ESQL: Remove extra block calls in evaluators and aggregators (elastic#135919)

2 implementers changed:
- Aggregators: Change `isNull() + getValueCount()` to a single `getValueCount()`. `getValueCount()` delegates to `isNull()` internally for Array blocks, and they're supposed to be in sync.
- Evaluators: Same change, also removing an impossible branch (`getValueCount() == 0` after the `isNull() == false`)

These changes were made to:
- Improve readability/branch sanity, specially in the evaluators case.
- Slight performance improvement. itable accesses have been problematic in similar cases, and this change removes up to 25% of them. There are 2 cases:
  - The position is null: We change a `isNull()` to a `getValueCount()` delegating to `isNull()` in some cases
  - The position is not null: We change a `isNull() + getValueCount()` by removing the `isNull()`, which could access a `BitSet`

Aggs microbenchmarks show no differences.

Author: ivancea

x-pack/plugin/esql/compute/gen/src/main/java/org/elasticsearch/compute/gen/AggregatorImplementer.java

@@ -428,7 +428,8 @@ private MethodSpec addRawBlock(boolean masked) {
                 builder.beginControlFlow("if (mask.getBoolean(p) == false)").addStatement("continue").endControlFlow();
             }
             for (AggregationParameter p : aggParams) {
-                builder.beginControlFlow("if ($L.isNull(p))", p.blockName());
+                builder.addStatement("int $LValueCount = $L.getValueCount(p)", p.name(), p.blockName());
+                builder.beginControlFlow("if ($LValueCount == 0)", p.name());
                 builder.addStatement("continue");
                 builder.endControlFlow();
             }
@@ -447,7 +448,7 @@ private MethodSpec addRawBlock(boolean masked) {
                 }
                 for (AggregationParameter p : aggParams) {
                     builder.addStatement("int $L = $L.getFirstValueIndex(p)", p.startName(), p.blockName());
-                    builder.addStatement("int $L = $L + $L.getValueCount(p)", p.endName(), p.startName(), p.blockName());
+                    builder.addStatement("int $L = $L + $LValueCount", p.endName(), p.startName(), p.name());
                     builder.beginControlFlow(
                         "for (int $L = $L; $L < $L; $L++)",
                         p.offsetName(),

x-pack/plugin/esql/compute/gen/src/main/java/org/elasticsearch/compute/gen/argument/StandardArgument.java

@@ -148,25 +148,23 @@ public void sumBaseRamBytesUsed(MethodSpec.Builder builder) {
     }
 
     static void skipNull(MethodSpec.Builder builder, String value) {
-        builder.beginControlFlow("if ($N.isNull(p))", value);
+        builder.beginControlFlow("switch ($N.getValueCount(p))", value);
         {
-            builder.addStatement("result.appendNull()");
-            builder.addStatement("continue position");
-        }
-        builder.endControlFlow();
-        builder.beginControlFlow("if ($N.getValueCount(p) != 1)", value);
-        {
-            builder.beginControlFlow("if ($N.getValueCount(p) > 1)", value);
-            {
-                builder.addStatement(
-                    // TODO: reflection on SingleValueQuery.MULTI_VALUE_WARNING?
-                    "warnings().registerException(new $T(\"single-value function encountered multi-value\"))",
-                    IllegalArgumentException.class
-                );
-            }
-            builder.endControlFlow();
-            builder.addStatement("result.appendNull()");
-            builder.addStatement("continue position");
+            builder.addCode("case 0:\n");
+            builder.addStatement("    result.appendNull()");
+            builder.addStatement("    continue position");
+
+            builder.addCode("case 1:\n");
+            builder.addStatement("    break");
+
+            builder.addCode("default:\n");
+            builder.addStatement(
+                // TODO: try to use SingleValueQuery.MULTI_VALUE_WARNING?
+                "    warnings().registerException(new $T(\"single-value function encountered multi-value\"))",
+                IllegalArgumentException.class
+            );
+            builder.addStatement("    result.appendNull()");
+            builder.addStatement("    continue position");
         }
         builder.endControlFlow();
     }