[Alt Impl] LLVM builtin arbitrary fixed precision int

[mratsim]

http://llvm.org/doxygen/APInt_8h_source.html

//===----------------------------------------------------------------------===//
 //                              APInt Class
 //===----------------------------------------------------------------------===//
 
 /// Class for arbitrary precision integers.
 ///
 /// APInt is a functional replacement for common case unsigned integer type like
 /// "unsigned", "unsigned long" or "uint64_t", but also allows non-byte-width
 /// integer sizes and large integer value types such as 3-bits, 15-bits, or more
 /// than 64-bits of precision. APInt provides a variety of arithmetic operators
 /// and methods to manipulate integer values of any bit-width. It supports both
 /// the typical integer arithmetic and comparison operations as well as bitwise
 /// manipulation.
 ///
 /// The class has several invariants worth noting:
 ///   * All bit, byte, and word positions are zero-based.
 ///   * Once the bit width is set, it doesn't change except by the Truncate,
 ///     SignExtend, or ZeroExtend operations.
 ///   * All binary operators must be on APInt instances of the same bit width.
 ///     Attempting to use these operators on instances with different bit
 ///     widths will yield an assertion.
 ///   * The value is stored canonically as an unsigned value. For operations
 ///     where it makes a difference, there are both signed and unsigned variants
 ///     of the operation. For example, sdiv and udiv. However, because the bit
 ///     widths must be the same, operations such as Mul and Add produce the same
 ///     results regardless of whether the values are interpreted as signed or
 ///     not.
 ///   * In general, the class tries to follow the style of computation that LLVM
 ///     uses in its IR. This simplifies its use for LLVM.

And https://llvm.org/docs/LangRef.html#integer-type

[mratsim]

This has been made significantly easier with Clang _ExtInt:

http://blog.llvm.org/2020/04/the-new-clang-extint-feature-provides.html

[mratsim]

Superceded by inline asembly #69