ART: Implement a fixed size string dex cache
Previously, the string dex cache was dex_file->NumStringIds() size, and
@ruhler found that only ~1% of that cache was ever getting filled. Since
many of these string dex caches were previously 100,000+ indices in
length, we're wasting a few hundred KB per app by storing null pointers.
The intent of this project was to reduce the space the string dex cache
is using, while not regressing on time that much. This is the first of a
few CLs, which implements the new fixed size array and disables the
compiled code so it always goes slow path. In four other CLs, I
implemented a "medium path" that regresses from the previous "fast path"
only a bit in assembly in the entrypoints. @vmarko will introduce new
compiled code in the future so that we ultimately won't be regressing on
time at all. Overall, space savings have been confirmed as on the order
of 100 KB per application.
A 4-5% slow down in art-opt on Golem, and no noticeable slow down in the
interpreter. The opt slow down should be diminished once the new
compiled code is introduced.
Test: m test-art-host
Bug: 20323084
Change-Id: Ic654a1fb9c1ae127dde59290bf36a23edb55ca8e
diff --git a/runtime/class_linker.cc b/runtime/class_linker.cc
index 4d48da6..9b0fb7d 100644
--- a/runtime/class_linker.cc
+++ b/runtime/class_linker.cc
@@ -66,6 +66,7 @@
#include "mirror/class.h"
#include "mirror/class-inl.h"
#include "mirror/class_loader.h"
+#include "mirror/dex_cache.h"
#include "mirror/dex_cache-inl.h"
#include "mirror/field.h"
#include "mirror/iftable-inl.h"
@@ -1271,7 +1272,10 @@
// If the oat file expects the dex cache arrays to be in the BSS, then allocate there and
// copy over the arrays.
DCHECK(dex_file != nullptr);
- const size_t num_strings = dex_file->NumStringIds();
+ size_t num_strings = mirror::DexCache::kDexCacheStringCacheSize;
+ if (dex_file->NumStringIds() < num_strings) {
+ num_strings = dex_file->NumStringIds();
+ }
const size_t num_types = dex_file->NumTypeIds();
const size_t num_methods = dex_file->NumMethodIds();
const size_t num_fields = dex_file->NumFieldIds();
@@ -1281,16 +1285,17 @@
CHECK_EQ(num_fields, dex_cache->NumResolvedFields());
DexCacheArraysLayout layout(image_pointer_size_, dex_file);
uint8_t* const raw_arrays = oat_dex_file->GetDexCacheArrays();
- // The space is not yet visible to the GC, we can avoid the read barriers and use
- // std::copy_n.
if (num_strings != 0u) {
- GcRoot<mirror::String>* const image_resolved_strings = dex_cache->GetStrings();
- GcRoot<mirror::String>* const strings =
- reinterpret_cast<GcRoot<mirror::String>*>(raw_arrays + layout.StringsOffset());
- for (size_t j = 0; kIsDebugBuild && j < num_strings; ++j) {
- DCHECK(strings[j].IsNull());
+ mirror::StringDexCacheType* const image_resolved_strings = dex_cache->GetStrings();
+ mirror::StringDexCacheType* const strings =
+ reinterpret_cast<mirror::StringDexCacheType*>(raw_arrays + layout.StringsOffset());
+ for (size_t j = 0; j < num_strings; ++j) {
+ DCHECK_EQ(strings[j].load(std::memory_order_relaxed).string_index, 0u);
+ DCHECK(strings[j].load(std::memory_order_relaxed).string_pointer.IsNull());
+ strings[j].store(image_resolved_strings[j].load(std::memory_order_relaxed),
+ std::memory_order_relaxed);
}
- std::copy_n(image_resolved_strings, num_strings, strings);
+ mirror::StringDexCachePair::Initialize(strings);
dex_cache->SetStrings(strings);
}
if (num_types != 0u) {
@@ -1473,14 +1478,14 @@
bool operator()(mirror::Class* klass) const SHARED_REQUIRES(Locks::mutator_lock_) {
if (forward_strings_) {
- GcRoot<mirror::String>* strings = klass->GetDexCacheStrings();
+ mirror::StringDexCacheType* strings = klass->GetDexCacheStrings();
if (strings != nullptr) {
DCHECK(
space_->GetImageHeader().GetImageSection(ImageHeader::kSectionDexCacheArrays).Contains(
reinterpret_cast<uint8_t*>(strings) - space_->Begin()))
<< "String dex cache array for " << PrettyClass(klass) << " is not in app image";
// Dex caches have already been updated, so take the strings pointer from there.
- GcRoot<mirror::String>* new_strings = klass->GetDexCache()->GetStrings();
+ mirror::StringDexCacheType* new_strings = klass->GetDexCache()->GetStrings();
DCHECK_NE(strings, new_strings);
klass->SetDexCacheStrings(new_strings);
}
@@ -2079,18 +2084,27 @@
// Zero-initialized.
raw_arrays = reinterpret_cast<uint8_t*>(linear_alloc->Alloc(self, layout.Size()));
}
- GcRoot<mirror::String>* strings = (dex_file.NumStringIds() == 0u) ? nullptr :
- reinterpret_cast<GcRoot<mirror::String>*>(raw_arrays + layout.StringsOffset());
+ mirror::StringDexCacheType* strings = (dex_file.NumStringIds() == 0u) ? nullptr :
+ reinterpret_cast<mirror::StringDexCacheType*>(raw_arrays + layout.StringsOffset());
GcRoot<mirror::Class>* types = (dex_file.NumTypeIds() == 0u) ? nullptr :
reinterpret_cast<GcRoot<mirror::Class>*>(raw_arrays + layout.TypesOffset());
ArtMethod** methods = (dex_file.NumMethodIds() == 0u) ? nullptr :
reinterpret_cast<ArtMethod**>(raw_arrays + layout.MethodsOffset());
ArtField** fields = (dex_file.NumFieldIds() == 0u) ? nullptr :
reinterpret_cast<ArtField**>(raw_arrays + layout.FieldsOffset());
+ size_t num_strings = mirror::DexCache::kDexCacheStringCacheSize;
+ if (dex_file.NumStringIds() < num_strings) {
+ num_strings = dex_file.NumStringIds();
+ }
+ DCHECK_ALIGNED(strings, alignof(mirror::StringDexCacheType)) <<
+ "Expected strings to align to StringDexCacheType.";
+ static_assert(alignof(mirror::StringDexCacheType) == 8u,
+ "Expected StringDexCacheType to have align of 8.");
if (kIsDebugBuild) {
// Sanity check to make sure all the dex cache arrays are empty. b/28992179
- for (size_t i = 0; i < dex_file.NumStringIds(); ++i) {
- CHECK(strings[i].Read<kWithoutReadBarrier>() == nullptr);
+ for (size_t i = 0; i < num_strings; ++i) {
+ CHECK_EQ(strings[i].load(std::memory_order_relaxed).string_index, 0u);
+ CHECK(strings[i].load(std::memory_order_relaxed).string_pointer.IsNull());
}
for (size_t i = 0; i < dex_file.NumTypeIds(); ++i) {
CHECK(types[i].Read<kWithoutReadBarrier>() == nullptr);
@@ -2102,10 +2116,13 @@
CHECK(mirror::DexCache::GetElementPtrSize(fields, i, image_pointer_size_) == nullptr);
}
}
+ if (strings != nullptr) {
+ mirror::StringDexCachePair::Initialize(strings);
+ }
dex_cache->Init(&dex_file,
location.Get(),
strings,
- dex_file.NumStringIds(),
+ num_strings,
types,
dex_file.NumTypeIds(),
methods,