Simple virtual machine in C - follow-up

Question

I've previously put my virtual machine for review and I've applied the changes and more to the code.

This is iteration 2 and I'd like to remind the reviewers that this virtual machine is meant to be embedded into a C++ game engine application as a compiled, exported C library (static, not dll).

The other reviews have told me not to use address label gotos because it's nonstandard but I'm compiling this entirely with Clang, Clang supports address label gotos.

I'm still planning out an assembler for the code as well as compacting the code into bytecode.

One question I do have to ask concerning this: would I have to segment the stack and data into bytes or can I leave them 8 byte arrays?

GitHub Code (if necessary)

#include <stdio.h>
#include <stdbool.h>
#include <stdint.h>
#include <inttypes.h>
#include <stdlib.h>
enum InstrSet {
 // push and pop are always assumed to hold a long int
 nop=0,
 push, pop, pushsp, popsp, // 1
 add, fadd, sub, fsub, // 5
 mul, fmul, idiv, fdiv, mod, // 9
 jmp, lt, gt, cmp, // 14
 jnz, jz, // 18
 inc, dec, shl, shr, and, or, xor, not, // 20
 cpy, swap, // 28
 load, store, // 30
 call, ret, // 32
 halt,
};
bool running = true;
#define STACKSIZE 256
struct vm_cpu {
 uint64_t memory[STACKSIZE << 2], stack[STACKSIZE], callstack[STACKSIZE >> 2];
 uint64_t *code;
 uint8_t ip, sp, callsp, callbp;
};
// don't forget to update this!
const char *opcode2str[] = {
 "nop","push","pop","pushsp", "popsp",
 "add","fadd","sub","fsub","mul","fmul","idiv","fdiv","mod",
 "jmp","lt","gt","cmp","jnz","jz",
 "inc","dec","shl","shr","and","or","xor","not",
 "cpy","swap","load","store","call","ret",
 "halt"
};
void vm_exec(struct vm_cpu *const vm)
{
 uint64_t b, a;
 double da, db;
 static const void *dispatch[] = {
 &&exec_nop,
 &&exec_push, &&exec_pop, &&exec_pushsp, &&exec_popsp,
 &&exec_add, &&exec_fadd, &&exec_sub, &&exec_fsub,
 &&exec_mul, &&exec_fmul, &&exec_idiv, &&exec_fdiv, &&exec_mod,
 &&exec_jmp, &&exec_lt, &&exec_gt, &&exec_cmp,
 &&exec_jnz, &&exec_jz,
 &&exec_inc, &&exec_dec, &&exec_shl, &&exec_shr, &&exec_and, &&exec_or, &&exec_xor, &&exec_not,
 &&exec_cpy, &&exec_swap, &&exec_load, &&exec_store,
 &&exec_call, &&exec_ret, 
 //&&exec_z,
 &&exec_halt
 };
 if( vm->code[vm->ip] > halt || vm->code[vm->ip] < nop ) {
 printf("illegal instruction exception! instruction == \'%" PRIu64 "\'\n", vm->code[vm->ip]);
 goto *dispatch[halt];
 return;
 }
 printf( "current instruction == \"%s\" @ ip == %u\n", opcode2str[vm->code[vm->ip]], vm->ip );
 goto *dispatch[ vm->code[vm->ip] ];
exec_nop:;
 vm->ip++;
 return;
exec_halt:;
 running = false;
 printf("vm done\n");
 return;
exec_cpy:; // makes a copy of the current value at the top of the stack and places the copy at the top.
 a = vm->stack[vm->sp];
 vm->stack[++vm->sp] = a;
 printf("copied %" PRIu64 ", top of stack: %" PRIu64 "\n", vm->stack[vm->sp-1], vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_swap:; // swaps two, topmost stack values.
 a = vm->stack[vm->sp--];
 b = vm->stack[vm->sp--];
 vm->stack[vm->sp++] = b;
 vm->stack[vm->sp++] = a;
 printf("swapped: a == %" PRIu64 " | b == %" PRIu64 "\n", vm->stack[vm->sp-2], vm->stack[vm->sp-1]);
 vm->ip++;
 return;
exec_load:; // stores a memory value into the top of the stack. pretty much push from memory.
 a = vm->code[++vm->ip];
 vm->stack[++vm->sp] = vm->memory[a];
 printf("loaded %" PRIu64 " from memory[%" PRIu64 "]\n", vm->stack[vm->sp], a);
 vm->ip++;
 return;
exec_store:; // pops value off stack into memory.
 a = vm->code[++vm->ip];
 vm->memory[a] = vm->stack[vm->sp--];
 printf("stored %" PRIu64 " to memory[%" PRIu64 "] | memory[%" PRIu64 "] = %" PRIu64 "\n", vm->memory[a], a, a, vm->stack[vm->sp+1]);
 vm->ip++;
 return;
// procedure instructions
exec_call:; // calling a procedure
 vm->ip++; // increment to function address
 printf("calling address: %u\n", vm->ip);
 vm->callstack[++vm->callsp] = vm->ip+1; // save post address so we can jump back to it after we finish.
