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A new Smalltalk dialect to replace Slang

Lowtalk is a new Smalltalk dialect that can work without a VM, by generating native machine into standard relocatable object files that can be linked with a platform specific linker. This dialect introduces syntax extensions for communicating with C function directly, to define variables and values with primitive types that can be implemented efficiently with machine instructions, and also to define the required

The Lowtalk compiler is completely implemented in Pharo, and it is implemented as a meta-circular evaluator that is used to bootstrap the target Smalltalk object model and runtime. The front-end is in this repository and its end result is the SSA intermediate representation of Slovim (Smalltalk Low Level Virtual Machine), a compiler infrastructure heavily inspired on LLVM but implemented completely in Pharo. The Slovim intermediate representation is converted into the SAsm intermediate representation which is inspired by the VirtualCPU for doing register allocation and generating x86 or x86_64 assembly code that is converted into the relocatable binary object file.

For debugging there is partial support for generating the debugging information in the DWARF standard, which allows to set breakpoints and inspect global variables in gdb. It is still missing the debugging support for inspecting local variables in gdb.

For garbage collection we are currently using automatic reference counting, which it is slow but allows to support real multi-threading easily, which of course we do.

Loading an Image with the Lowtalk compiler.

For loading an image with the Lowtalk compiler, you should clone this repository and execute the script.

Compiler usage

The Lowtalk compiler can be invoked from a Pharo Playground, or from the command line.

Compiler usage example in the playground

For compiling the Hello World sample into a relocatable object file, you can run this script on the playground:

compiler := LowtalkCompiler compilationTarget:
    SLVMSAsmCompilationTarget x86 withDebugInformation.
	evaluateFileNamed: 'runtime/runtime.ltk';
    evaluateFileNamed: 'samples/HelloWorld.ltk';
	optimizationLevel: 1;
	writeObjectToFileNamed: 'hello.o'

Compiler usage example in the command line

The same Hello World sample can be compiled from the command line by using the following command:

./ltkc -m32 -c -o hello.o -O1 -g runtime/runtime.ltk samples/HelloWorld.ltk

Linking example

The resulting hello.o can be linked in Linux into an executable with the following bash command:

gcc -m32 -no-pie -pthread -o hello hello.o

Because we do not have support position independent code yet, on OS X for linking it is required to use the following command:

clang -m32 -Wl,-no_pie -read_only_relocs suppress -o hello hello.o

Syntax extensions

Variables with types

[<Int32> :a<Int64> :b |
    | c c<Int32>|

let variable definition expression

In addition to the normal way of defining variables in Smalltalk, there is a let expression that allows to define variable. Unlike a normal variable definition, a let definition without an explicit type will try to infer its type according to the initial value that is given to it. For literal values, it will try to infer a primitive type if possible.

let a := 5. "Int32 type"
let b := 1.0. "Float64 type"
let c<Float32> := 1.0. "Float64 type"

C string style literal (e.g: c'Hello World\n')

By prefixing a string literal with the c character, it is possible to use C escape codes in the string literal.

#{} C function style calling expression

LibC printf #{c'An Integer: %d\n' . 2}.

Method AST expression

Because Smalltalk does not have an uniform syntax for method definitions, or DoIt expression, in Lowtalk to be able to keep everything in plain text files, there is an expression that returns the AST of a method:

:[addWith: other
    ^ self + other

This is typically used in the following way:

Object category: 'sample' methods: {
:[addWith: other
    ^ self + other

Type System

In Lowtalk there two main categories of types: native types, and dynamic object types. Native types are the types that have a direct correlation with a C type. Dynamic object types, are used to represent a generic dynamic object (_DynamicObject), or a specific dynamic object that is used in very special circumstances, such as implementing self.

Native Type C Type
Void Void
Int8 char
Int16 short
Int32 int
Int64 long/long long
IntPointer intptr_t
UInt8 unsigned char
UInt16 unsigned short
UInt32 unsigned int
UInt64 unsigned long/unsigned long long
UIntPointer uintptr_t
Float32 long/long long
Float64 long/long long
Void pointer void*
UInt8 const pointer const char *
UInt8 array char[]
UInt8 array: 64 char[64]

Structures, unions, and packed structures (structure with alignment of 1) can be defined in the following ways:

Structure <<< #MyPoint
    slots: {
        #x => Float32.
        #y => Float64.

Union <<< #MyUnion
    slots: {
        #p => MyPoint.
        #pt => (MyPoint pointer).
        #f => Float32.
        #d => Float64.

PackedStructure <<< #MyPackedStructure
    slots: {
        #a => UInt8.
        #b => UInt16.

The evaluation of the slots of these types is done lazily to be able to resolve circular dependencies between types, e.g:

Structure <<< #LinkedListNode
    slots: {
        #next => LinkedListNode pointer.
        #value => Void pointer.

Future Work

This is a non-exhaustive list of future work stuff to do:

  • Slovim was originally created to generate Spir-V and graphics API specific shader code, so it should be possible
  • A Lowcode backend for Slovim should make it possible to use Lowtalk in a Pharo image.
  • Replace the automatic reference counting with a concurrent garbage collector.
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