[libc] Add sub_with_borrow to builtin_wrapper.h

[lntue]

Summary

Add sub_with_borrow to builtin_wrapper.h to be used in UInt.

[sivachandra]

libc/src/__support/builtin_wrappers.h

127

Why not use number_pair for the return value?

[lntue]

libc/src/__support/builtin_wrappers.h

127

Since these are kind of built-in wrappers, I want to kind of preserve how the builtins are defined. Also the number_pair for subtraction is actually a bit odd, since then the carry/borrow/high part is actually of negative sign, while the result/low part is of positive sign.

[lntue]

Add DEFINE_NAMED_PAIR_TEMPLATE and update return values for builtins wrappers.

[sivachandra]

Thanks for cleaning up other places also.

libc/src/__support/builtin_wrappers.h

 inline unsigned long long
 add_with_carry<unsigned long long>(unsigned long long a, unsigned long long b,
                                    unsigned long long carry_in,
                                    unsigned long long &carry_out) {
   return __builtin_addcll(a, b, carry_in, &carry_out);
 }
 #endif // __has_builtin(__builtin_addc)
 
+// Subtract with borrow
+template <typename T>
+inline constexpr cpp::enable_if_t<
+    cpp::is_integral_v<T> && cpp::is_unsigned_v<T>, T>
+sub_with_borrow(T a, T b, T borrow_in, T &borrow_out) {
+  T tmp = a - b;
+  T diff = tmp - borrow_in;
+  borrow_out = (diff > tmp) || (tmp > a);
+  return diff;
+}
+
+#if __has_builtin(__builtin_subc)
+// https://clang.llvm.org/docs/LanguageExtensions.html#multiprecision-arithmetic-builtins
+
+template <>
+inline unsigned char sub_with_borrow<unsigned char>(unsigned char a,
+                                                    unsigned char b,
+                                                    unsigned char borrow_in,
+                                                    unsigned char &borrow_out) {
+  return __builtin_subcb(a, b, borrow_in, &borrow_out);
+}
+
+template <>
+inline unsigned short
+sub_with_borrow<unsigned short>(unsigned short a, unsigned short b,
+                                unsigned short borrow_in,
+                                unsigned short &borrow_out) {
+  return __builtin_subcs(a, b, borrow_in, &borrow_out);
+}
+
+template <>
+inline unsigned int sub_with_borrow<unsigned int>(unsigned int a,
+                                                  unsigned int b,
+                                                  unsigned int borrow_in,
+                                                  unsigned int &borrow_out) {
+  return __builtin_subc(a, b, borrow_in, &borrow_out);
+}
+
+template <>
+inline unsigned long sub_with_borrow<unsigned long>(unsigned long a,
+                                                    unsigned long b,
+                                                    unsigned long borrow_in,
+                                                    unsigned long &borrow_out) {
+  return __builtin_subcl(a, b, borrow_in, &borrow_out);
+}
+
+template <>
+inline unsigned long long
+sub_with_borrow<unsigned long long>(unsigned long long a, unsigned long long b,
+                                    unsigned long long borrow_in,
+                                    unsigned long long &borrow_out) {
+  return __builtin_subcll(a, b, borrow_in, &borrow_out);
+}
+#endif // __has_builtin(__builtin_subc)
+
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_SUPPORT_BUILTIN_WRAPPERS_H