 vm->callbp = vm->callsp; // save stack pointer to frame pointer so we can make a stack frame
 vm->ip = vm->code[vm->ip]; // jump to function address.
 printf("call return addr: %" PRIu64 " | frame ptr == %u\n", vm->callstack[vm->callsp], vm->callbp);
 return;
exec_ret:;
 vm->callsp = vm->callbp;
 printf("callsp set to callbp, callsp == %u\n", vm->callsp);
 vm->ip = vm->callstack[vm->callsp--];
 vm->callbp = vm->callsp;
 printf("returning to address: %u\n", vm->ip);
 return;
// various jumps
exec_jmp:; // unconditional jump
 vm->ip = vm->code[vm->ip+1];
 printf("jumping to... %u\n", vm->ip);
 return;
exec_jnz:; // Jump if Not Zero = JNZ
 ++vm->ip;
 vm->ip = (vm->stack[vm->sp]) ? vm->code[vm->ip] : vm->ip+1;
 printf("jnz'ing to... %u\n", vm->ip);
 return;
exec_jz:; // Jump if Zero = JZ
 ++vm->ip;
 vm->ip = (!vm->stack[vm->sp]) ? vm->code[vm->ip] : vm->ip+1;
 printf("jz'ing to... %u\n", vm->ip);
 return;
// conditional stuff. Conditionals are always done signed I believe.
exec_lt:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = (int64_t)a < (int64_t)b;
 printf("less than result %" PRIu64 " < %" PRIu64 " == %" PRIu64 "\n", a, b, vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_gt:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = (int64_t)a > (int64_t)b;
 printf("greater than result %" PRIu64 " > %" PRIu64 " == %" PRIu64 "\n", a, b, vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_cmp:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = (int64_t)a == (int64_t)b;
 printf("compare result %" PRIu64 " == %" PRIu64 " %" PRIu64 "\n", a, b, vm->stack[vm->sp]);
 vm->ip++;
 return;
// pushes and pops
exec_push:; // put an item on the top of the stack
 ++vm->sp;
 if( !vm->sp ) { // if we increment sp and sp is 0, we ran out of stack memory.
 printf("stack overflow!\n");
 goto *dispatch[halt];
 }
 vm->stack[vm->sp] = vm->code[++vm->ip];
 printf("pushing %" PRIu64 "\n", vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_pushsp:; // pushes value of sp to the top of the stack
 ++vm->sp;
 if( !vm->sp ) { // if we increment sp and sp is 0, we ran out of stack memory.
 printf("stack overflow!\n");
 goto *dispatch[halt];
 }
 vm->stack[vm->sp] = vm->sp-1;
 printf("pushing sp val of %" PRIu64 "\n", vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_pop:; // reduce stack
 if( vm->sp ) // make sure that there's something in the stack before popping.
 vm->sp--;
 if( vm->sp==255 ) { // if we decrement sp and sp's bits went all 1, we popped too much!
 printf("stack underflow!\n");
 goto *dispatch[halt];
 }
 printf("popped, stack pointer %x\n", vm->sp);
 vm->ip++;
 return;
exec_popsp:; // Pops value off top of stack and sets SP to that value
 if( vm->sp )
 vm->sp = vm->stack[vm->sp];
 printf("popped sp, stack pointer %x\n", vm->sp);
 vm->ip++;
 return;
// arithmetic maths. order: int math, float math is last.
exec_add:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 // we then add the result and push it to the stack
 vm->stack[++vm->sp] = a + b; // set the value to the top of the stack
 printf("add result %" PRIu64 "\n", vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_sub:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = a - b;
 // 0x8... is uint64_t's sign bit
 if( vm->stack[vm->sp] & 0x8000000000000000 )
 printf( "sub result %lli\n", (int64_t)vm->stack[vm->sp] );
 else printf( "sub result %" PRIu64 "\n", vm->stack[vm->sp] );
 vm->ip++;
 return;
exec_mul:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = a * b;
 printf("mul result %" PRIu64 "\n", vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_idiv:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 if( b==0 ) {
 printf("div by 0 not allowed, restoring stack\n");
 goto *dispatch[halt];
 }
 vm->stack[++vm->sp] = a / b;
 printf("div result %" PRIu64 "\n", vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_mod:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 if( b==0 ) {
 printf("mod by 0 not allowed, restoring stack\n");
 goto *dispatch[halt];
 }
 vm->stack[++vm->sp] = a % b;
