janet/vm.c

#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include "vm.h"
#include "value.h"
#include "ds.h"

static const char OOM[] = "Out of memory";
static const char NO_UPVALUE[] = "No upvalue";
static const char EXPECTED_FUNCTION[] = "Expected function";
static const char VMS_EXPECTED_NUMBER_ROP[] = "Expected right operand to be number";
static const char VMS_EXPECTED_NUMBER_LOP[] = "Expected left operand to be number";

/* The size of a StackFrame in units of Values. */
#define FRAME_SIZE ((sizeof(GstStackFrame) + sizeof(GstValue) - 1) / sizeof(GstValue))

/* Get the stack frame pointer for a thread */
static GstStackFrame *thread_frame(GstThread * thread) {
    return (GstStackFrame *)(thread->data + thread->count - FRAME_SIZE);
}

/* Ensure that a thread has enough space in it */
static void thread_ensure(Gst *vm, GstThread *thread, uint32_t size) {
	if (size > thread->capacity) {
    	uint32_t newCap = size * 2;
		GstValue *newData = gst_alloc(vm, sizeof(GstValue) * newCap);
		memcpy(newData, thread->data, thread->capacity * sizeof(GstValue));
		thread->data = newData;
		thread->capacity = newCap;
	}
}

/* Push a stack frame onto a thread */
static void thread_push(Gst *vm, GstThread *thread, GstValue callee, uint32_t size) {
    uint16_t oldSize;
    uint32_t nextCount, i;
    GstStackFrame *frame;
    if (thread->count) {
        frame = thread_frame(thread);
        oldSize = frame->size;
    } else {
        oldSize = 0;
    }
    nextCount = thread->count + oldSize + FRAME_SIZE;
    thread_ensure(vm, thread, nextCount + size);
    thread->count = nextCount;
    /* Ensure values start out as nil so as to not confuse
     * the garabage collector */
    for (i = nextCount; i < nextCount + size; ++i)
        thread->data[i].type = GST_NIL;
    vm->base = thread->data + thread->count;
    vm->frame = frame = (GstStackFrame *)(vm->base - FRAME_SIZE);
    /* Set up the new stack frame */
    frame->prevSize = oldSize;
    frame->size = size;
    frame->env = NULL;
    frame->callee = callee;
}

/* Copy the current function stack to the current closure
   environment. Call when exiting function with closures. */
static void thread_split_env(Gst *vm) {
    GstStackFrame *frame = vm->frame;
    GstFuncEnv *env = frame->env;
    /* Check for closures */
    if (env) {
        GstThread *thread = vm->thread;
        uint32_t size = frame->size;
        env->thread = NULL;
        env->stackOffset = size;
        env->values = gst_alloc(vm, sizeof(GstValue) * size);
        memcpy(env->values, thread->data + thread->count, size * sizeof(GstValue));
    }
}

/* Pop the top-most stack frame from stack */
static void thread_pop(Gst *vm) {
    GstThread *thread = vm->thread;
    GstStackFrame *frame = vm->frame;
    uint32_t delta = FRAME_SIZE + frame->prevSize;
    if (thread->count) {
        thread_split_env(vm);
    } else {
        gst_error(vm, "Nothing to pop from stack.");
    }
    thread->count -= delta;
    vm->base -= delta;
    vm->frame = (GstStackFrame *)(vm->base - FRAME_SIZE);
}


/* The metadata header associated with an allocated block of memory */
#define gc_header(mem) ((GCMemoryHeader *)(mem) - 1)

/* Memory header struct. Node of a linked list of memory blocks. */
typedef struct GCMemoryHeader GCMemoryHeader;
struct GCMemoryHeader {
    GCMemoryHeader * next;
    uint32_t color : 1;
};

/* Forward declaration */
static void gst_mark(Gst *vm, GstValue *x);

