[libc-commits] [libc] [libc] Optimize BigInt→decimal in IntegerToString (PR #123580)

[via libc-commits]

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@@ -164,6 +164,168 @@ template <size_t radix> using Custom = details::Fmt<radix>;
 
 } // namespace radix
 
+// Extract the low-order decimal digit from a value of integer type T. The
+// returned value is the digit itself, from 0 to 9. The input value is passed
+// by reference, and modified by dividing by 10, so that iterating this
+// function extracts all the digits of the original number one at a time from
+// low to high.
+template <typename T, cpp::enable_if_t<cpp::is_integral_v<T>, int> = 0>
+LIBC_INLINE uint8_t extract_decimal_digit(T &value) {
+  const uint8_t digit(static_cast<uint8_t>(value % 10));
+  // For built-in integer types, we assume that an adequately fast division is
+  // available. If hardware division isn't implemented, then with a divisor
+  // known at compile time the compiler might be able to generate an optimized
+  // sequence instead.
+  value /= 10;
+  return digit;
+}
+
+// A specialization of extract_decimal_digit for the BigInt type in big_int.h,
+// avoiding the use of general-purpose BigInt division which is very slow.
+template <typename T, cpp::enable_if_t<is_big_int_v<T>, int> = 0>
+LIBC_INLINE uint8_t extract_decimal_digit(T &value) {
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lntue wrote:

ditto.

https://github.com/llvm/llvm-project/pull/123580