add LongDecimal->ShortDecimal conversion

velox/functions/prestosql/aggregates/SimpleNumericAggregate.h

@@ -147,20 +147,7 @@ class SimpleNumericAggregate : public exec::Aggregate {
       if (!decoded.isNullAt(0)) {
         updateDuplicateValues(
             initialValue, decoded.valueAt<TInput>(0), rows.countSelected());
-        // Some DECIMAL type aggregations requires conversion from LongDecimal
-        // to ShortDecimal. However, this conversion is not desired as it
-        // requires a safety check. Specialize this case instead.
-        if constexpr (
-            std::is_same<TInput, ShortDecimal>::value &&
-            std::is_same<TData, LongDecimal>::value) {
-          updateNonNullValue<true, TData>(
-              group,
-              static_cast<LongDecimal>(initialValue).unscaledValue(),
-              updateSingleValue);
-        } else {
-          updateNonNullValue<true, TData>(
-              group, initialValue, updateSingleValue);
-        }
+        updateNonNullValue<true, TData>(group, initialValue, updateSingleValue);
       }
     } else if (decoded.mayHaveNulls()) {
       rows.applyToSelected([&](vector_size_t i) {

velox/type/LongDecimal.cpp

@@ -34,5 +34,4 @@ string to_string(facebook::velox::int128_t x) {
   reverse(ans.begin(), ans.end());
   return ans;
 }
-
 } // namespace std

velox/type/LongDecimal.h

@@ -37,8 +37,7 @@ struct LongDecimal {
   // Default required for creating vector with NULL values.
   LongDecimal() = default;
   constexpr LongDecimal(int128_t value) : unscaledValue_(value) {}
-  constexpr LongDecimal(ShortDecimal value)
-      : unscaledValue_(value.unscaledValue()) {}
+  LongDecimal(ShortDecimal& value) : unscaledValue_(value.unscaledValue()) {}
 
   int128_t unscaledValue() const {
     return unscaledValue_;
@@ -78,6 +77,12 @@ static inline LongDecimal operator*(const int value, const LongDecimal& other) {
   return LongDecimal(value * other.unscaledValue());
 }
 
+inline ShortDecimal::ShortDecimal(LongDecimal& other) {
+  // Ensure the value fits.
+  VELOX_DCHECK((other.unscaledValue() >> 64) == 0);
+  unscaledValue_ = other.unscaledValue();
+}
+
 } // namespace facebook::velox
 
 namespace folly {

velox/type/ShortDecimal.h

@@ -23,11 +23,14 @@
 
 namespace facebook::velox {
 
+struct LongDecimal;
+
 struct ShortDecimal {
  public:
   // Default required for creating vector with NULL values.
   ShortDecimal() = default;
   constexpr ShortDecimal(int64_t value) : unscaledValue_(value) {}
+  ShortDecimal(LongDecimal& value);
 
   int64_t unscaledValue() const {
     return unscaledValue_;