/* Helper to mark function environments */
static void gst_mark_funcenv(Gst *vm, GstFuncEnv *env) {
    if (gc_header(env)->color != vm->black) {
        GstValue temp;
        gc_header(env)->color = vm->black;
        if (env->thread) {
            temp.type = GST_THREAD;
            temp.data.thread = env->thread;
            gst_mark(vm, &temp);
        }
        if (env->values) {
            uint32_t count = env->stackOffset;
            uint32_t i;
            gc_header(env->values)->color = vm->black;
            for (i = 0; i < count; ++i)
                gst_mark(vm, env->values + i);
        }
    }
}

/* GC helper to mark a FuncDef */
static void gst_mark_funcdef(Gst *vm, GstFuncDef *def) {
    if (gc_header(def)->color != vm->black) {
        gc_header(def)->color = vm->black;
        gc_header(def->byteCode)->color = vm->black;
        uint32_t count, i;
        if (def->literals) {
            count = def->literalsLen;
            gc_header(def->literals)->color = vm->black;
            for (i = 0; i < count; ++i) {
                /* If the literal is a NIL type, it actually
                 * contains a FuncDef */
               	if (def->literals[i].type == GST_NIL) {
					gst_mark_funcdef(vm, (GstFuncDef *) def->literals[i].data.pointer);
               	} else {
                    gst_mark(vm, def->literals + i);
               	}
            }
        }
    }
}

/* Helper to mark a stack frame. Returns the next frame. */
static GstStackFrame *gst_mark_stackframe(Gst *vm, GstStackFrame *frame) {
    uint32_t i;
    GstValue *stack = (GstValue *)frame + FRAME_SIZE;
    gst_mark(vm, &frame->callee);
    if (frame->env)
        gst_mark_funcenv(vm, frame->env);
    for (i = 0; i < frame->size; ++i)
        gst_mark(vm, stack + i);
    return (GstStackFrame *)(stack + frame->size);
}

/* Mark allocated memory associated with a value. This is
 * the main function for doing the garbage collection mark phase. */
static void gst_mark(Gst *vm, GstValue *x) {
    switch (x->type) {
        case GST_NIL:
        case GST_BOOLEAN:
        case GST_NUMBER:
        case GST_CFUNCTION:
            break;

        case GST_STRING:
            gc_header(gst_string_raw(x->data.string))->color = vm->black;
            break;

        case GST_BYTEBUFFER:
            gc_header(x->data.buffer)->color = vm->black;
            gc_header(x->data.buffer->data)->color = vm->black;
            break;

        case GST_ARRAY:
            if (gc_header(x->data.array)->color != vm->black) {
                uint32_t i, count;
                count = x->data.array->count;
                gc_header(x->data.array)->color = vm->black;
                gc_header(x->data.array->data)->color = vm->black;
                for (i = 0; i < count; ++i)
                    gst_mark(vm, x->data.array->data + i);
            }
            break;

        case GST_THREAD:
            if (gc_header(x->data.thread)->color != vm->black) {
                GstThread *thread = x->data.thread;
                GstStackFrame *frame = (GstStackFrame *)thread->data;
                GstStackFrame *end = thread_frame(thread);
                gc_header(thread)->color = vm->black;
                gc_header(thread->data)->color = vm->black;
                while (frame <= end)
                    frame = gst_mark_stackframe(vm, frame);
            }
            break;

        case GST_FUNCTION:
            if (gc_header(x->data.function)->color != vm->black) {
                GstFunction *f = x->data.function;
                gc_header(f)->color = vm->black;
                gst_mark_funcdef(vm, f->def);
                if (f->env)
                    gst_mark_funcenv(vm, f->env);
                if (f->parent) {
                    GstValue temp;
                    temp.type = GST_FUNCTION;
                    temp.data.function = f->parent;
                    gst_mark(vm, &temp);
                }
            }
            break;

        case GST_OBJECT:
            if (gc_header(x->data.object)->color != vm->black) {
                uint32_t i;
                GstBucket *bucket;
                gc_header(x->data.object)->color = vm->black;
                gc_header(x->data.object->buckets)->color = vm->black;
                for (i = 0; i < x->data.object->capacity; ++i) {
					bucket = x->data.object->buckets[i];
					while (bucket) {
						gst_mark(vm, &bucket->key);
						gst_mark(vm, &bucket->value);
						bucket = bucket->next;
					}
                }
            }
            break;

