Optimize XML Codec

[adwsingh]

What behavior changes?

• New native (StAX-free) XML serializer/deserializer behind Builder.useNative(true) or -Dsmithy-java.xml-provider=smithy. StAX remains default.
• New codec-commons module with shared NumberCodec, TimestampCodec, and StripedPool. JSON/XML and Query codecs now use these instead of inlined copies.
• QueryFormSerializer rewritten with inline byte buffer + StripedPool, replacing FormUrlEncodedSink.

Why is this change needed?

Performance. StAX imposes per-element overhead from stream state machines and String allocations. The native codec does byte-level parsing/writing inline with pre-computed element name bytes and speculative in-order member lookup.

How was this validated?

• DifferentialXmlFuzzTest — native vs StAX reference produce identical output on fuzz inputs
• GeneratedModelSerdeTest — all Smithy types through code-generated shapes, parameterized over both providers
• NumberCodecTest, TimestampCodecTest, and their fuzz counterparts
• JMH benchmarks on m7i.xlarge (JDK 25, G1GC, 1G heap)

Benchmark Results

Environment: m7i.xlarge, OpenJDK 25, G1GC, 1G heap, nanosecond precision

Summary

Protocol / Direction          Median Improvement
─────────────────────────────────────────────────────────────────────
XML Deserialization           ████████████████████████████████████  -84%
XML Serialization             ██████████████████████████            -63%
Query Serialization           ██████████████████████                -54%
Query Deserialization         ████████████████████████████████████  -85%
JSON (codec-commons)          ████                                  -10%
CBOR (codec-commons)          ████                                  -10%

restXml

Benchmark                        Baseline    After     Change
────────────────────────────────────────────────────────────────────────────────
CopyObjectOutput_M                 8,169     1,258     ████████████████████ -85%
CopyObjectOutput_OutOfOrder       10,179     1,726     ████████████████████ -83%
PutObject_S                          580       207     ███████████████      -64%
PutObject_M                          603       218     ███████████████      -64%
PutObject_L                          598       225     ███████████████      -62%
GetObject_S                          508       459     ██                   -10%
GetObject_M                          486       456     █                     -6%
GetObject_L                          492       490     ░                     -1%
CopyObjectOutput_Baseline            150       140     █                     -7%
CopyObjectRequest_Baseline           252       237     █                     -6%
CopyObjectRequest_M                1,252     1,187     █                     -5%

awsQuery — Deserialization (XML response parsing)

Benchmark                        Baseline    After     Change
────────────────────────────────────────────────────────────────────────────────
GetMetricDataResponse_S           25,133     2,596     █████████████████████ -90%
GetMetricDataResponse_M           49,116     7,295     ████████████████████  -85%
GetMetricDataResponse_OutOfOrder  50,996     7,921     ████████████████████  -84%
GetMetricDataResponse_L          424,828    78,873     ███████████████████   -81%

awsQuery — Serialization (form-urlencoded request)

Benchmark                        Baseline    After     Change
────────────────────────────────────────────────────────────────────────────────
PutMetricDataRequest_M             5,074     1,911     ███████████████      -62%
GetMetricDataRequest_S             1,098       468     █████████████        -57%
PutMetricDataRequest_S               609       287     ████████████         -53%
PutMetricDataRequest_L            53,696    26,308     ████████████         -51%
GetMetricDataRequest_M             5,364     3,430     ████████             -36%
GetMetricDataRequest_L             9,337     7,506     ████                 -20%
PutMetricDataRequest_Baseline        109        99     █                     -9%

awsJson1.0

Benchmark                        Baseline    After     Change
────────────────────────────────────────────────────────────────────────────────
PutItemRequest_ShallowMap_L       12,186     8,067     ████████             -34%
PutItemRequest_MixedItem_S           656       569     ███                  -13%
PutItemRequest_ShallowMap_S          653       568     ███                  -13%
PutItemRequest_BinaryData_S          294       256     ███                  -13%
GetItemOutput_S                      831       736     ██                   -11%
GetItemOutputBinary_M              3,634     3,233     ██                   -11%
PutItemRequest_MixedItem_M         2,950     2,645     ██                   -10%
PutItemRequest_Nested_L            1,203     1,082     ██                   -10%
GetItemOutput_M                    3,198     2,887     ██                   -10%
GetItemOutputBinary_S                905       821     ██                    -9%
GetItemOutputBinary_L             25,236    22,983     ██                    -9%
GetItemOutput_L                   21,601    19,877     █                     -8%
PutItemRequest_Baseline              225       206     █                     -9%
PutItemRequest_Nested_M              446       416     █                     -7%
PutItemRequest_MixedItem_L         8,108     8,810     ░                    +9%

restJson1

Benchmark                        Baseline    After     Change
────────────────────────────────────────────────────────────────────────────────
GetObject_M                          512       450     ██                   -12%
CopyObjectOutput_M                 1,269     1,145     ██                   -10%
GetObject_S                          505       456     ██                   -10%
CopyObjectOutput_OutOfOrder        1,827     1,661     ██                    -9%
CopyObjectRequest_M                1,328     1,219     █                     -8%
PutObject_L                          958       888     █                     -7%
PutObject_S                          945       880     █                     -7%
PutObject_M                          943       880     █                     -7%
CopyObjectRequest_Baseline           252       238     █                     -6%
CopyObjectOutput_Baseline            163       155     █                     -5%
GetObject_L                          507       484     █                     -4%

rpcv2Cbor

Benchmark                        Baseline    After     Change
────────────────────────────────────────────────────────────────────────────────
PutItemRequest_Nested_L            1,160       947     ████                 -18%
PutItemRequest_Baseline              236       197     ████                 -17%
GetItemOutput_M                    2,165     1,843     ███                  -15%
PutItemRequest_MixedItem_S           608       525     ███                  -14%
GetItemOutput_L                   13,377    11,832     ██                   -12%
PutItemRequest_ShallowMap_L        8,039     7,195     ██                   -11%
GetItemOutputBinary_S                497       451     ██                    -9%
PutItemRequest_ShallowMap_S          571       521     ██                    -9%
PutItemRequest_ShallowMap_M        2,599     2,397     █                     -8%
PutItemRequest_Nested_M              434       408     █                     -6%
PutItemRequest_BinaryData_M        1,164     1,100     █                     -6%
GetItemOutput_OutOfOrder           2,290     2,172     █                     -5%
GetItemOutputBinary_M              1,931     1,834     █                     -5%
GetItemOutputBinary_L             12,028    11,553     █                     -4%
PutItemRequest_MixedItem_L         6,989     6,715     █                     -4%
PutItemRequest_BinaryData_L       19,267    18,612     ░                     -3%
GetItemOutput_S                      476       467     ░                     -2%
PutItemRequest_BinaryData_S          239       241     ░                    +1%
PutItemRequest_MixedItem_M         2,497     2,619     ░                    +5%
GetItemOutput_Baseline                52        57     ░                   +11%

What should reviewers focus on?

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