Add support for 128-bit integers

Author: Amanieu

text/0000-int128.md

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+- Feature Name: int128
+- Start Date: 21-02-2016
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+This RFC adds the `i128` and `u128` types to Rust. Because these types are not available on all platforms, a new target flag (`target_has_int128`) is added to allow users to check whether 128-bit integers are supported. The `i128` and `u128` are not added to the prelude, and must instead be explicitly imported with `use core::{i128, u128}`.
+
+# Motivation
+[motivation]: #motivation
+
+Some algorithms need to work with very large numbers that don't fit in 64 bits, such as certain cryptographic algorithms. One possibility would be to use a BigNum library, but these use heap allocation and tend to have high overhead. LLVM has support for very efficient 128-bit integers, which are exposed by Clang in C as the `__int128` type.
+
+# Detailed design
+[design]: #detailed-design
+
+From a quick look at Clang's source, 128-bit integers are supported on all 64-bit platforms and a few 32-bit ones (those with 64-bit registers: x32 and MIPS n32). To allow users to determine whether 128-bit integers are available, a `target_has_int128` cfg is added. The `i128` and `u128` types are only available when this flag is set.
+
+The actual `i128` and `u128` types are not added to the Rust prelude since that would break compatibility. Instead they must be explicitly imported with `use core::{i128, u128}` or `use std::{i128, u128}`. This will also catch attempts to use 128-bit integers when they are not supported by the underlying platform since the import will fail if `target_has_int128` is not defined.
+
+Implementation-wise, this should just be a matter of adding a new primitive type to the compiler and adding trait implementations for `i128`/`u128` in libcore. A new entry will need to be added to target specifications to specify whether the target supports 128-bit integers.
+
+One possible complication is that primitive types aren't currently part of the prelude, instead they are directly added to the global namespace by the compiler. The new `i128` and `u128` types will behave differently and will need to be explicitly imported.
+
+Another possible issue is that a `u128` can hold a very large number that doesn't fit in a `f32`. We need to make sure this doesn't lead to any `undef`s from LLVM.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+It adds a type to the language that may or may not be present depending on the target architecture. This could lead to surprises, bu
+
+# Alternatives
+[alternatives]: #alternatives
+
+There have been several attempts to create `u128`/`i128` wrappers based on two `u64` values, but these can't match the performance of LLVM's native 128-bit integers.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+How should 128-bit literals be handled? The easiest solution would be to limit integer literals to 64 bits, which is what GCC does (no support for `__int128` literals).