    }

}

/* Iterate over all allocated memory, and free memory that is not
 * marked as reachable. Flip the gc color flag for next sweep. */
static void gst_sweep(Gst *vm) {
    GCMemoryHeader *previous = NULL;
    GCMemoryHeader *current = vm->blocks;
    GCMemoryHeader *next;
    while (current) {
        next = current->next;
        if (current->color != vm->black) {
            if (previous) {
                previous->next = next;
            } else {
                vm->blocks = next;
            }
            free(current);
        } else {
            previous = current;
        }
        current = next;
    }
    /* Rotate flag */
    vm->black = !vm->black;
}

/* Prepare a memory block */
static void *gst_alloc_prepare(Gst *vm, char *rawBlock, uint32_t size) {
    GCMemoryHeader *mdata;
    if (rawBlock == NULL) {
        gst_crash(vm, OOM);
    }
    vm->nextCollection += size;
    mdata = (GCMemoryHeader *)rawBlock;
    mdata->next = vm->blocks;
    vm->blocks = mdata;
    mdata->color = !vm->black;
    return rawBlock + sizeof(GCMemoryHeader);
}

/* Allocate some memory that is tracked for garbage collection */
void *gst_alloc(Gst *vm, uint32_t size) {
    uint32_t totalSize = size + sizeof(GCMemoryHeader);
    return gst_alloc_prepare(vm, malloc(totalSize), totalSize);
}

/* Allocate some zeroed memory that is tracked for garbage collection */
void *gst_zalloc(Gst *vm, uint32_t size) {
    uint32_t totalSize = size + sizeof(GCMemoryHeader);
    return gst_alloc_prepare(vm, calloc(1, totalSize), totalSize);
}

/* Run garbage collection */
void gst_collect(Gst *vm) {
    if (vm->lock > 0) return;
    /* Thread can be null */
    if (vm->thread) {
        GstValue thread;
        thread.type = GST_THREAD;
        thread.data.thread = vm->thread;
        gst_mark(vm, &thread);
    }
    gst_mark(vm, &vm->ret);
    gst_sweep(vm);
    vm->nextCollection = 0;
}

/* Run garbage collection if needed */
void gst_maybe_collect(Gst *vm) {
    if (vm->nextCollection >= vm->memoryInterval)
        gst_collect(vm);
}

/* Get an upvalue */
static GstValue *gst_vm_upvalue_location(Gst *vm, GstFunction *fn, uint16_t level, uint16_t index) {
    GstFuncEnv *env;
    GstValue *stack;
    if (!level)
        return vm->base + index;
    while (fn && --level)
        fn = fn->parent;
    gst_assert(vm, fn, NO_UPVALUE);
    env = fn->env;
    if (env->thread)
        stack = env->thread->data + env->stackOffset;
    else
        stack = env->values;
    return stack + index;
}

/* Get a literal */
static GstValue gst_vm_literal(Gst *vm, GstFunction *fn, uint16_t index) {
    if (index > fn->def->literalsLen) {
        gst_error(vm, NO_UPVALUE);
    }
    return fn->def->literals[index];
}

/* Boolean truth definition */
static int truthy(GstValue v) {
    return v.type != GST_NIL && !(v.type == GST_BOOLEAN && !v.data.boolean);
}

/* Return from the vm */
static void gst_vm_return(Gst *vm, GstValue ret) {
    thread_pop(vm);
    if (vm->thread->count == 0) {
        gst_exit(vm, ret);
    }
    vm->pc = vm->frame->pc;
    vm->base[vm->frame->ret] = ret;
}

