start of x86-64 implementation; stack pushing, sinking, add with overflow, procedure return ops

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 = "nmvm",
+        // 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);
+}

idea.md

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+# .nmvm Near Metal Virtual Machine
+Exercise in building low overhead VM via architecture specific means.

src/arch/x86-64.zig

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+// Execution thread convention:
+// rdi <- binary thread
+
+// Resources used:
+// https://mort.coffee/home/fast-interpreters/
+// https://blog.reverberate.org/2021/04/21/musttail-efficient-interpreters.html
+// https://en.wikibooks.org/wiki/X86_Assembly/GNU_assembly_syntax
+// https://www.cs.princeton.edu/courses/archive/spr18/cos217/lectures/15_AssemblyFunctions.pdf
+// https://ziglang.org/documentation/master/#toc-Assembly
+// https://csiflabs.cs.ucdavis.edu/~ssdavis/50/att-syntax.htm
+
+pub const Word = u64;
+
+// todo: Variant that pushes array of words.
+/// (iw | -- iw)
+pub fn opPushWord() callconv(.Naked) noreturn {
+    asm volatile (
+        \\ add $0x10, %%rdi
+        \\ pushq -8(%%rdi)
+        \\ jmpq *(%%rdi)
+    );
+}
+
+// todo: Variant that discards array of words.
+/// (w --)
+pub fn opSinkWord() callconv(.Naked) noreturn {
+    asm volatile (
+        \\ add $0x08, %%rdi
+        \\ addq $0x08, %%rsp
+        \\ jmpq *(%%rdi)
+    );
+}
+
+/// (iw | -- (iw'nth word from stack) )
+// fn opTakeWord(binary: [*]const Word, cond: bool) noreturn {
+//     @setRuntimeSafety(false);
+//     takeWord(binary[1].word);
+//     @call(.always_tail, binary[2].function, .{ &binary[2], cond });
+// }
+
+/// (iw | w)
+// fn opSetWord(binary: [*]const Word, cond: bool) noreturn {
+//     @setRuntimeSafety(false);
+//     setWord(binary[1].word, popWord());
+//     @call(.always_tail, binary[2].function, .{ &binary[2], cond });
+// }
+
+// todo: Generate operation permutations procedurally.
+// todo: Jump on overflow instead of cond setting?
+/// (w1 w2 -- sum overflow)
+pub fn opSumWordsWithOverflow() callconv(.Naked) noreturn {
+    // https://www.felixcloutier.com/x86/adc
+    // https://www.felixcloutier.com/x86/setcc
+    // idea: Could https://www.felixcloutier.com/x86/cmovcc be better for overflow push?
+    asm volatile (
+        \\ movq (%%rsp), %%rax
+        \\ adcq 8(%%rsp), %%rax
+        \\ movq %%rax, 8(%%rsp)
+        \\ setc %%al
+        \\ movb %%al, 7(%%rsp)
+        \\ addq $0x08, %%rdi
+        \\ jmpq *(%%rdi)
+    );
+}
+
+// todo: Generate operation permutations procedurally.
+// todo: We might not need cond register if conditions and jumps are combined?
+/// (w1 w2)
+// fn opRelativeJumpIfGreaterThan(binary: [*]const Word, cond: bool) noreturn {
+//     @setRuntimeSafety(false);
+//     const offset = if (popWord() > popWord()) binary[1].word else 2;
+//     @call(.always_tail, binary[offset].function, .{ &binary[offset], cond });
+// }
+
+/// (addr)
+pub fn opReturn() callconv(.Naked) noreturn {
+    // https://www.felixcloutier.com/x86/ret
+    asm volatile ("ret");
+}
+
+pub fn execute(binary: []const Word, entry_addr: usize) void {
+    // todo: Ensure correctness.
+    // https://wiki.osdev.org/System_V_ABI
+    // https://www.felixcloutier.com/x86/call
+    asm volatile (
+        \\ call *(%%rdi)
+        :
+        : [thread] "rdi" (&binary[entry_addr]),
+        : "rflags", "rax", "rbx", "rsp", "rbp", "r12", "r13", "r14", "r15", "rsi", "rdx", "rcx", "r8", "r9", "r10", "r11", "memory"
+    );
+}

src/interpreter.zig

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+// todo: Interpreter context as binary local variable.
+//       It would hold memory mappings, as well as error stack.
+// todo: Define procedure call for user code.
+// todo: Instruction set extensions, such as memory management schemes, non-exhaustive logging,
+//       exception mechanism, coroutines via yield/resume and etc.
+// todo: Threading scheme.
+// todo: Extension for native floating point stack ops.
+// todo: Try using small code model with nopie/nopic binary.
+
+// idea: Specialized opcodes that have side effects on read and write, such as
+//       zero-check on push/pop, or jump if condition bit met. This would create a lot
+//       of permutations tho, we might try to discover which code devices are most used.
+
+// idea: 'JIT' could be done by simple op* compiled binary copying up until `jmpq *(%%rdi)`,
+//       with immediate operand prelude modified, which could be done procedurally.
+
+usingnamespace @import("arch/x86-64.zig");

src/main.zig

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+const int = @import("interpreter.zig");
+
+pub fn main() !void {
+    const binary = [_]int.Word{
+        @as(int.Word, @intFromPtr(&int.opPushWord)),
+        ~@as(int.Word, 1),
+        @as(int.Word, @intFromPtr(&int.opPushWord)),
+        ~@as(int.Word, 1),
+        @as(int.Word, @intFromPtr(&int.opSumWordsWithOverflow)),
+        @as(int.Word, @intFromPtr(&int.opSinkWord)),
+        @as(int.Word, @intFromPtr(&int.opSinkWord)),
+        @as(int.Word, @intFromPtr(&int.opReturn)),
+    };
+
+    int.execute(&binary, 0);
+}