Monday, January 2, 2023

Bflat – a single ahead of time crosscompiler and runtime for C#

bflat

C# as you know it but with Go-inspired tooling that produces small, selfcontained, and native executables out of the box.

$ echo 'System.Console.WriteLine("Hello World");' > hello.cs
$ bflat build hello.cs
$ ./hello
Hello World
$ bflat build hello.cs --os:windows
$ file ./hello.exe
hello.exe: PE32+ executable (console) x86-64, for MS Windows

🎻 What exactly is bflat

bflat is a concoction of Roslyn - the "official" C# compiler that produces .NET executables - and NativeAOT (née CoreRT) - the ahead of time compiler for .NET based on CoreCLR. Thanks to this, you get access to the latest C# features using the high performance CoreCLR GC and native code generator (RyuJIT).

bflat merges the two components together into a single ahead of time crosscompiler and runtime for C#.

bflat can currently target:

x64/arm64 glibc-based Linux (2.17 or later on x64 (~CentOS 7), or 2.27 or later on arm64 (~Ubuntu 18.04))
arm64 bionic-based Linux (Android API level 21)
x64/arm64 Windows (Windows 7 or later)
x64 UEFI (only with --stdlib:zero)

Support for musl-based Linux is in the works.

bflat can either produce native executables, or native shared libraries that can be called from other languages through FFI.

🥁 Where to get bflat

Look at the Releases tab of this repo and download a compiler that matches your host system. These are all crosscompilers and can target any of the supported OSes/architectures.

Unzip the archive to a convenient location and add the root to your PATH. You're all set. See the samples directory for a couple samples.

The binary releases are licensed under the MIT license.

🎷 I don't see dotnet, MSBuild, or NuGet

That's the point. bflat is to dotnet as VS Code is to VS.

🎙 Where is the source code

The source code is split between this repo and bflattened/runtime. The bflattened/runtime repo is a regularly updated fork of the dotnet/runtime repo that contains non-upstreamable bflat-specific changes. The bflattened/runtime repo produces compiler and runtime binaries that this repo consumes.

📚 Two standard libraries

bflat comes with two standard libraries. The first one (called DotNet) is the default and comes from the dotnet/runtime repo fork. It includes everything you know and love from .NET. The second one (called Zero) is a minimal standard library that doesn't have much more than just primitive types. The source code for it lives in this repo. Switch between those with --stdlib:zero argument.

📻 How to stay up-to-date on bflat?

Follow me on Twitter.

🎺 Optimizing output for size

By default, bflat produces executables that are between 2 MB and 3 MB in size, even for the simplest apps. There are multiple reasons for this:

bflat includes stack trace data about all compiled methods so that it can print pretty exception stack traces
even the simplest apps might end up calling into reflection (to e.g. get the name of the OutOfMemoryException class), globalization, etc.
method bodies are aligned at 16-byte boundaries to optimize CPU cache line utilization
(Doesn't apply to Windows) DWARF debug information is included in the executable

The "bigger" defaults are chosen for friendliness and convenience. To get an experience that more closely matches low level programming languages, specify --no-reflection, --no-stacktrace-data, --no-globalization, and --no-exception-messages arguments to bflat build.

Best to show an example. Following program:

using System.Diagnostics;
using static System.Console;

WriteLine($"NullReferenceException message is: {new NullReferenceException().Message}");
WriteLine($"The runtime type of int is named: {typeof(int)}");
WriteLine($"Type of boxed integer is{(123.GetType() == typeof(int) ? "" : " not")} equal to typeof(int)");
WriteLine($"Type of boxed integer is{(123.GetType() == typeof(byte) ? "" : " not")} equal to typeof(byte)");
WriteLine($"Upper case of 'Вторник' is '{"Вторник".ToUpper()}'");
WriteLine($"Current stack frame is {new StackTrace().GetFrame(0)}");

will print this by default:

NullReferenceException message is: Object reference not set to an instance of an object.
The runtime type of int is named: System.Int32
Type of boxed integer is equal to typeof(int)
Type of boxed integer is not equal to typeof(byte)
Upper case of 'Вторник' is 'ВТОРНИК'
Current stack frame is <Program>$.<Main>$(String[]) + 0x154 at offset 340 in file:line:column <filename unknown>:0:0

But it will print this with all above arguments specified:

NullReferenceException message is: Arg_NullReferenceException
The runtime type of int is named: EETypeRva:0x00048BD0
Type of boxed integer is equal to typeof(int)
Type of boxed integer is not equal to typeof(byte)
Upper case of 'Вторник' is 'Вторник'
Current stack frame is ms!<BaseAddress>+0xb82d4 at offset 340 in file:line:column <filename unknown>:0:0

With all options turned on, one can comfortably fit useful programs under 1 MB. The above program is 735 kB on Windows at the time of writing this. The output executables are executables like any other. You can use a tool like UPX to compress them further (to ~300 kB range).

If you're targeting a Unix-like system, you might want to pass --separate-symbols to place debug information into a separate file (debug information is big!). This is not needed on Windows because the platform convention is to place debug information in a separate PDB file already.

🎸 Preprocessor definitions

Besides the preprocessor definitions provided at the command line, bflat defines several other symbols: BFLAT (defined always), DEBUG (defined when not optimizing), WINDOWS/LINUX/UEFI (when the corresponding operating system is the target), X64/ARM64 (when the corresponding architecture is targeted).

🎹 Debugging bflat apps

Apps compiled with bflat debug same as any other native code. Launch the produced executable under your favorite debugger (gdb or lldb on Linux, or Visual Studio or WinDbg on Windows) and you'll be able to set breakpoints, step, and see local variables.

☝ Samples

The repo has samples with README in the samples directory. Clone the repo and try the samples yourself!

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