[libc-commits] [libc] [libc] Make add_with_carry and sub_with_borrow constexpr. (PR #81898)

[via libc-commits]

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@@ -69,48 +67,68 @@ add_with_carry(T a, T b, T carry_in) {
 // https://clang.llvm.org/docs/LanguageExtensions.html#multiprecision-arithmetic-builtins
 
 template <>
-LIBC_INLINE SumCarry<unsigned char>
+LIBC_INLINE constexpr SumCarry<unsigned char>
 add_with_carry<unsigned char>(unsigned char a, unsigned char b,
                               unsigned char carry_in) {
-  SumCarry<unsigned char> result{0, 0};
-  result.sum = __builtin_addcb(a, b, carry_in, &result.carry);
-  return result;
+  if (__builtin_is_constant_evaluated()) {
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lntue wrote:

I've checked on godbolt, and seems like it works fine on clang 9.0+ and gcc 11+ in all x86-64, arm32, and riscv-32 

https://github.com/llvm/llvm-project/pull/81898