chromium/third_party/protobuf/src/google/protobuf/compiler/cpp/padding_optimizer.cc

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#include <google/protobuf/compiler/cpp/padding_optimizer.h>

#include <google/protobuf/compiler/cpp/helpers.h>

namespace google {
namespace protobuf {
namespace compiler {
namespace cpp {

namespace {

// FieldGroup is just a helper for PaddingOptimizer below. It holds a vector of
// fields that are grouped together because they have compatible alignment, and
// a preferred location in the final field ordering.
class FieldGroup { … };

}  // namespace

// Reorder 'fields' so that if the fields are output into a c++ class in the new
// order, fields of similar family (see below) are together and within each
// family, alignment padding is minimized.
//
// We try to do this while keeping each field as close as possible to its field
// number order so that we don't reduce cache locality much for function that
// access each field in order.  Originally, OptimizePadding used declaration
// order for its decisions, but generated code minus the serializer/parsers uses
// the output of OptimizePadding as well (stored in
// MessageGenerator::optimized_order_).  Since the serializers use field number
// order, we use that as a tie-breaker.
//
// We classify each field into a particular "family" of fields, that we perform
// the same operation on in our generated functions.
//
// REPEATED is placed first, as the C++ compiler automatically initializes
// these fields in layout order.
//
// STRING is grouped next, as our Clear/SharedCtor/SharedDtor walks it and
// calls ArenaStringPtr::Destroy on each.
//
// LAZY_MESSAGE is grouped next, as it interferes with the ability to memset
// non-repeated fields otherwise.
//
// MESSAGE is grouped next, as our Clear/SharedDtor code walks it and calls
// delete on each.  We initialize these fields with a NULL pointer (see
// MessageFieldGenerator::GenerateConstructorCode), which allows them to be
// memset.
//
// ZERO_INITIALIZABLE is memset in Clear/SharedCtor
//
// OTHER these fields are initialized one-by-one.
void PaddingOptimizer::OptimizeLayout(
    std::vector<const FieldDescriptor*>* fields, const Options& options,
    MessageSCCAnalyzer* scc_analyzer) { … }

}  // namespace cpp
}  // namespace compiler
}  // namespace protobuf
}  // namespace google