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bors f502bd3abd Auto merge of #86761 - Alexhuszagh:master, r=estebank

Update Rust Float-Parsing Algorithms to use the Eisel-Lemire algorithm.

# Summary

Rust, although it implements a correct float parser, has major performance issues in float parsing. Even for common floats, the performance can be 3-10x [slower](https://arxiv.org/pdf/2101.11408.pdf) than external libraries such as [lexical](https://github.com/Alexhuszagh/rust-lexical) and [fast-float-rust](https://github.com/aldanor/fast-float-rust).

Recently, major advances in float-parsing algorithms have been developed by Daniel Lemire, along with others, and implement a fast, performant, and correct float parser, with speeds up to 1200 MiB/s on Apple's M1 architecture for the [canada](0e2b5d163d/data/canada.txt) dataset, 10x faster than Rust's 130 MiB/s.

In addition, [edge-cases](https://github.com/rust-lang/rust/issues/85234) in Rust's [dec2flt](868c702d0c/library/core/src/num/dec2flt) algorithm can lead to over a 1600x slowdown relative to efficient algorithms. This is due to the use of Clinger's correct, but slow [AlgorithmM and Bellepheron](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.45.4152&rep=rep1&type=pdf), which have been improved by faster big-integer algorithms and the Eisel-Lemire algorithm, respectively.

Finally, this algorithm provides substantial improvements in the number of floats the Rust core library can parse. Denormal floats with a large number of digits cannot be parsed, due to use of the `Big32x40`, which simply does not have enough digits to round a float correctly. Using a custom decimal class, with much simpler logic, we can parse all valid decimal strings of any digit count.

```rust
// Issue in Rust's dec2fly.
"2.47032822920623272088284396434110686182e-324".parse::<f64>();   // Err(ParseFloatError { kind: Invalid })
```

# Solution

This pull request implements the Eisel-Lemire algorithm, modified from [fast-float-rust](https://github.com/aldanor/fast-float-rust) (which is licensed under Apache 2.0/MIT), along with numerous modifications to make it more amenable to inclusion in the Rust core library. The following describes both features in fast-float-rust and improvements in fast-float-rust for inclusion in core.

**Documentation**

Extensive documentation has been added to ensure the code base may be maintained by others, which explains the algorithms as well as various associated constants and routines. For example, two seemingly magical constants include documentation to describe how they were derived as follows:

```rust
    // Round-to-even only happens for negative values of q
    // when q ≥ −4 in the 64-bit case and when q ≥ −17 in
    // the 32-bitcase.
    //
    // When q ≥ 0,we have that 5^q ≤ 2m+1. In the 64-bit case,we
    // have 5^q ≤ 2m+1 ≤ 2^54 or q ≤ 23. In the 32-bit case,we have
    // 5^q ≤ 2m+1 ≤ 2^25 or q ≤ 10.
    //
    // When q < 0, we have w ≥ (2m+1)×5^−q. We must have that w < 2^64
    // so (2m+1)×5^−q < 2^64. We have that 2m+1 > 2^53 (64-bit case)
    // or 2m+1 > 2^24 (32-bit case). Hence,we must have 2^53×5^−q < 2^64
    // (64-bit) and 2^24×5^−q < 2^64 (32-bit). Hence we have 5^−q < 2^11
    // or q ≥ −4 (64-bit case) and 5^−q < 2^40 or q ≥ −17 (32-bitcase).
    //
    // Thus we have that we only need to round ties to even when
    // we have that q ∈ [−4,23](in the 64-bit case) or q∈[−17,10]
    // (in the 32-bit case). In both cases,the power of five(5^|q|)
    // fits in a 64-bit word.
    const MIN_EXPONENT_ROUND_TO_EVEN: i32;
    const MAX_EXPONENT_ROUND_TO_EVEN: i32;
```

This ensures maintainability of the code base.

**Improvements for Disguised Fast-Path Cases**

The fast path in float parsing algorithms attempts to use native, machine floats to represent both the significant digits and the exponent, which is only possible if both can be exactly represented without rounding. In practice, this means that the significant digits must be 53-bits or less and the then exponent must be in the range `[-22, 22]` (for an f64). This is similar to the existing dec2flt implementation.

However, disguised fast-path cases exist, where there are few significant digits and an exponent above the valid range, such as `1.23e25`. In this case, powers-of-10 may be shifted from the exponent to the significant digits, discussed at length in https://github.com/rust-lang/rust/issues/85198.

