ART: Compress LengthPrefixedArray on 32-bit targets.
Previously, the LengthPrefixedArray<ArtMethod> on 32-bit
targets contained a 64-bit length field followed by the
ArtMethod elements with size only a multiple of 4, not 8.
Consequently, an odd-length array broke the alignment for
the following array which would have the 64-bit length
placed at an unaligned address.
To fix that, we make the length field 32-bit and explicitly
pass the alignment information to the LengthPrefixedArray.
This also makes the 32-bit boot image a bit smaller.
On Nexus 5, AOSP, ToT, the field section is 11528B smaller
and the method section is 21036B smaller. 64-bit targets
should see the same savings for the field section but no
difference for the methods section.
Change-Id: I3e03e7b94129025c8a1c117c27645a34dec516d2
diff --git a/runtime/length_prefixed_array.h b/runtime/length_prefixed_array.h
index 2b2e8d3..d9bc656 100644
--- a/runtime/length_prefixed_array.h
+++ b/runtime/length_prefixed_array.h
@@ -21,6 +21,8 @@
#include "linear_alloc.h"
#include "stride_iterator.h"
+#include "base/bit_utils.h"
+#include "base/casts.h"
#include "base/iteration_range.h"
namespace art {
@@ -28,29 +30,35 @@
template<typename T>
class LengthPrefixedArray {
public:
- explicit LengthPrefixedArray(uint64_t length) : length_(length) {}
+ explicit LengthPrefixedArray(size_t length)
+ : length_(dchecked_integral_cast<uint32_t>(length)) {}
- T& At(size_t index, size_t element_size = sizeof(T)) {
+ T& At(size_t index, size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
DCHECK_LT(index, length_);
- return *reinterpret_cast<T*>(&data_[0] + index * element_size);
+ return AtUnchecked(index, element_size, alignment);
}
- StrideIterator<T> Begin(size_t element_size = sizeof(T)) {
- return StrideIterator<T>(reinterpret_cast<T*>(&data_[0]), element_size);
+ StrideIterator<T> Begin(size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
+ return StrideIterator<T>(&AtUnchecked(0, element_size, alignment), element_size);
}
- StrideIterator<T> End(size_t element_size = sizeof(T)) {
- return StrideIterator<T>(reinterpret_cast<T*>(&data_[0] + element_size * length_),
- element_size);
+ StrideIterator<T> End(size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
+ return StrideIterator<T>(&AtUnchecked(length_, element_size, alignment), element_size);
}
- static size_t OffsetOfElement(size_t index, size_t element_size = sizeof(T)) {
- return offsetof(LengthPrefixedArray<T>, data_) + index * element_size;
+ static size_t OffsetOfElement(size_t index,
+ size_t element_size = sizeof(T),
+ size_t alignment = alignof(T)) {
+ DCHECK_ALIGNED_PARAM(element_size, alignment);
+ return RoundUp(offsetof(LengthPrefixedArray<T>, data), alignment) + index * element_size;
}
- // Alignment is the caller's responsibility.
- static size_t ComputeSize(size_t num_elements, size_t element_size = sizeof(T)) {
- return OffsetOfElement(num_elements, element_size);
+ static size_t ComputeSize(size_t num_elements,
+ size_t element_size = sizeof(T),
+ size_t alignment = alignof(T)) {
+ size_t result = OffsetOfElement(num_elements, element_size, alignment);
+ DCHECK_ALIGNED_PARAM(result, alignment);
+ return result;
}
uint64_t Length() const {
@@ -58,21 +66,26 @@
}
// Update the length but does not reallocate storage.
- void SetLength(uint64_t length) {
- length_ = length;
+ void SetLength(size_t length) {
+ length_ = dchecked_integral_cast<uint32_t>(length);
}
private:
- uint64_t length_; // 64 bits for 8 byte alignment of data_.
- uint8_t data_[0];
+ T& AtUnchecked(size_t index, size_t element_size, size_t alignment) {
+ return *reinterpret_cast<T*>(
+ reinterpret_cast<uintptr_t>(this) + OffsetOfElement(index, element_size, alignment));
+ }
+
+ uint32_t length_;
+ uint8_t data[0];
};
// Returns empty iteration range if the array is null.
template<typename T>
IterationRange<StrideIterator<T>> MakeIterationRangeFromLengthPrefixedArray(
- LengthPrefixedArray<T>* arr, size_t element_size) {
+ LengthPrefixedArray<T>* arr, size_t element_size = sizeof(T), size_t alignment = alignof(T)) {
return arr != nullptr ?
- MakeIterationRange(arr->Begin(element_size), arr->End(element_size)) :
+ MakeIterationRange(arr->Begin(element_size, alignment), arr->End(element_size, alignment)) :
MakeEmptyIterationRange(StrideIterator<T>(nullptr, 0));
}