perf: Optimize some decimal expressions

[andygrove]

Summary

For some decimal operations, we cast the inputs to Decimal256, perform the math operation, and then cast the result back to Decimal128:

let left = Arc::new(Cast::new(
    left,
    DataType::Decimal256(p1, s1),
    SparkCastOptions::new_without_timezone(EvalMode::Legacy, false),
));
let right = Arc::new(Cast::new(
    right,
    DataType::Decimal256(p2, s2),
    SparkCastOptions::new_without_timezone(EvalMode::Legacy, false),
));
let child = Arc::new(BinaryExpr::new(left, op, right));
Ok(Arc::new(Cast::new(
    child,
    data_type,
    SparkCastOptions::new_without_timezone(EvalMode::Legacy, false),
)))

This causes intermediate allocations and can be optimized with a custom WideDecimalBinaryExpr that performs i256 register arithmetic directly, reducing per-batch allocation from 4 intermediate arrays (3 Decimal256 @ 32 bytes/elem + 1 Decimal128 @ 16 bytes/elem = 112 bytes/elem) to 1 output array (16 bytes/elem)

TPC-H q1

Before:

        "durations": [
            8.183,
            4.728,
            4.597,
            4.562,
            4.538
        ],

After:

        "durations": [
            7.874,
            4.327,
            4.185,
            4.135,
            4.069
        ],

Criterion benchmark results (8192 element batches)

Case	Old	Fused	Speedup
add (same scale)	171 µs	57 µs	3.0x
add (diff scale)	173 µs	57 µs	3.0x
multiply	361 µs	305 µs	1.2x
subtract	173 µs	58 µs	3.0x

How it works

WideDecimalBinaryExpr evaluates left/right children, performs add/sub/mul using i256 intermediates via arrow::compute::kernels::arity::try_binary, applies scale adjustment with HALF_UP rounding, checks precision bounds, and outputs a single Decimal128 array. Follows the same pattern as decimal_div in div.rs.

Overflow handling matches existing behavior:

• Ansi mode: returns ArrowError::ComputeError
• Legacy/Try mode: uses i128::MAX sentinel + null_if_overflow_precision

[andygrove]

@sqlbenchmark run tpch --iterations 3

[andygrove]

@sqlbenchmark run tpch --iterations 3

[andygrove]

@sqlbenchmark run tpch --iterations 3

[sqlbenchmark]

Benchmark job comet-pr-3619-c3986837690 failed due to an error.

[coderfender]

This is awesome !

[parthchandra]

Don't we need to scale up to s_out if s_out > max_scale?