Make({{10, 15}, {20, 25}, {30, 35}});
+  a.Release(Make({{25, 30}}));
+  CheckLooksLike(a, {{10, 15}, {20, 35}});
+}
+
+TEST_F(DisjointAllocationPoolTest, MergingSkipLargerSrc) {
+  DisjointAllocationPool a = Make({{10, 15}, {20, 25}, {30, 35}});
+  a.Release(Make({{25, 30}, {35, 40}}));
+  CheckLooksLike(a, {{10, 15}, {20, 40}});
+}
+
+TEST_F(DisjointAllocationPoolTest, MergingSkipLargerSrcWithGap) {
+  DisjointAllocationPool a = Make({{10, 15}, {20, 25}, {30, 35}});
+  a.Release(Make({{25, 30}, {36, 40}}));
+  CheckLooksLike(a, {{10, 15}, {20, 35}, {36, 40}});
+}
+
+}  // namespace wasm
+}  // namespace internal
+}  // namespace v8