 printf("mod result %" PRIu64 "\n", vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_inc:;
 vm->stack[vm->sp]++;
 printf("increment result %" PRIu64 "\n", vm->stack[vm->sp]);
 vm->ip++;
 return;
exec_dec:;
 vm->stack[vm->sp]--;
 printf("decrement result %" PRIu64 "\n", vm->stack[vm->sp]);
 vm->ip++;
 return;
// bit wise maths
exec_shl:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = b << a;
 printf( "bit shift left result %" PRIu64 "\n", vm->stack[vm->sp] );
 vm->ip++;
 return;
exec_shr:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = b >> a;
 printf( "bit shift right result %" PRIu64 "\n", vm->stack[vm->sp] );
 vm->ip++;
 return;
exec_and:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = b & a;
 printf( "bitwise and result %" PRIu64 "\n", vm->stack[vm->sp] );
 vm->ip++;
 return;
exec_or:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = b | a;
 printf( "bitwise or result %" PRIu64 "\n", vm->stack[vm->sp] );
 vm->ip++;
 return;
exec_xor:;
 b = vm->stack[vm->sp--];
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = b ^ a;
 printf( "bitwise xor result %" PRIu64 "\n", vm->stack[vm->sp] );
 vm->ip++;
 return;
exec_not:;
 a = vm->stack[vm->sp--];
 vm->stack[++vm->sp] = ~a;
 printf( "bitwise not result %" PRIu64 "\n", vm->stack[vm->sp] );
 vm->ip++;
 return;
// floating point maths
exec_fadd:;
 db = *(double *)(&vm->stack[vm->sp--]);
 da = *(double *)(&vm->stack[vm->sp--]);
 printf("da %f | db %f\n", da, db);
 db += da;
 vm->stack[++vm->sp] = *(uint64_t *)(&db);
 printf("f add result %f\n", db);
 vm->ip++;
 return;
exec_fsub:;
 db = *(double *)(&vm->stack[vm->sp--]);
 da = *(double *)(&vm->stack[vm->sp--]);
 //printf("da %f | db %f\n", da, db);
 db -= da;
 vm->stack[++vm->sp] = *(uint64_t *)(&db);
 printf("f sub result %f\n", db);
 vm->ip++;
 return;
exec_fmul:;
 db = *(double *)(&vm->stack[vm->sp--]);
 da = *(double *)(&vm->stack[vm->sp--]);
 //printf("da %f | db %f\n", da, db);
 db *= da;
 vm->stack[++vm->sp] = *(uint64_t *)(&db);
 printf("f mul result %f\n", db);
 vm->ip++;
 return;
exec_fdiv:;
 db = *(double *)(&vm->stack[vm->sp--]);
 da = *(double *)(&vm->stack[vm->sp--]);
 printf("da %f | db %f\n", da, db);
 if( !db ) {
 printf("fdiv by 0 not allowed, restoring stack\n");
 goto *dispatch[halt];
 }
 db /= da;
 vm->stack[++vm->sp] = *(uint64_t *)(&db);
 printf("f div result %f\n", db);
 vm->ip++;
 return;
}
uint64_t get_file_size(FILE *pFile)
{
 uint64_t size = 0;
 if( !pFile )
 return size;
 if( !fseek(pFile, 0, SEEK_END) ) {
 size = ( uint64_t )ftell(pFile);
 rewind(pFile);
 }
 return size;
}
int main(void)
{
 typedef uint64_t casm[] ;
 /*
 uint i = 10;
 uint n = 0;
 while( n<i )
 ++n;
 */
 casm loop = {
 push, 10, // push 10
 store, 0, // store 10 to memory address 0
 push, 0, // push 0
 store, 1, // store 0 to address #1
 load, 1, // push 0 from address #1
 load, 0, // push 10 from address #0
 lt, // 0 < 10?
 //jz, 24, // jump to halt if 0.
 jz, 22,
 load, 1, // push 0 from memory
 //push, 1, // push 1,
 //add, // increment by 1, possibly change to inc?
 inc, // increment by 1
 store, 1, // store result to mem address #1.
 jmp, 8, // jump to loading 0x01 into stack.
 halt
 };
 /*
 uint a = 10;
 if( a )
 a = 15;
 */
 casm ifcond = {
 push, 10,
 store, 0x0,
 load, 0x0,
 jz, 12,
 push, 15,
 store, 0x0,
 halt
 };
 // test call and ret opcodes
 casm func = {
 nop,
 call, 5, // 1
 jmp, 11,
 push, 10,
 push, 15,
 add,
 ret, // 4
 halt, // 3
 };
 // test calls within calls and returning.
 uint8_t func1=4, func2=9, func3=18;
 casm callercalling = {
 nop,
 call, func1,
 halt,
 // func1:
 push, 9, // 4
 call, func2,
 ret,
 // func2:
 push, 5, // 10
 push, 10,
 mul,
 mul,
 call, func3, // 15
 ret, 
 // func3:
 push, 40,
 idiv, // 20
 ret,
 };
 casm test_pushsppopsp = {
 nop,
 push, 10,
 push, 10,
 push, 2,
 popsp,
 pushsp,
 halt
 };
 casm callcallcall = {
 nop,
 call, 4,
 halt, // 3
 call, 7, // 4
 ret,
 call, 10, // 6
 ret,
 call, 13, // 8
 ret,
 call, 15, // 10
 ret
 };
 struct vm_cpu *p_vm = &(struct vm_cpu){ 0 };
 p_vm->code = callcallcall;
 while( running ) {
 vm_exec( p_vm );
 }
 return 0;
}

