2b82b7e50a
APFloat: Rewrite It In Rust and use it for deterministic floating-point CTFE.
As part of the CTFE initiative, we're forced to find a solution for floating-point operations.
By design, IEEE-754 does not explicitly define everything in a deterministic manner, and there is some variability between platforms, at the very least (e.g. NaN payloads).
If types are to evaluate constant expressions involving type (or in the future, const) generics, that evaluation needs to be *fully deterministic*, even across `rustc` host platforms.
That is, if `[T; T::X]` was used in a cross-compiled library, and the evaluation of `T::X` executed a floating-point operation, that operation has to be reproducible on *any other host*, only knowing `T` and the definition of the `X` associated const (as either AST or HIR).
Failure to uphold those rules allows an associated type (e.g. `<Foo as Iterator>::Item`) to be seen as two (or more) different types, depending on the current host, and such type safety violations typically allow writing of a `transmute` in safe code, given enough generics.
The options considered by @rust-lang/compiler were:
1. Ban floating-point operations in generic const-evaluation contexts
2. Emulate floating-point operations in an uniformly deterministic fashion
The former option may seem appealing at first, but floating-point operations *are allowed today*, so they can't be banned wholesale, a distinction has to be made between the code that already works, and future generic contexts. *Moreover*, every computation that succeeded *has to be cached*, otherwise the generic case can be reproduced without any generics. IMO there are too many ways it can go wrong, and a single violation can be enough for an unsoundness hole.
Not to mention we may end up really wanting floating-point operations *anyway*, in CTFE.
I went with the latter option, and seeing how LLVM *already* has a library for this exact purpose (as it needs to perform optimizations independently of host floating-point capabilities), i.e. `APFloat`, that was what I ended up basing this PR on.
But having been burned by the low reusability of bindings that link to LLVM, and because I would *rather* the floating-point operations to be wrong than not deterministic or not memory-safe (`APFloat` does far more pointer juggling than I'm comfortable with), I decided to RIIR.
This way, we have a guarantee of *no* `unsafe` code, a bit more control over the where native floating-point might accidentally be involved, and non-LLVM backends can share it.
I've also ported all the testcases over, *before* any functionality, to catch any mistakes.
Currently the PR replaces all CTFE operations to go through `apfloat::ieee::{Single,Double}`, keeping only the bits of the `f32` / `f64` memory representation in between operations.
Converting from a string also double-checks that `core::num` and `apfloat` agree on the interpretation of a floating-point number literal, in case either of them has any bugs left around.
r? @nikomatsakis
f? @nagisa @est31
<hr/>
Huge thanks to @edef1c for first demoing usable `APFloat` bindings and to @chandlerc for fielding my questions on IRC about `APFloat` peculiarities (also upstreaming some bugfixes).
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| CODE_OF_CONDUCT.md | ||
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| LICENSE-MIT | ||
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The Rust Programming Language
This is the main source code repository for Rust. It contains the compiler, standard library, and documentation.
Quick Start
Read "Installation" from The Book.
Building from Source
-
Make sure you have installed the dependencies:
g++4.7 or later orclang++3.x or laterpython2.7 (but not 3.x)- GNU
make3.81 or later cmake3.4.3 or latercurlgit
-
Clone the source with
git:$ git clone https://github.com/rust-lang/rust.git $ cd rust
-
Build and install:
$ ./x.py build && sudo ./x.py installNote: Install locations can be adjusted by copying the config file from
./src/bootstrap/config.toml.exampleto./config.toml, and adjusting theprefixoption under[install]. Various other options, such as enabling debug information, are also supported, and are documented in the config file.When complete,
sudo ./x.py installwill place several programs into/usr/local/bin:rustc, the Rust compiler, andrustdoc, the API-documentation tool. This install does not include Cargo, Rust's package manager, which you may also want to build.
Building on Windows
There are two prominent ABIs in use on Windows: the native (MSVC) ABI used by Visual Studio, and the GNU ABI used by the GCC toolchain. Which version of Rust you need depends largely on what C/C++ libraries you want to interoperate with: for interop with software produced by Visual Studio use the MSVC build of Rust; for interop with GNU software built using the MinGW/MSYS2 toolchain use the GNU build.
MinGW
MSYS2 can be used to easily build Rust on Windows:
-
Grab the latest MSYS2 installer and go through the installer.
