Avoid ListFields and heap allocation in proto2::internal::IsDescendant. - #1204
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IsDescendant is called twice by ABSL_DCHECK in every Message::CopyFrom and generated CopyFrom in non-opt builds (2.15% mean / 3.10% pooled inclusive CPU across k8-fastbuild Forge tests). Previously, IsDescendant allocated a std::vector<const FieldDescriptor*> and called Reflection::ListFields to collect and sort all set fields (including scalar, enum, and string fields, and looking up extension FieldDescriptors in DescriptorPool), only to skip all non-message fields. Instead, short-circuit on default instances, scan descriptor fields directly skipping non-CPPTYPE_MESSAGE fields before checking presence, and inspect ExtensionSet entries in place via AnyOfNoPrefetch without allocating or sorting a field vector. PiperOrigin-RevId: 988534153
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Avoid ListFields and heap allocation in proto2::internal::IsDescendant.
IsDescendant is called twice by ABSL_DCHECK in every Message::CopyFrom and
generated CopyFrom in non-opt builds (2.15% mean / 3.10% pooled inclusive CPU
across k8-fastbuild Forge tests). Previously, IsDescendant allocated a
std::vector<const FieldDescriptor*> and called Reflection::ListFields to
collect and sort all set fields (including scalar, enum, and string fields, and
looking up extension FieldDescriptors in DescriptorPool), only to skip all
non-message fields.
Instead, short-circuit on default instances, scan descriptor fields directly
skipping non-CPPTYPE_MESSAGE fields before checking presence, and inspect
ExtensionSet entries in place via AnyOfNoPrefetch without allocating or sorting
a field vector.