[PR] perf: Optimize `lcd`, `gcd` NULL handling [datafusion]

["neilconway (via GitHub)" <gi...@apache.org>]

neilconway opened a new pull request, #21468:
URL: https://github.com/apache/datafusion/pull/21468

   ## Which issue does this PR close?
   
   - Closes #21467.
   
   ## Rationale for this change
   
   This PR implements three distinct optimizations:
   
   1. `lcm` was computing the result NULL buffer iteratively. This is relatively slow. Switching to Arrow's `try_binary` kernel both makes the implementation more concise and also improves performance by computing the result NULL buffer via the bitwise union of the input NULL buffers.
   2. The `gcd` scalar arg path was doing similarly; switching to Arrow's `try_unary` yields a similar speedup.
   3. For the `gcd` scalar path, computing the GCD can only fail in a few edge cases (e.g., `gcd(i64::MIN, i64::MIN)`). It is cheap to check for these edge-cases; for most `gcd` inputs, we can use Arrow's `unary` kernel instead of `try_unary`. The former is more efficient because it allows LLVM to vectorize the code more effectively.
   
   Benchmarks (ARM64):
   ```
     - gcd array and scalar: 2.9ms → 2.2ms, -25% faster
     - lcm both array: 2.7ms → 2.0ms, -26% faster
   ```
   
   ## What changes are included in this PR?
   
   * Add benchmark for `lcd`
   * Improve SLT test coverage
   * Move Rust unit test for `lcd` to SLT
   * Optimize `lcd` and `gcm` NULL handling
   * Optimize `gcm` to avoid overhead for edge cases
   
   ## Are these changes tested?
   
   Yes.
   
   ## Are there any user-facing changes?
   
   No.
   


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Re: [PR] perf: Optimize NULL handling in `lcm`, `gcd` [datafusion]

["mbutrovich (via GitHub)" <gi...@apache.org>]

mbutrovich merged PR #21468:
URL: https://github.com/apache/datafusion/pull/21468


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Re: [PR] perf: Optimize NULL handling in `lcm`, `gcd` [datafusion]

["mbutrovich (via GitHub)" <gi...@apache.org>]

mbutrovich commented on code in PR #21468:
URL: https://github.com/apache/datafusion/pull/21468#discussion_r3065352325


##########
datafusion/functions/src/math/gcd.rs:
##########
@@ -126,18 +126,23 @@ fn compute_gcd_for_arrays(a: &ArrayRef, b: &ArrayRef) -> Result<ColumnarValue> {
 }
 
 fn compute_gcd_with_scalar(arr: &ArrayRef, scalar: Option<i64>) -> Result<ColumnarValue> {
+    let prim = arr.as_primitive::<Int64Type>();
     match scalar {
+        Some(scalar_value) if scalar_value != 0 && scalar_value != i64::MIN => {
+            // The gcd result divides both inputs' absolute values. When the

Review Comment:
   This is a slick optimization.



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