From 4ddab8901cb7d8bafb7ad6fe15bab46635734e32 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Thu, 17 Feb 2022 21:24:10 -0800
Subject: [PATCH] Merge accumulator into int128_fallback

---
 include/fmt/format-inl.h | 29 ++---------------------------
 include/fmt/format.h     | 17 +++++++++++++----
 test/format-impl-test.cc | 17 -----------------
 3 files changed, 15 insertions(+), 48 deletions(-)

diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 99be0d4d..c4340cf4 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -142,7 +142,7 @@ FMT_FUNC std::system_error vsystem_error(int error_code, string_view format_str,
 namespace detail {
 
 template <> FMT_FUNC int count_digits<4>(detail::uintptr_fallback n) {
-  // fallback_uintptr is always stored in little endian.
+  // uintptr_fallback is always stored in little endian.
   int i = static_cast<int>(sizeof(void*)) - 1;
   while (i > 0 && n.value[i] == 0) --i;
   auto char_digits = std::numeric_limits<unsigned char>::digits / 4;
@@ -347,29 +347,6 @@ FMT_CONSTEXPR inline fp get_cached_power(int min_exponent,
           impl_data::pow10_exponents[index]};
 }
 
-// A simple accumulator to hold the sums of terms in bigint::square if uint128_t
-// is not available.
-struct accumulator {
-  uint64_t lower;
-  uint64_t upper;
-
-  constexpr accumulator() : lower(0), upper(0) {}
-  constexpr explicit operator uint32_t() const {
-    return static_cast<uint32_t>(lower);
-  }
-
-  FMT_CONSTEXPR void operator+=(uint64_t n) {
-    lower += n;
-    if (lower < n) ++upper;
-  }
-  FMT_CONSTEXPR void operator>>=(int shift) {
-    FMT_ASSERT(shift == 32, "");
-    (void)shift;
-    lower = (upper << 32) | (lower >> 32);
-    upper >>= 32;
-  }
-};
-
 class bigint {
  private:
   // A bigint is stored as an array of bigits (big digits), with bigit at index
@@ -558,9 +535,7 @@ class bigint {
     int num_result_bigits = 2 * num_bigits;
     basic_memory_buffer<bigit, bigits_capacity> n(std::move(bigits_));
     bigits_.resize(to_unsigned(num_result_bigits));
-    using accumulator_t =
-        conditional_t<FMT_USE_INT128, uint128_opt, accumulator>;
-    auto sum = accumulator_t();
+    auto sum = uint128_t();
     for (int bigit_index = 0; bigit_index < num_bigits; ++bigit_index) {
       // Compute bigit at position bigit_index of the result by adding
       // cross-product terms n[i] * n[j] such that i + j == bigit_index.
diff --git a/include/fmt/format.h b/include/fmt/format.h
index a32c2b53..2ac466ff 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -346,8 +346,12 @@ class uint128_fallback {
 
  public:
   constexpr uint128_fallback(uint64_t value = 0) : lo_(value), hi_(0) {}
-  explicit operator int() const { return static_cast<int>(lo_); }
-  explicit operator uint64_t() const { return lo_; }
+
+  template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+  constexpr explicit operator T() const {
+    return static_cast<T>(lo_);
+  }
+
   friend auto operator==(const uint128_fallback& lhs,
                          const uint128_fallback& rhs) -> bool {
     return lhs.hi_ == rhs.hi_ && lhs.lo_ == rhs.lo_;
@@ -361,14 +365,19 @@ class uint128_fallback {
     FMT_ASSERT(lhs.lo_ >= rhs, "");
     return {lhs.hi_, lhs.lo_ - rhs};
   }
-  auto operator>>(int shift) const -> uint128_fallback {
+  FMT_CONSTEXPR auto operator>>(int shift) const -> uint128_fallback {
     if (shift == 64) return {0, hi_};
     return {hi_ >> shift, (hi_ << (64 - shift)) | (lo_ >> shift)};
   }
-  auto operator<<(int shift) const -> uint128_fallback {
+  FMT_CONSTEXPR auto operator<<(int shift) const -> uint128_fallback {
     if (shift == 64) return {lo_, 0};
     return {hi_ << shift | (lo_ >> (64 - shift)), (lo_ << shift)};
   }
+  FMT_CONSTEXPR void operator>>=(int shift) { *this = *this >> shift; }
+  FMT_CONSTEXPR void operator+=(uint64_t n) {
+    lo_ += n;
+    if (lo_ < n) ++hi_;
+  }
 };
 
 using uint128_t = conditional_t<FMT_USE_INT128, uint128_opt, uint128_fallback>;
diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
index 90105b5a..194cfa92 100644
--- a/test/format-impl-test.cc
+++ b/test/format-impl-test.cc
@@ -96,23 +96,6 @@ TEST(bigint_test, multiply) {
   EXPECT_EQ("fffffffffffffffe0000000000000001", fmt::format("{}", bigmax));
 }
 
-TEST(bigint_test, accumulator) {
-  fmt::detail::accumulator acc;
-  EXPECT_EQ(acc.lower, 0);
-  EXPECT_EQ(acc.upper, 0);
-  acc.upper = 12;
-  acc.lower = 34;
-  EXPECT_EQ(static_cast<uint32_t>(acc), 34);
-  acc += 56;
-  EXPECT_EQ(acc.lower, 90);
-  acc += max_value<uint64_t>();
-  EXPECT_EQ(acc.upper, 13);
-  EXPECT_EQ(acc.lower, 89);
-  acc >>= 32;
-  EXPECT_EQ(acc.upper, 0);
-  EXPECT_EQ(acc.lower, 13 * 0x100000000);
-}
-
 TEST(bigint_test, square) {
   bigint n0(0);
   n0.square();