Pinch

https://github.com/amshafer/pinch

A simple borrow-checked language built using LLVM's MLIR

Multi-level Intermediate Representation (MLIR) is an LLVM project designed to provide insfrastructure for developing high level languages. Languages create a MLIR "dialect" and lower it to LLVM IR for code generation.

Pinch is a borrow checked language inspired by Rust. It supports very few operations and isn't intended for any real use. It was completed as a course project for my code optimization class.

Running the Pinch Compiler

Once you have compiled LLVM, you should have a directory named build. The pinch executable is the newly built compiler:

# see pinch -h for the full set of flags
$ ./build/bin/pinch ~/source.pinch -emit=mlir

The -emit flag controls what pass of the compiler should generate output. Here are the different passes:

• -emit=mlir - generate Pinch MLIR dialect
• -emit=mlir-borrow-checked - generate Pinch MLIR, but generate compiler errors if the borrow checker fails.
• -emit=mlir-std - partially lower to the Standard dialect
• -emit=llvm - lower to LLVM IR
• -emit=jit - execute the program

Example Programs

Here's a couple example programs and their output. More programs can be found in the examples directory.

Basic Borrow Checking

fn bad_function() -> &u32 {
   let ret = 3;
   return &ret;
}

fn main() {
   let a = 2;
   let b = &mut a;
   let c = &a;

   let d = bad_function();
}

The MLIR generated from this is:

module {
  func @bad_function() -> memref<1xui32> attributes {src = [], sym_visibility = "private"} {
    %0 = pinch.constant {dst = "ret"} 3
    %1 = "pinch.borrow"(%0) {dst = "return", src = "ret"} : (ui32) -> memref<1xui32>
    pinch.return %1 : memref<1xui32> {src = "return"}
  }
  func @main() attributes {src = []} {
    %0 = pinch.constant {dst = "a"} 2
    %1 = "pinch.borrow_mut"(%0) {dst = "b", src = "a"} : (ui32) -> memref<1xui32>
    %2 = "pinch.borrow"(%0) {dst = "c", src = "a"} : (ui32) -> memref<1xui32>
    %3 = pinch.generic_call @bad_function() {dst = "d"} : () -> ui32
    pinch.return {src = ""}
  }
}

If we compile with -emit=mlir-borrow-checked we see the following error:

loc("/home/ashafer/bin/test.pinch":5:4): error: returning reference to variable whose lifetime is ending

In this example we have a function bad_function which returns an address to a stack variable, which is unsafe. We also have variable a being borrowed invalidly in main. If a mutable reference is borrowed then we cannot create any shared references. The compiler generates an error in bad_function and exits before it can process this second error.

Boxed Variables

In Rust Box is a special type which is a smart pointer to heap allocated data. Pinch isn't as functional, but we do support boxes:

fn test2(a: Box) -> u32 {
   return 3;
}

fn test(a: Box) -> u32 {
   let b = *a;
   test2(a);
   print(b);
   return b;
}

fn main() {
   let a = box(3);
   test(a);
}

In this example we create a heap allocated integer, and move it into some subroutines. The result is just a printed integer, not very impressive. The MLIR output however is pretty interesting:

module {
  func @test2(%arg0: !pinch.box) -> ui32 attributes {src = ["a"], sym_visibility = "private"} {
    "pinch.drop"(%arg0) : (!pinch.box) -> ()
    %0 = pinch.constant {dst = "return"} 3
    pinch.return %0 : ui32 {src = ""}
  }
  func @test(%arg0: !pinch.box) -> ui32 attributes {src = ["a"], sym_visibility = "private"} {
    %0 = "pinch.deref"(%arg0) {dst = "b", src = "a"} : (!pinch.box) -> ui32
    %1 = "pinch.move"(%arg0) {dst = "", src = "a"} : (!pinch.box) -> !pinch.box
    %2 = pinch.generic_call @test2(%1) {dst = ""} : (!pinch.box) -> ui32
    pinch.print %0 : ui32
    pinch.return %0 : ui32 {src = "b"}
  }
  func @main() attributes {src = []} {
    %0 = "pinch.box"() {dst = "a", value = 3 : ui32} : () -> !pinch.box
    %1 = "pinch.move"(%0) {dst = "", src = "a"} : (!pinch.box) -> !pinch.box
    %2 = pinch.generic_call @test(%1) {dst = ""} : (!pinch.box) -> ui32
    pinch.return {src = ""}
  }
}

You can see that the boxed value isn't dropped until test2 is called. Based on the borrow checking rules related to moving variables this is the final place the box is used. Both of the other functions move the box, which is why they do not generate calls to the pinch.drop operation.

Compiling

This repository is designed to be dropped into the examples folder in the MLIR project. You will need to grab the latest llvm project, and add this directory to it.

Steps:

$ git clone https://github.com/llvm/llvm-project.git
$ cd llvm-project/mlir/examples
$ git clone https://github.com/amshafer/pinch

You then need to add the following line to the CMakeLists.txt in mlir/examples:

add_subdirectory(pinch)

At this point you can go ahead and build the llvm-project as normal. Pinch will be picked up by the build system the same way the Toy examples are.

See the README in llvm-project for steps to build LLVM.

Example Output

Below are the expected results for all the examples in examples when executed. If you would like to get the other IR outputs please consult the flags to -emit= mentioned above.

valid.pinch

ashafer@wolfgang:git/llvm-project % ./build/bin/pinch mlir/examples/pinch/examples/valid.pinch -emit=jit 
3 

bad_return.pinch

ashafer@wolfgang:git/llvm-project % ./build/bin/pinch mlir/examples/pinch/examples/bad_return.pinch -emit=jit
loc("mlir/examples/pinch/examples/bad_return.pinch":5:4): error: returning reference to variable whose lifetime is ending
loc("mlir/examples/pinch/examples/bad_return.pinch":11:12): error: Cannot borrow a shared reference while a mutable reference is active

box.pinch

ashafer@wolfgang:git/llvm-project % ./build/bin/pinch mlir/examples/pinch/examples/box.pinch -emit=jit 
3