-
Run
mingw32_shell.batormingw64_shell.batfrom wherever you installed MSYS2 (i.e.C:\msys64), depending on whether you want 32-bit or 64-bit Rust. (As of the latest version of MSYS2 you have to runmsys2_shell.cmd -mingw32ormsys2_shell.cmd -mingw64from the command line instead) -
From this terminal, install the required tools:
# Update package mirrors (may be needed if you have a fresh install of MSYS2) $ pacman -Sy pacman-mirrors # Install build tools needed for Rust. If you're building a 32-bit compiler, # then replace "x86_64" below with "i686". If you've already got git, python, # or CMake installed and in PATH you can remove them from this list. Note # that it is important that you do **not** use the 'python2' and 'cmake' # packages from the 'msys2' subsystem. The build has historically been known # to fail with these packages. $ pacman -S git \ make \ diffutils \ tar \ mingw-w64-x86_64-python2 \ mingw-w64-x86_64-cmake \ mingw-w64-x86_64-gcc -
Navigate to Rust's source code (or clone it), then build it:
$ ./x.py build && ./x.py install
MSVC
MSVC builds of Rust additionally require an installation of Visual Studio 2013
(or later) so rustc can use its linker. Make sure to check the “C++ tools”
option.
With these dependencies installed, you can build the compiler in a cmd.exe
shell with:
> python x.py build
Currently building Rust only works with some known versions of Visual Studio. If you have a more recent version installed the build system doesn't understand then you may need to force rustbuild to use an older version. This can be done by manually calling the appropriate vcvars file before running the bootstrap.
CALL "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin\amd64\vcvars64.bat"
python x.py build
Specifying an ABI
Each specific ABI can also be used from either environment (for example, using the GNU ABI in powershell) by using an explicit build triple. The available Windows build triples are:
- GNU ABI (using GCC)
i686-pc-windows-gnux86_64-pc-windows-gnu
- The MSVC ABI
i686-pc-windows-msvcx86_64-pc-windows-msvc
The build triple can be specified by either specifying --build=ABI when
invoking x.py commands, or by copying the config.toml file (as described
in Building From Source), and modifying the build option under the [build]
section.
Configure and Make
While it's not the recommended build system, this project also provides a
configure script and makefile (the latter of which just invokes x.py).
$ ./configure
$ make && sudo make install
When using the configure script, the generated config.mk file may override the
config.toml file. To go back to the config.toml file, delete the generated
config.mk file.
Building Documentation
If you’d like to build the documentation, it’s almost the same:
$ ./x.py doc
The generated documentation will appear under doc in the build directory for
the ABI used. I.e., if the ABI was x86_64-pc-windows-msvc, the directory will be
build\x86_64-pc-windows-msvc\doc.
Notes
Since the Rust compiler is written in Rust, it must be built by a precompiled "snapshot" version of itself (made in an earlier state of development). As such, source builds require a connection to the Internet, to fetch snapshots, and an OS that can execute the available snapshot binaries.
Snapshot binaries are currently built and tested on several platforms:
| Platform / Architecture | x86 | x86_64 |
|---|---|---|
| Windows (7, 8, Server 2008 R2) | ✓ | ✓ |
| Linux (2.6.18 or later) | ✓ | ✓ |
| OSX (10.7 Lion or later) | ✓ | ✓ |
You may find that other platforms work, but these are our officially supported build environments that are most likely to work.
Rust currently needs between 600MiB and 1.5GiB to build, depending on platform. If it hits swap, it will take a very long time to build.
There is more advice about hacking on Rust in CONTRIBUTING.md.
Getting Help
The Rust community congregates in a few places:
- Stack Overflow - Direct questions about using the language.
- users.rust-lang.org - General discussion and broader questions.
- /r/rust - News and general discussion.
Contributing
To contribute to Rust, please see CONTRIBUTING.
Rust has an IRC culture and most real-time collaboration happens in a variety of channels on Mozilla's IRC network, irc.mozilla.org. The most popular channel is #rust, a venue for general discussion about Rust. And a good place to ask for help would be #rust-beginners.
License
Rust is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0), with portions covered by various BSD-like licenses.
See LICENSE-APACHE, LICENSE-MIT, and COPYRIGHT for details.