opCall, return stack, r12 for thread pointer

idea.md

@@ -1,2 +1,8 @@
 # .nmvm Near Metal Virtual Machine
 Exercise in building low overhead VM via architecture specific means.
+
+## Cases
+- Native stack usage for virtual machine.
+- Specific permutations of instructions for case optimizations.
+- Array processing instructions.
+- Absolute addressing for interpreter state, in TLS.

src/arch/x86-64.zig

@@ -1,5 +1,10 @@
+// todo: Use r12, r13, ... instead? They're preserved in Sys V abi which might make it more confortable,
+//       but they might increase binary size, gotta test.
+
+// todo: Try using something else that has lesser opcode size.
 // Execution thread convention:
-// rdi <- binary thread
+// r12 <- binary thread pointer
+// r13 <- return stack pointer
 
 // Resources used:
 // https://mort.coffee/home/fast-interpreters/
@@ -10,14 +15,17 @@
 // https://csiflabs.cs.ucdavis.edu/~ssdavis/50/att-syntax.htm
 
 pub const Word = u64;
+pub const RecursionLimit = 1024;
+
+threadlocal var return_stack: [RecursionLimit + 1]Word = undefined;
 
 // 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)
+        \\ add $0x10, %%r12
+        \\ pushq -8(%%r12)
+        \\ jmpq *(%%r12)
     );
 }
 
@@ -25,9 +33,9 @@ pub fn opPushWord() callconv(.Naked) noreturn {
 /// (w --)
 pub fn opSinkWord() callconv(.Naked) noreturn {
     asm volatile (
-        \\ add $0x08, %%rdi
+        \\ add $0x08, %%r12
         \\ addq $0x08, %%rsp
-        \\ jmpq *(%%rdi)
+        \\ jmpq *(%%r12)
     );
 }
 
@@ -53,13 +61,13 @@ pub fn opSumWordsWithOverflow() callconv(.Naked) noreturn {
     // https://www.felixcloutier.com/x86/setcc
     // idea: Could https://www.felixcloutier.com/x86/cmovcc be better for overflow push?
     asm volatile (
+        \\ addq $0x08, %%r12
         \\ movq (%%rsp), %%rax
         \\ adcq 8(%%rsp), %%rax
         \\ movq %%rax, 8(%%rsp)
         \\ setc %%al
-        \\ movb %%al, 7(%%rsp)
-        \\ addq $0x08, %%rdi
-        \\ jmpq *(%%rdi)
+        \\ movb %%al, (%%rsp)
+        \\ jmpq *(%%r12)
     );
 }
 
@@ -72,20 +80,47 @@ pub fn opSumWordsWithOverflow() callconv(.Naked) noreturn {
 //     @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
+// todo: Complex call op that would receive immediate mask that would tell
+//       which positions of stack to duplicate, as well as mixing of plain immediate operands.
+//       Or we could decouple it from call, it might be useful at other places.
+/// (iw |)
+pub fn opCall() callconv(.Naked) noreturn {
     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"
+        \\ leaq 16(%%r12), %%rax
+        \\ subq $0x8, %%r13
+        \\ movq %%rax, (%%r13)
+        \\ movq 8(%%r12), %%r12
+        \\ jmpq *(%%r12)
+    );
+}
+
+/// (addr)
+pub fn opReturn() callconv(.Naked) noreturn {
+    asm volatile (
+        \\ movq (%%r13), %%r12
+        \\ addq $0x08, %%r13
+        \\ jmpq *(%%r12)
+    );
+}
+
+// todo: Make sure it's non reentry in one given thread.
+pub fn execute(binary: []const Word, entry_addr: usize) void {
+    @setCold(true);
+    // todo: Ensure correctness.
+    // https://wiki.osdev.org/System_V_ABI
+
+    // todo: Use remaining stack as return.
+
+    // Such device is used so that opReturn could be used for return.
+    asm volatile (
+        \\     movq $0f, 8(%%r13)
+        \\     leaq 8(%%r13), %%rax
+        \\     movq %%rax, (%%r13)
+        \\     jmpq *(%%r12)
+        \\ 0:
+        :
+        : [thread] "r" (&binary[entry_addr]),
+          [retstk] "r" (&return_stack[return_stack.len - 2]),
+        : "rflags", "rax", "rbx", "rsp", "rdi", "rbp", "r14", "r15", "rsi", "rdx", "rcx", "r8", "r9", "r10", "r11", "memory"
     );
 }

src/interpreter.zig

@@ -14,4 +14,4 @@
 // 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");
+pub usingnamespace @import("arch/x86-64.zig");

src/main.zig

@@ -1,16 +1,23 @@
 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),
+    // todo: Mixing return addresses in stack poses a challenge, hm.
+    const add = [_]int.Word{
         @as(int.Word, @intFromPtr(&int.opSumWordsWithOverflow)),
+        @as(int.Word, @intFromPtr(&int.opReturn)),
+    };
+
+    const entry = [_]int.Word{
+        @as(int.Word, @intFromPtr(&int.opPushWord)),
+        1,
+        @as(int.Word, @intFromPtr(&int.opPushWord)),
+        2,
+        @as(int.Word, @intFromPtr(&int.opCall)),
+        @as(int.Word, @intFromPtr(&add)),
         @as(int.Word, @intFromPtr(&int.opSinkWord)),
         @as(int.Word, @intFromPtr(&int.opSinkWord)),
         @as(int.Word, @intFromPtr(&int.opReturn)),
     };
 
-    int.execute(&binary, 0);
+    int.execute(&entry, 0);
 }