• chux: What is the purpose of locating the comment // push and pop are always assumed to hold a long int in enum InstrSet {? Why is it there?
• chux: Little left to add after @deduplicator extensive answer. if( vm->sp==255 ) --> if( vm->sp==STACKSIZE-1)

Answer 1

1. There's no reason, good or otherwise, why you wrote your own jump-table and dispatch yourself using the gcc-extension of computed goto instead of simply using the standard feature switch. It's more verbose, and certainly no more efficient than letting the compiler do it for you.

2. You know that the first enumerator, unless given a different value manually, is always zero?

3. The comments on the enum InstrSet are quite cryptic. As is their grouping.

4. Do you know the X-Macro-Trick? That lets you generate both the enum InstrSet and the array opcode2str from a common source.

   #define INSTR_SET \
    X(nop) \
    X(push) X(pop) X(pushsp) X(popsp) \
    X(add) X(fadd) X(sub) X(fsub) \
    X(mul) X(fmul) X(idiv) X(fdiv) X(mod) \
    X(jmp) X(lt) X(gt) X(cmp) \
    X(jnz) X(jz) \
    X(inc) X(dec) X(shl) X(shr) X(and) X(or) X(xor) X(not) \
    X(cpy) X(swap) \
    X(load) X(store) \
    X(call) X(ret) \
    X(halt)
   #define X(x) x,
   enum InstrSet { INSTR_SET };
   #undef X
   #define X(x) # x ,
   const char* opcode2str[] = { INSTR_SET };
   #undef X
   #undef INSTR_SET
   

5. Any code in the same block after goto or return is somewhat redundant without intervening goto-labels.

6. You know that an unsigned number cannot be smaller than zero? You explicitly made sure nop was zero (it would have been anyway) and vm->code[vm->ip] is a uint64_t.

   if( vm->code[vm->ip] > halt || vm->code[vm->ip] < nop ) {
   

7. Any reason you hardcode that halt is the last instruction? Use sizeof to derive it from opcode2str. Or change the output-format slightly and use a default-case.

8. Why don't you output the position of an illegal instruction?

9. Your output on encountering an illegal instruction is fixed, even though you tried to output the offending instruction. Anyway, why don't you print the position too? %% results in output of a literal %.

   printf("illegal instruction exception! instruction == \'%"
    PRIu64 "\'\n", vm->code[vm->ip]);
   

10. Try to limit the line-length to something sensible. Horizontal scrolling is murder on maintainability.

11. Loose redundant casts. Even non-redundant ones normally indicate an error.

12. Use a union if you want to treat a uint64_t as a double, or its an aliasing-violation. And you should assert they use the same amount of memory somewhere.

13. You should automate your tests. And also test bad inputs. See point 8.

14. I don't see why you are using a compound-literal in main. Just put it in an automatic variable, and take the address where needed.

• Nergal: for #12, previous review told me NOT to use a union and use the pointer method haha. 1st version of this vm used a union to convert but reviewer said that's bad.
• Deduplicator: Well, the union way is defined in C, in C++ you need to go over std::byte or (unsiged) char. en.cppreference.com/w/cpp/language/reinterpret_cast Whoever told you anything else...