**Digit Parsing Improvements**

Typically, integers are parsed from string 1-at-a-time, requiring unnecessary multiplications which can slow down parsing. An approach to parse 8 digits at a time using only 3 multiplications is described in length [here](https://johnnylee-sde.github.io/Fast-numeric-string-to-int/). This leads to significant performance improvements, and is implemented for both big and little-endian systems.

**Unsafe Changes**

Relative to fast-float-rust, this library makes less use of unsafe functionality and clearly documents it. This includes the refactoring and documentation of numerous unsafe methods undesirably marked as safe. The original code would look something like this, which is deceptively marked as safe for unsafe functionality.

```rust
impl AsciiStr {
    #[inline]
    pub fn step_by(&mut self, n: usize) -> &mut Self {
        unsafe { self.ptr = self.ptr.add(n) };
        self
    }
}

...

#[inline]
fn parse_scientific(s: &mut AsciiStr<'_>) -> i64 {
    // the first character is 'e'/'E' and scientific mode is enabled
    let start = *s;
    s.step();
    ...
}
```

The new code clearly documents safety concerns, and does not mark unsafe functionality as safe, leading to better safety guarantees.

```rust
impl AsciiStr {
    /// Advance the view by n, advancing it in-place to (n..).
    pub unsafe fn step_by(&mut self, n: usize) -> &mut Self {
        // SAFETY: same as step_by, safe as long n is less than the buffer length
        self.ptr = unsafe { self.ptr.add(n) };
        self
    }
}

...

/// Parse the scientific notation component of a float.
fn parse_scientific(s: &mut AsciiStr<'_>) -> i64 {
    let start = *s;
    // SAFETY: the first character is 'e'/'E' and scientific mode is enabled
    unsafe {
        s.step();
    }
    ...
}
```

This allows us to trivially demonstrate the new implementation of dec2flt is safe.

**Inline Annotations Have Been Removed**

In the previous implementation of dec2flt, inline annotations exist practically nowhere in the entire module. Therefore, these annotations have been removed, which mostly does not impact [performance](https://github.com/aldanor/fast-float-rust/issues/15#issuecomment-864485157).

**Fixed Correctness Tests**

Numerous compile errors in `src/etc/test-float-parse` were present, due to deprecation of `time.clock()`, as well as the crate dependencies with `rand`. The tests have therefore been reworked as a [crate](https://github.com/Alexhuszagh/rust/tree/master/src/etc/test-float-parse), and any errors in `runtests.py` have been patched.

**Undefined Behavior**

An implementation of `check_len` which relied on undefined behavior (in fast-float-rust) has been refactored, to ensure that the behavior is well-defined. The original code is as follows:

```rust
    #[inline]
    pub fn check_len(&self, n: usize) -> bool {
        unsafe { self.ptr.add(n) <= self.end }
    }
```

And the new implementation is as follows:

```rust
    /// Check if the slice at least `n` length.
    fn check_len(&self, n: usize) -> bool {
        n <= self.as_ref().len()
    }
```

Note that this has since been fixed in [fast-float-rust](https://github.com/aldanor/fast-float-rust/pull/29).

**Inferring Binary Exponents**

Rather than explicitly store binary exponents, this new implementation infers them from the decimal exponent, reducing the amount of static storage required. This removes the requirement to store [611 i16s](868c702d0c/library/core/src/num/dec2flt/table.rs (L8)).

# Code Size

The code size, for all optimizations, does not considerably change relative to before for stripped builds, however it is **significantly** smaller prior to stripping the resulting binaries. These binary sizes were calculated on x86_64-unknown-linux-gnu.

**new**

Using rustc version 1.55.0-dev.

opt-level|size|size(stripped)
|:-:|:-:|:-:|
0|400k|300K
1|396k|292K
2|392k|292K
3|392k|296K
s|396k|292K
z|396k|292K

**old**

Using rustc version 1.53.0-nightly.

opt-level|size|size(stripped)
|:-:|:-:|:-:|
0|3.2M|304K
1|3.2M|292K
2|3.1M|284K
3|3.1M|284K
s|3.1M|284K
z|3.1M|284K

# Correctness

The dec2flt implementation passes all of Rust's unittests and comprehensive float parsing tests, along with numerous other tests such as Nigel Toa's comprehensive float [tests](https://github.com/nigeltao/parse-number-fxx-test-data) and Hrvoje Abraham  [strtod_tests](https://github.com/ahrvoje/numerics/blob/master/strtod/strtod_tests.toml). Therefore, it is unlikely that this algorithm will incorrectly round parsed floats.

