Introduction

nlvm (the nim-level virtual machine?) is an LLVM-based compiler for the Nim language.

From Nim's point of view, it's a backend just like C or JavaScript - from LLVM's point of view, it's a language frontend that emits IR.

When I started on this little project, I knew neither llvm nor Nim. Therefore, I'd specially like to thank the friendly folks at the #nim channel that never seemed to tire of my nooby questions. Also, thanks to all tutorial writers out there, on llvm, programming and other topics for providing such fine sources of copy-pa... er, inspiration!

Questions, patches, improvement suggestions and reviews welcome. When you find bugs, feel free to fix them as well :)

Fork and enjoy!

Jacek Sieka (arnetheduck on gmail point com)

Status

nlvm is generally at par with nim in terms of features, with the following notable differences:

• Fast compile times - no intermediate C compiler step
• DWARF ("zero-cost") exception handling
• High-quality gdb/lldb debug information with source stepping, type information etc
• Smart code generation - compiler intrinsics for overflow checking, smart constant initialization, etc
• Native wasm32 support with no extra tooling

Most things from nim work just fine (see notes below however!):

• the same standard library is used
• similar command line options are supported (just change nim to nlvm!)
• importc works without needing C header files - the declaration in the .nim file needs to be accurate

Test coverage is not too bad either:

• bootstrapping and compiling itself
• ~95% of all upstream tests - most failures can be traced to the standard library and compiler relying on C implementation details - see skipped-tests.txt for an updated list of issues
• compiling most applications
• platforms: linux/x86_64, wasm32 (pre-alpha!)
• majority of the nim standard library (the rest can be fixed easily - requires upstream changes however)

How you could contribute:

• work on making skipped-tests.txt smaller
• improve platform support (osx and windows should be easy, arm would be nice)
• help nlvm generate better IR - optimizations, builtins, exception handling..
• help upstream make std library smaller and more nlvm-compatible
• send me success stories :)
• leave the computer for a bit and do something real for your fellow earthlings

nlvm does not:

• understand C - as a consequence, header, emit and similar pragmas will not work - neither will the fancy importcpp/C++ features
• support all nim compiler flags and features - do file bugs for anything useful that's missing

Compile instructions

To do what I do, you will need:

• Linux
• A C/C++ compiler (ironically, I happen to use gcc most of the time)

Start with a clone:

cd $SRC
git clone https://github.com/arnetheduck/nlvm.git
cd nlvm && git submodule update --init

We will need a few development libraries installed, mainly due to how nlvm processes library dependencies (see dynlib section below):

# Fedora
sudo dnf install pcre-devel openssl-devel sqlite-devel ninja-build

# Debian, ubuntu etc
sudo apt-get install libpcre3-dev libssl-dev libsqlite3-dev ninja-build

Compile nlvm (if needed, this will also build nim and llvm):

make

Compile with itself and compare:

make compare

Run test suite:

make test
make stats

You can link statically to LLVM to create a stand-alone binary - this will use a more optimized version of LLVM as well, but takes longer to build:

make STATIC_LLVM=1

If you want a faster nlvm, you can also try the release build - it will be called nlvmr:

make STATIC_LLVM=1 nlvmr

When you update nlvm from git, don't forget the submodule:

git pull && git submodule update

To build a docker image, use:

make docker

To run built nlvm docker image use:

docker run -v $(pwd):/code/ nlvm c -r /code/test.nim

Compiling your code

On the command line, nlvm is mostly compatible with nim.

When compiling, nlvm will generate a single .o file with all code from your project and link it using $CC - this helps it pick the right flags for linking with the C library.

cd $SRC/nlvm/Nim/examples
../../nlvm/nlvm c fizzbuzz

If you want to see the generated LLVM IR, use the -c option:

cd $SRC/nlvm/Nim/examples
../../nlvm/nlvm c -c fizzbuzz
less fizzbuzz.ll

You can then run the LLVM optimizer on it:

opt -Os fizzbuzz.ll | llvm-dis

... or compile it to assembly (.s):

llc fizzbuzz.ll
less fizzbuzz.s

Apart from the code of your .nim files, the compiler will also mix in the compatibility found library in nlvm-lib/.

