[GitHub] [arrow] emkornfield commented on a change in pull request #8279: ARROW-10102: [C++] Refactor BasicDecimal128 Multiplication to use unsigned helper

[GitBox <gi...@apache.org>]

emkornfield commented on a change in pull request #8279:
URL: https://github.com/apache/arrow/pull/8279#discussion_r497242687



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File path: cpp/src/arrow/util/basic_decimal.cc
##########
@@ -248,40 +252,67 @@ BasicDecimal128& BasicDecimal128::operator>>=(uint32_t bits) {
   return *this;
 }
 
-BasicDecimal128& BasicDecimal128::operator*=(const BasicDecimal128& right) {
-  // Break the left and right numbers into 32 bit chunks
-  // so that we can multiply them without overflow.
-  const uint64_t L0 = static_cast<uint64_t>(high_bits_) >> 32;
-  const uint64_t L1 = static_cast<uint64_t>(high_bits_) & kIntMask;
-  const uint64_t L2 = low_bits_ >> 32;
-  const uint64_t L3 = low_bits_ & kIntMask;
+namespace {
 
-  const uint64_t R0 = static_cast<uint64_t>(right.high_bits_) >> 32;
-  const uint64_t R1 = static_cast<uint64_t>(right.high_bits_) & kIntMask;
-  const uint64_t R2 = right.low_bits_ >> 32;
-  const uint64_t R3 = right.low_bits_ & kIntMask;
+// TODO: Remove this guard once it's used by BasicDecimal256
+#ifndef ARROW_USE_NATIVE_INT128
+void ExtendAndMultiplyUint64(uint64_t x, uint64_t y, uint64_t* hi, uint64_t* lo) {

Review comment:
       docs on what hi and lo would be useful.




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