/* Implementation of the opcode for function calls */
static void gst_vm_call(Gst *vm) {
    GstThread *thread = vm->thread;
    GstValue callee = vm->base[vm->pc[1]];
    uint32_t arity = vm->pc[3];
    uint32_t oldCount = thread->count;
    uint32_t i;
    GstValue *oldBase;
    vm->frame->pc = vm->pc + 4 + arity;
    vm->frame->ret = vm->pc[2];
    if (callee.type == GST_FUNCTION) {
        GstFunction *fn = callee.data.function;
        thread_push(vm, thread, callee, fn->def->locals);
    } else if (callee.type == GST_CFUNCTION) {
        thread_push(vm, thread, callee, arity);
    } else {
        gst_error(vm, EXPECTED_FUNCTION);
    }
    oldBase = thread->data + oldCount;
    if (callee.type == GST_CFUNCTION) {
        for (i = 0; i < arity; ++i)
            vm->base[i] = oldBase[vm->pc[4 + i]];
        ++vm->lock;
        gst_vm_return(vm, callee.data.cfunction(vm));
        --vm->lock;
    } else {
        GstFunction *f = callee.data.function;
        uint32_t locals = f->def->locals;
        for (i = 0; i < arity; ++i)
            vm->base[i] = oldBase[vm->pc[4 + i]];
        for (; i < locals; ++i)
            vm->base[i].type = GST_NIL;
        vm->pc = f->def->byteCode;
    }
}

/* Implementation of the opcode for tail calls */
static void gst_vm_tailcall(Gst *vm) {
    GstThread *thread = vm->thread;
    GstValue callee = vm->base[vm->pc[1]];
    uint32_t arity = vm->pc[2];
    uint16_t newFrameSize, currentFrameSize;
    uint32_t i;
    /* Check for closures */
    thread_split_env(vm);
    if (callee.type == GST_CFUNCTION) {
        newFrameSize = arity;
    } else if (callee.type == GST_FUNCTION) {
        GstFunction * f = callee.data.function;
        newFrameSize = f->def->locals;
    } else {
        gst_error(vm, EXPECTED_FUNCTION);
    }
    /* Ensure stack has enough space for copies of arguments */
    currentFrameSize = vm->frame->size;
    thread_ensure(vm, thread, thread->count + currentFrameSize + arity);
    vm->base = thread->data + thread->count;
    /* Copy the arguments into the extra space */
    for (i = 0; i < arity; ++i)
        vm->base[currentFrameSize + i] = vm->base[vm->pc[3 + i]];
    /* Copy the end of the stack to the parameter position */
    memcpy(vm->base, vm->base + currentFrameSize, arity * sizeof(GstValue));
    /* nil the non argument part of the stack for gc */
    for (i = arity; i < newFrameSize; ++i)
        vm->base[i].type = GST_NIL;
    /* Update the stack frame */
    vm->frame->size = newFrameSize;
    vm->frame->callee = callee;
    vm->frame->env = NULL;
    if (callee.type == GST_CFUNCTION) {
        ++vm->lock;
        gst_vm_return(vm, callee.data.cfunction(vm));
        --vm->lock;
    } else {
        GstFunction *f = callee.data.function;
        vm->pc = f->def->byteCode;
    }
}

/* Instantiate a closure */
static GstValue gst_vm_closure(Gst *vm, uint16_t literal) {
    GstThread *thread = vm->thread;
    if (vm->frame->callee.type != GST_FUNCTION) {
        gst_error(vm, EXPECTED_FUNCTION);
    } else {
        GstValue constant, ret;
        GstFunction *fn, *current;
        GstFuncEnv *env = vm->frame->env;
        if (!env) {
            env = gst_alloc(vm, sizeof(GstFuncEnv));
            env->thread = thread;
            env->stackOffset = thread->count;
            env->values = NULL;
            vm->frame->env = env;
        }
        current = vm->frame->callee.data.function;
        constant = gst_vm_literal(vm, current, literal);
        if (constant.type != GST_NIL) {
            gst_error(vm, "Error trying to create closure");
        }
        fn = gst_alloc(vm, sizeof(GstFunction));
        fn->def = (GstFuncDef *) constant.data.pointer;
        fn->parent = current;
        fn->env = env;
        ret.type = GST_FUNCTION;
        ret.data.function = fn;
        return ret;
    }
}