# Issues Addressed

This will fix and close the following issues:

- resolves #85198
- resolves #85214
- resolves #85234
- fixes #31407
- fixes #31109
- fixes #53015
- resolves #68396
- closes https://github.com/aldanor/fast-float-rust/issues/15

2021-07-17 12:56:22 +00:00

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README.md

The Rust Programming Language

This is the main source code repository for Rust. It contains the compiler, standard library, and documentation.

Note: this README is for users rather than contributors. If you wish to contribute to the compiler, you should read the Getting Started section of the rustc-dev-guide instead.

Quick Start

Read "Installation" from The Book.

Installing from Source

The Rust build system uses a Python script called x.py to build the compiler, which manages the bootstrapping process. It lives in the root of the project.

The x.py command can be run directly on most systems in the following format:

./x.py <subcommand> [flags]

This is how the documentation and examples assume you are running x.py.

Systems such as Ubuntu 20.04 LTS do not create the necessary python command by default when Python is installed that allows x.py to be run directly. In that case you can either create a symlink for python (Ubuntu provides the python-is-python3 package for this), or run x.py using Python itself:

# Python 3
python3 x.py <subcommand> [flags]

# Python 2.7
python2.7 x.py <subcommand> [flags]

More information about x.py can be found by running it with the --help flag or reading the rustc dev guide.

Building on a Unix-like system

Make sure you have installed the dependencies:
- g++ 5.1 or later or clang++ 3.5 or later
- python 3 or 2.7
- GNU make 3.81 or later
- cmake 3.13.4 or later
- ninja
- curl
- git
- ssl which comes in libssl-dev or openssl-devel
- pkg-config if you are compiling on Linux and targeting Linux

Clone the source with git:

git clone https://github.com/rust-lang/rust.git
cd rust

Configure the build settings:

The Rust build system uses a file named config.toml in the root of the source tree to determine various configuration settings for the build. Copy the default config.toml.example to config.toml to get started.
```
cp config.toml.example config.toml
```
If you plan to use x.py install to create an installation, it is recommended that you set the prefix value in the [install] section to a directory.

Create install directory if you are not installing in default directory
Build and install:
```
./x.py build && ./x.py install
```
When complete, ./x.py install will place several programs into $PREFIX/bin: rustc, the Rust compiler, and rustdoc, the API-documentation tool. This install does not include Cargo, Rust's package manager. To build and install Cargo, you may run ./x.py install cargo or set the build.extended key in config.toml to true to build and install all tools.

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.bat or mingw64_shell.bat from 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 run msys2_shell.cmd -mingw32 or msys2_shell.cmd -mingw64 from 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', 'cmake' and 'ninja'
# 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-python \
            mingw-w64-x86_64-cmake \
            mingw-w64-x86_64-gcc \
            mingw-w64-x86_64-ninja

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 2017 (or later) so rustc can use its linker. The simplest way is to get the Visual Studio, check the “C++ build tools” and “Windows 10 SDK” workload.

(If you're installing cmake yourself, be careful that “C++ CMake tools for Windows” doesn't get included under “Individual components”.)

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 and the build system doesn't understand, 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\2019\Community\VC\Auxiliary\Build\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-gnu
- x86_64-pc-windows-gnu
The MSVC ABI
- i686-pc-windows-msvc
- x86_64-pc-windows-msvc

The build triple can be specified by either specifying --build=<triple> when invoking x.py commands, or by copying the config.toml file (as described in Installing 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 stage 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, 10, ...)	✓	✓
Linux (kernel 2.6.32, glibc 2.11 or later)	✓	✓
macOS (10.7 Lion or later)	(*)	✓

(*): Apple dropped support for running 32-bit binaries starting from macOS 10.15 and iOS 11. Due to this decision from Apple, the targets are no longer useful to our users. Please read our blog post for more info.

You may find that other platforms work, but these are our officially supported build environments that are most likely to work.

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

If you are interested in contributing to the Rust project, please take a look at the Getting Started guide in the rustc-dev-guide.

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.

Trademark

The Rust Foundation owns and protects the Rust and Cargo trademarks and logos (the “Rust Trademarks”).

If you want to use these names or brands, please read the media guide.

Third-party logos may be subject to third-party copyrights and trademarks. See Licenses for details.

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README.md Unescape Escape

The Rust Programming Language

Quick Start

Installing from Source

Building on a Unix-like system

Building on Windows

MinGW

MSVC

Specifying an ABI

Configure and Make

Building Documentation

Notes

Getting Help

Contributing

License

Trademark

README.md