First commit

.gitattributes

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+# Default behavior, if core.autocrlf is unset.
+* text=auto
+
+# Files to be converted to native line endings on checkout.
+*.cpp text
+*.h text
+
+# Text files to always have LF (unix) line endings on checkout.
+*.zig text eol=lf

.github/workflows/ci.yml

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+name: CI
+
+on:
+  pull_request:
+    paths:
+      - ".github/workflows/ci.yml"
+      - "**.zig"
+  push:
+    paths:
+      - ".github/workflows/ci.yml"
+      - "**.zig"
+    branches:
+      - main
+  schedule:
+    - cron: "0 0 * * *"
+concurrency:
+  # Cancels pending runs when a PR gets updated.
+  group: ${{ github.head_ref || github.run_id }}-${{ github.actor }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [
+          ubuntu-latest,
+        ]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 15
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Set up Zig
+        uses: goto-bus-stop/setup-zig@v2
+        with:
+          version: master
+      - name: Zig environment variables
+        run: zig env
+      - name: Test
+        run: zig build test

.gitignore

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+zig-cache
+zig-out

build.zig

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+const std = @import("std");
+
+// Although this function looks imperative, note that its job is to
+// declaratively construct a build graph that will be executed by an external
+// runner.
+pub fn build(b: *std.Build) void {
+    // Standard target options allows the person running `zig build` to choose
+    // what target to build for. Here we do not override the defaults, which
+    // means any target is allowed, and the default is native. Other options
+    // for restricting supported target set are available.
+    const target = b.standardTargetOptions(.{});
+
+    // Standard optimization options allow the person running `zig build` to select
+    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall. Here we do not
+    // set a preferred release mode, allowing the user to decide how to optimize.
+    const optimize = b.standardOptimizeOption(.{});
+
+    const exe = b.addExecutable(.{
+        .name = "compiler",
+        // In this case the main source file is merely a path, however, in more
+        // complicated build scripts, this could be a generated file.
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    // This declares intent for the executable to be installed into the
+    // standard location when the user invokes the "install" step (the default
+    // step when running `zig build`).
+    b.installArtifact(exe);
+
+    // This *creates* a Run step in the build graph, to be executed when another
+    // step is evaluated that depends on it. The next line below will establish
+    // such a dependency.
+    const run_cmd = b.addRunArtifact(exe);
+
+    // By making the run step depend on the install step, it will be run from the
+    // installation directory rather than directly from within the cache directory.
+    // This is not necessary, however, if the application depends on other installed
+    // files, this ensures they will be present and in the expected location.
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    // This allows the user to pass arguments to the application in the build
+    // command itself, like this: `zig build run -- arg1 arg2 etc`
+    if (b.args) |args| {
+        run_cmd.addArgs(args);
+    }
+
+    // This creates a build step. It will be visible in the `zig build --help` menu,
+    // and can be selected like this: `zig build run`
+    // This will evaluate the `run` step rather than the default, which is "install".
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+
+    // Creates a step for unit testing. This only builds the test executable
+    // but does not run it.
+    const unit_tests = b.addTest(.{
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    const run_unit_tests = b.addRunArtifact(unit_tests);
+
+    // Similar to creating the run step earlier, this exposes a `test` step to
+    // the `zig build --help` menu, providing a way for the user to request
+    // running the unit tests.
+    const test_step = b.step("test", "Run unit tests");
+    test_step.dependOn(&run_unit_tests.step);
+}