/* Start running the VM */
int gst_start(Gst *vm) {

    /* Set jmp_buf to jump back to for return. */
    {
        int n;
        if ((n = setjmp(vm->jump))) {
            vm->lock = 0;
            /* Good return */
            if (n == 1) {
                return 0;
            } else {
                /* Error or crash. Handling TODO. */
                return n;
            }
        }
    }

    for (;;) {
        GstValue temp, v1, v2;
        uint16_t opcode = *vm->pc;

        switch (opcode) {

        #define DO_BINARY_MATH(op) \
            v1 = vm->base[vm->pc[2]]; \
            v2 = vm->base[vm->pc[3]]; \
            gst_assert(vm, v1.type == GST_NUMBER, VMS_EXPECTED_NUMBER_LOP); \
            gst_assert(vm, v2.type == GST_NUMBER, VMS_EXPECTED_NUMBER_ROP); \
            temp.type = GST_NUMBER; \
            temp.data.number = v1.data.number op v2.data.number; \
            vm->base[vm->pc[1]] = temp; \
            vm->pc += 4; \
            break;

        case GST_OP_ADD: /* Addition */
            DO_BINARY_MATH(+)

        case GST_OP_SUB: /* Subtraction */
            DO_BINARY_MATH(-)

        case GST_OP_MUL: /* Multiplication */
            DO_BINARY_MATH(*)

        case GST_OP_DIV: /* Division */
            DO_BINARY_MATH(/)

        #undef DO_BINARY_MATH

        case GST_OP_NOT: /* Boolean unary (Boolean not) */
            temp.type = GST_BOOLEAN;
            temp.data.boolean = !truthy(vm->base[vm->pc[2]]);
            vm->base[vm->pc[1]] = temp;
            vm->pc += 3;
            break;

        case GST_OP_LD0: /* Load 0 */
            temp.type = GST_NUMBER;
            temp.data.number = 0;
            vm->base[vm->pc[1]] = temp;
            vm->pc += 2;
            break;

        case GST_OP_LD1: /* Load 1 */
            temp.type = GST_NUMBER;
            temp.data.number = 1;
            vm->base[vm->pc[1]] = temp;
            vm->pc += 2;
            break;

        case GST_OP_FLS: /* Load False */
            temp.type = GST_BOOLEAN;
            temp.data.boolean = 0;
            vm->base[vm->pc[1]] = temp;
            vm->pc += 2;
            break;

        case GST_OP_TRU: /* Load True */
            temp.type = GST_BOOLEAN;
            temp.data.boolean = 1;
            vm->base[vm->pc[1]] = temp;
            vm->pc += 2;
            break;

        case GST_OP_NIL: /* Load Nil */
            temp.type = GST_NIL;
            vm->base[vm->pc[1]] = temp;
            vm->pc += 2;
            break;

        case GST_OP_I16: /* Load Small Integer */
            temp.type = GST_NUMBER;
            temp.data.number = ((int16_t *)(vm->pc))[2];
            vm->base[vm->pc[1]] = temp;
            vm->pc += 3;
            break;

        case GST_OP_UPV: /* Load Up Value */
            temp = vm->frame->callee;
            gst_assert(vm, temp.type == GST_FUNCTION, EXPECTED_FUNCTION);
            vm->base[vm->pc[1]] = *gst_vm_upvalue_location(vm, temp.data.function, vm->pc[2], vm->pc[3]);
            vm->pc += 4;
            break;

        case GST_OP_JIF: /* Jump If */
            if (truthy(vm->base[vm->pc[1]])) {
                vm->pc += 4;
            } else {
                vm->pc += *((int32_t *)(vm->pc + 2));
            }
            break;

        case GST_OP_JMP: /* Jump */
            vm->pc += *((int32_t *)(vm->pc + 1));
            break;

        case GST_OP_CAL: /* Call */
            gst_vm_call(vm);
            break;

