GH-51268: [C++][Parquet] Avoid reloading DELTA_BINARY_PACKED decoder state for every value - #51249
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GH-51268: [C++][Parquet] Avoid reloading DELTA_BINARY_PACKED decoder state for every value#51249prtkgaur wants to merge 1 commit into
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…every value min_delta_ and last_value_ have the same type as GetInternal's output buffer, and that buffer points into memory the caller owns, so the compiler cannot prove the prefix-sum store does not land on either member: it reloads the frame and stores the running value on every value. On aarch64 with GCC 11.5 the loop body is 8 instructions with 4 memory operations per value, where 6 and 2 are enough. Hold the running value and the frame in locals across the loop and write last_value_ back once when it ends. The arithmetic is unchanged - every term stays in the unsigned type, so the wrapping the existing comment documents is preserved and decoded values are identical. On the DELTA_BINARY_PACKED decode benchmarks already in the tree this is 1.26x to 1.28x wherever the running sum is a meaningful share of the work.
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Rationale for this change
min_delta_andlast_value_are members of the same type asGetInternal's output buffer,and that buffer points into memory the caller owns, so the compiler cannot prove the prefix-sum
store does not land on either member. It reloads the frame and stores the running value on
every value: on aarch64 the loop body is 8 instructions with 4 memory operations per value,
where 6 and 2 are enough.
What changes are included in this PR?
In the non-zero-bit-width branch of
DeltaBitPackDecoder<DType>::GetInternal, hold the runningvalue and the frame in locals across the loop and write
last_value_back once when the loopends. The arithmetic is unchanged -- every term stays in the unsigned type, so the documented
wrapping behaviour is preserved and no decoded value changes.
Are these changes tested?
By the existing tests: this is a compilation concern, not a behavioural one, and
parquet-encoding-testalready round trips DELTA_BINARY_PACKED over both integer widthsincluding the overflow cases that exercise the wrapping.
Benchmark
parquet-encoding-benchmark, the DELTA_BINARY_PACKED decode arms already in the tree. AWSGraviton4, GCC 11.5,
Release, pinned to one core, 9 repetitions, medians, 65,536 values; mainand this PR built and measured in the same sitting. The two
Fixedarms are thedelta_bit_width_ == 0fast path, which this diff cannot execute.Decode_Int32_NarrowDecode_Int32_WideDecode_Int64_NarrowDecode_Int64_WideDecode_Int32_FixedDecode_Int64_FixedAre there any user-facing changes?
No. No API change, no format change, and decoded values are identical.