src/main.zig

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+const std = @import("std");
+const page_size = std.mem.page_size;
+const assert = std.debug.assert;
+const expectEqual = std.testing.expectEqual;
+
+const Section = struct {
+    content: []align(page_size) u8,
+    index: usize = 0,
+};
+
+const Image = struct {
+    text: Section,
+    rodata: Section,
+    data: Section,
+    entry_point: u32 = 0,
+
+    fn create() !Image {
+        return Image{
+            .text = .{ .content = try mmap(page_size, .{ .executable = true }) },
+            .rodata = .{ .content = try mmap(page_size, .{ .executable = false }) },
+            .data = .{ .content = try mmap(page_size, .{ .executable = false }) },
+        };
+    }
+
+    inline fn mmap(size: usize, flags: packed struct {
+        executable: bool,
+    }) ![]align(page_size) u8 {
+        const protection_flags = std.os.PROT.READ | std.os.PROT.WRITE | if (flags.executable) std.os.PROT.EXEC else 0;
+        const mmap_flags = std.os.MAP.ANONYMOUS | std.os.MAP.PRIVATE;
+
+        return std.os.mmap(null, size, protection_flags, mmap_flags, -1, 0);
+    }
+
+    fn appendCode(image: *Image, code: []const u8) void {
+        const destination = image.text.content[image.text.index..][0..code.len];
+        @memcpy(destination, code);
+        image.text.index += code.len;
+    }
+
+    fn getEntryPoint(image: *const Image, comptime Function: type) *const Function {
+        comptime {
+            assert(@typeInfo(Function) == .Fn);
+        }
+
+        assert(image.text.content.len > 0);
+        return @as(*const Function, @ptrCast(&image.text.content[image.entry_point]));
+    }
+};
+
+const Rex = enum(u8) {
+    b = upper_4_bits | (1 << 0),
+    x = upper_4_bits | (1 << 1),
+    r = upper_4_bits | (1 << 2),
+    w = upper_4_bits | (1 << 3),
+
+    const upper_4_bits = 0b100_0000;
+};
+
+const prefix_rex_w = [1]u8{@intFromEnum(Rex.w)};
+const prefix_16_bit_operand = [1]u8{0x66};
+
+const ret = [1]u8{0xc3};
+const movabs_to_register_a = [1]u8{0xb8};
+
+inline fn intToArrayOfBytes(integer: anytype) [@sizeOf(@TypeOf(integer))]u8 {
+    comptime {
+        assert(@typeInfo(@TypeOf(integer)) == .Int);
+    }
+
+    return @as([@sizeOf(@TypeOf(integer))]u8, @bitCast(integer));
+}
+
+inline fn movU16ToA(integer: u16) [4]u8 {
+    return prefix_16_bit_operand ++ movabs_to_register_a ++ intToArrayOfBytes(integer);
+}
+
+inline fn movU32ToA(integer: u32) [5]u8 {
+    return movabs_to_register_a ++ intToArrayOfBytes(integer);
+}
+
+inline fn movU64ToA(integer: u64) [10]u8 {
+    return prefix_rex_w ++ movabs_to_register_a ++ intToArrayOfBytes(integer);
+}
+
+test "ret void" {
+    var image = try Image.create();
+    image.appendCode(&ret);
+
+    const function_pointer = image.getEntryPoint(fn () callconv(.C) void);
+    function_pointer();
+}
+
+test "ret unsigned integer 16-bit" {
+    var image = try Image.create();
+    const expected_number = 0xffff;
+    image.appendCode(&movU16ToA(expected_number));
+    image.appendCode(&ret);
+
+    const function_pointer = image.getEntryPoint(fn () callconv(.C) u16);
+    const result = function_pointer();
+    try expectEqual(result, expected_number);
+}
+
+test "ret unsigned integer 32-bit" {
+    var image = try Image.create();
+    const expected_number = 0xffff_ffff;
+    image.appendCode(&movU32ToA(expected_number));
+    image.appendCode(&ret);
+
+    const function_pointer = image.getEntryPoint(fn () callconv(.C) u32);
+    const result = function_pointer();
+    try expectEqual(result, expected_number);
+}
+
+test "ret unsigned integer 64-bit" {
+    var image = try Image.create();
+    const expected_number = 0xffff_ffff_ffff_ffff;
+    image.appendCode(&movU64ToA(expected_number));
+    image.appendCode(&ret);
+
+    const function_pointer = image.getEntryPoint(fn () callconv(.C) u64);
+    const result = function_pointer();
+    try expectEqual(result, expected_number);
+}