Pipeline

Generally, the nim compiler pipeline looks something like this:

nim --> c files --> IR --> object files --> executable

In nlvm, we remove one step and bunch all the code together:

nim --> IR --> single object file --> executable

Going straight to the IR means it's possible to express nim constructs more clearly, allowing llvm to understand the code better and thus do a better job at optimization. It also helps keep compile times down, because the c-to-IR step can be avoided.

The practical effect of generating a single object file is similar to gcc -fwhole-program -flto - it is expensive in terms of memory, but results in slightly smaller and faster binaries. Notably, the IR-to-machine-code step, including any optimizations, is repeated in full for each recompile.

Common issues

dynlib

nim uses a runtime dynamic library loading scheme to gain access to shared libraries. When compiling, no linking is done - instead, when running your application, nim will try to open anything the user has installed.

nlvm does not support the {.dynlib.} pragma - instead you can use {.passL.} using normal system linking.

# works with `nim`
proc f() {. importc, dynlib: "mylib" .}

# works with both `nim` and `nlvm`
{.passL: "-lmylib".}
proc f() {. importc .}

header and emit

When nim compiles code, it will generate c code which may include other c code, from headers or directly via emit statements. This means nim has direct access do symbols declared in the c file, which can be both a feature and a problem.

In nlvm, {.header.} directives are ignored - nlvm looks strictly at the signature of the declaration, meaning the declaration must exactly match the c header file or subtly ABI issues and crashes ensue!

# When `nim` encounters this, it will emit `jmp_buf` in the `c` code without
# knowing the true size of the type, letting the `c` compiler determine it
# instead.
type C_JmpBuf {.importc: "jmp_buf", header: "<setjmp.h>".} = object

# nlvm instead ignores the `header` directive completely and will use the
# declaration as written. Failure to correctly declare the type will result
# in crashes and subtle bugs - memory will be overwritten or fields will be
# read from the wrong offsets.
#
# The following works with both `nim` and `nlvm`, but requires you to be
# careful to match the binary size and layout exactly (note how `bycopy`
# sometimes help to further nail down the ABI):

when defined(linux) and defined(amd64):
  type
    C_JmpBuf {.importc: "jmp_buf", bycopy.} = object
      abi: array[200 div sizeof(clong), clong]

# In `nim`, `C` constant defines are often imported using the following trick,
# which makes `nim` emit the right `C` code that the value from the header
# can be read (no writing of course, even though it's a `var`!)
#
# assuming a c header with: `#define RTLD_NOW 2`
# works for nim:
var RTLD_NOW* {.importc: "RTLD_NOW", header: "<dlfcn.h>".}: cint

# both nlvm and nim (note how these values often can be platform-specific):
when defined(linux) and defined(amd64):
  const RTLD_NOW* = cint(2)

wasm32 support

wasm32 support is still very bare-bones, so you will need to do a bit of tinkering to get it to work.

Presently, the wasm32-unknown-unknown target is mapped to --os:standalone and --cpu:wasm32 - this choice represents a very raw wasm engine with 32-bit little-endian integers and pointers - in the future, the nim standard library and system.nim will need to be updated to support WASM system interfaces like emscripten or WASI.

To compile wasm files, you will thus need a panicoverride.nim - a minimal example looks like this and discards any errors:

proc rawoutput(s: string) = discard
proc panic(s: string) {.noreturn.} = discard

After placing the above code in your project folder, you can compile .nim code to wasm32:

nim c -c --nlvm.target=wasm32-unknown-unkown myfile.nim
less myfile.ll

To go from there, follow the steps found here.

Random notes

• Upstream is pinned using a submodule - nlvm relies heavily on internals that keep changing - it's unlikely that it works with any other versions, patches welcome to update it
• The nim standard library likes to import C headers directly which works because the upstream nim compiler uses a C compiler underneath - ergo, large parts of the standard library don't work with nlvm.
• Happy to take patches for anything, including better platform support!
• For development, it's convenient to build LLVM with assertions turned on - the API is pretty unforgiving