        case GST_OP_RET: /* Return */
            gst_vm_return(vm, vm->base[vm->pc[1]]);
            break;

        case GST_OP_SUV: /* Set Up Value */
            temp = vm->frame->callee;
            gst_assert(vm, temp.type == GST_FUNCTION, EXPECTED_FUNCTION);
            *gst_vm_upvalue_location(vm, temp.data.function, vm->pc[2], vm->pc[3]) = vm->base[vm->pc[1]];
            vm->pc += 4;
            break;

        case GST_OP_CST: /* Load constant value */
            temp = vm->frame->callee;
            gst_assert(vm, temp.type == GST_FUNCTION, EXPECTED_FUNCTION);
            vm->base[vm->pc[1]] = gst_vm_literal(vm, temp.data.function, vm->pc[2]);
            vm->pc += 3;
            break;

        case GST_OP_I32: /* Load 32 bit integer */
            temp.type = GST_NUMBER;
            temp.data.number = *((int32_t *)(vm->pc + 2));
            vm->base[vm->pc[1]] = temp;
            vm->pc += 4;
            break;

        case GST_OP_F64: /* Load 64 bit float */
            temp.type = GST_NUMBER;
            temp.data.number = (GstNumber) *((double *)(vm->pc + 2));
            vm->base[vm->pc[1]] = temp;
            vm->pc += 6;
            break;

        case GST_OP_MOV: /* Move Values */
            vm->base[vm->pc[1]] = vm->base[vm->pc[2]];
            vm->pc += 3;
            break;

        case GST_OP_CLN: /* Create closure from constant FuncDef */
            vm->base[vm->pc[1]] = gst_vm_closure(vm, vm->pc[2]);
            vm->pc += 3;
            break;

        case GST_OP_EQL: /* Equality */
            temp.type = GST_BOOLEAN;
            temp.data.boolean = gst_equals(vm->base[vm->pc[2]], vm->base[vm->pc[3]]);
            vm->base[vm->pc[1]] = temp;
            vm->pc += 4;
            break;

        case GST_OP_LTN: /* Less Than */
            temp.type = GST_BOOLEAN;
            temp.data.boolean = (gst_compare(vm->base[vm->pc[2]], vm->base[vm->pc[3]]) == -1);
            vm->base[vm->pc[1]] = temp;
            vm->pc += 4;
            break;

        case GST_OP_LTE: /* Less Than or Equal to */
            temp.type = GST_BOOLEAN;
            temp.data.boolean = (gst_compare(vm->base[vm->pc[2]], vm->base[vm->pc[3]]) != 1);
            vm->base[vm->pc[1]] = temp;
            vm->pc += 4;
            break;

        case GST_OP_ARR: /* Array literal */
            {
                uint32_t i;
                uint32_t arrayLen = vm->pc[2];
                GstArray *array = gst_array(vm, arrayLen);
                array->count = arrayLen;
                for (i = 0; i < arrayLen; ++i)
                    array->data[i] = vm->base[vm->pc[3 + i]];
                temp.type = GST_ARRAY;
                temp.data.array = array;
                vm->base[vm->pc[1]] = temp;
                vm->pc += 3 + arrayLen;
            }
            break;

        case GST_OP_DIC: /* Object literal */
            {
                uint32_t i = 3;
                uint32_t kvs = vm->pc[2];
                GstObject *o = gst_object(vm, kvs + 2);
                kvs = kvs + 3;
                while (i < kvs) {
                    v1 = vm->base[vm->pc[i++]];
                    v2 = vm->base[vm->pc[i++]];
                    gst_object_put(vm, o, v1, v2);
                }
                temp.type = GST_OBJECT;
                temp.data.object = o;
                vm->base[vm->pc[1]] = temp;
                vm->pc += kvs;
            }
            break;

        case GST_OP_TCL: /* Tail call */
            gst_vm_tailcall(vm);
            break;

