emkornfield commented on a change in pull request #8279:
URL: https://github.com/apache/arrow/pull/8279#discussion_r497247327
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File path: cpp/src/arrow/util/basic_decimal.cc
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@@ -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) {
+#ifdef ARROW_USE_NATIVE_INT128
+ const __uint128_t r = static_cast<__uint128_t>(x) * y;
+ *lo = r & kInt64Mask;
+ *hi = r >> 64;
+#else
+ const uint64_t x_lo = x & kIntMask;
+ const uint64_t y_lo = y & kIntMask;
+ const uint64_t x_hi = x >> 32;
+ const uint64_t y_hi = y >> 32;
- uint64_t product = L3 * R3;
- low_bits_ = product & kIntMask;
+ const uint64_t t = x_lo * y_lo;
+ const uint64_t t_lo = t & kIntMask;
+ const uint64_t t_hi = t >> 32;
- uint64_t sum = product >> 32;
+ const uint64_t u = x_hi * y_lo + t_hi;
+ const uint64_t u_lo = u & kIntMask;
+ const uint64_t u_hi = u >> 32;
- product = L2 * R3;
- sum += product;
- high_bits_ = static_cast<int64_t>(sum < product ? kCarryBit : 0);
+ const uint64_t v = x_lo * y_hi + u_lo;
+ const uint64_t v_hi = v >> 32;
- product = L3 * R2;
- sum += product;
+ *hi = x_hi * y_hi + u_hi + v_hi;
+ *lo = (v << 32) + t_lo;
+#endif
+}
+#endif
- low_bits_ += sum << 32;
+void MultiplyUint128(uint64_t x_hi, uint64_t x_lo, uint64_t y_hi, uint64_t
y_lo,
+ uint64_t* hi, uint64_t* lo) {
+#ifdef ARROW_USE_NATIVE_INT128
+ const __uint128_t x = (static_cast<__uint128_t>(x_hi) << 64) | x_lo;
+ const __uint128_t y = (static_cast<__uint128_t>(y_hi) << 64) | y_lo;
+ const __uint128_t r = x * y;
+ *lo = r & kInt64Mask;
+ *hi = r >> 64;
+#else
+ ExtendAndMultiplyUint64(x_lo, y_lo, hi, lo);
+ *hi += (x_hi * y_lo) + (x_lo * y_hi);
Review comment:
can you comment on why hi is hi is being reset here (and lo isn't being
used at all?)
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