[llvm] [Support] Add constexpr versions of Add/Sub/MulOverflow (PR #210404)

[Eli Friedman via llvm-commits]

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@@ -696,64 +697,85 @@ SaturatingMultiplyAdd(T X, T Y, T A, bool *ResultOverflowed = nullptr) {
 LLVM_ABI extern const float huge_valf;
 
 /// Add two signed integers, computing the two's complement truncated result,
-/// returning true if overflow occurred.
+/// returning a pair {result, overflow}, where "overflow" is a boolean value
+/// indicating whether an overflow occurred.
 template <typename T>
-std::enable_if_t<std::is_signed_v<T>, T> AddOverflow(T X, T Y, T &Result) {
-#if __has_builtin(__builtin_add_overflow)
-  return __builtin_add_overflow(X, Y, &Result);
-#else
+constexpr std::enable_if_t<std::is_signed_v<T>, std::pair<T, bool>>
+AddOverflow(T X, T Y) {
   // Perform the unsigned addition.
   using U = std::make_unsigned_t<T>;
   const U UX = static_cast<U>(X);
   const U UY = static_cast<U>(Y);
   const U UResult = UX + UY;
 
   // Convert to signed.
-  Result = static_cast<T>(UResult);
+  auto Result = static_cast<T>(UResult);
 
   // Adding two positive numbers should result in a positive number.
   if (X > 0 && Y > 0)
-    return Result <= 0;
+    return {Result, Result <= 0};
   // Adding two negatives should result in a negative number.
   if (X < 0 && Y < 0)
-    return Result >= 0;
-  return false;
+    return {Result, Result >= 0};
+  return {Result, false};
+}
+
+/// Add two signed integers, computing the two's complement truncated result,
+/// returning true if overflow occurred.
+template <typename T>
+std::enable_if_t<std::is_signed_v<T>, T> AddOverflow(T X, T Y, T &Result) {
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efriedma-quic wrote:

Ping

https://github.com/llvm/llvm-project/pull/210404