        /* Macro for generating some math operators */
        #define DO_MULTI_MATH(op, start) { \
            uint16_t count = vm->pc[2]; \
            uint16_t i; \
            GstNumber accum = start; \
            for (i = 0; i < count; ++i) { \
                v1 = vm->base[vm->pc[3 + i]]; \
                gst_assert(vm, v1.type == GST_NUMBER, "Expected number"); \
                accum = accum op v1.data.number; \
            } \
            temp.type = GST_NUMBER; \
            temp.data.number = accum; \
            vm->base[vm->pc[1]] = temp; \
            vm->pc += 3 + count; \
            break; \
        }

        /* Vectorized math */
        case GST_OP_ADM:
            DO_MULTI_MATH(+, 0)

        case GST_OP_SBM:
            DO_MULTI_MATH(-, 0)

        case GST_OP_MUM:
            DO_MULTI_MATH(*, 1)

        case GST_OP_DVM:
            DO_MULTI_MATH(/, 1)

        #undef DO_MULTI_MATH

        case GST_OP_RTN: /* Return nil */
            temp.type = GST_NIL;
            gst_vm_return(vm, temp);
            break;

        case GST_OP_GET:
			temp = gst_get(vm, vm->base[vm->pc[2]], vm->base[vm->pc[3]]);
			vm->base[vm->pc[1]] = temp;
			vm->pc += 4;
            break;

        case GST_OP_SET:
			gst_set(vm, vm->base[vm->pc[1]], vm->base[vm->pc[2]], vm->base[vm->pc[3]]);
			vm->pc += 4;
            break;

        default:
           	gst_error(vm, "Unknown opcode");
            break;
        }

        /* Move collection only to places that allocate memory */
        /* This, however, is good for testing */
        gst_maybe_collect(vm);
    }
}

/* Get an argument from the stack */
GstValue gst_arg(Gst *vm, uint16_t index) {
    uint16_t frameSize = vm->frame->size;
    gst_assert(vm, frameSize > index, "Cannot get arg out of stack bounds");
    return vm->base[index];
}

/* Put a value on the stack */
void gst_set_arg(Gst* vm, uint16_t index, GstValue x) {
    uint16_t frameSize = vm->frame->size;
    gst_assert(vm, frameSize > index, "Cannot set arg out of stack bounds");
    vm->base[index] = x;
}

/* Get the size of the VMStack */
uint16_t gst_count_args(Gst *vm) {
    return vm->frame->size;
}

/* Initialize the VM */
void gst_init(Gst *vm) {
    vm->ret.type = GST_NIL;
    vm->base = NULL;
    vm->frame = NULL;
    vm->pc = NULL;
    vm->error = NULL;
    /* Garbage collection */
    vm->blocks = NULL;
    vm->nextCollection = 0;
    /* Setting memoryInterval to zero currently forces
     * a collection pretty much every cycle, which is
     * obviously horrible for performance. It helps ensure
     * there are no memory bugs during dev */
    vm->memoryInterval = 0;
    vm->black = 0;
    vm->lock = 0;
    /* Add thread */
    vm->thread = NULL;
}

/* Load a function into the VM. The function will be called with
 * no arguments when run */
void gst_load(Gst *vm, GstValue callee) {
    uint32_t startCapacity = 100;
    GstThread *thread = gst_alloc(vm, sizeof(GstThread));
    thread->data = gst_alloc(vm, sizeof(GstValue) * startCapacity);
    thread->capacity = startCapacity;
    thread->count = 0;
    vm->thread = thread;
    if (callee.type == GST_FUNCTION) {
        GstFunction *fn = callee.data.function;
        thread_push(vm, thread, callee, fn->def->locals);
        vm->pc = fn->def->byteCode;
    } else if (callee.type == GST_CFUNCTION) {
        thread_push(vm, thread, callee, 0);
        vm->pc = NULL;
    } else {
        return;
    }
}

/* Clear all memory associated with the VM */
void gst_deinit(Gst *vm) {
    GCMemoryHeader *current = vm->blocks;
    while (current) {
        GCMemoryHeader *next = current->next;
        free(current);
        current = next;
    }
    vm->blocks = NULL;
}