Merge branch 'stack-refactor-merge'

4 files changed, 299 insertions, 401 deletions

diff --git a/exec_stack.h b/exec_stack.h
new file mode 100644
index 00000000..95fca713
--- /dev/null
+++ b/exec_stack.h
@@ -0,0 +1,82 @@
+#ifndef EXEC_STACK_H
+#define EXEC_STACK_H
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct determine_alignment {
+  char x;
+  union { int i; double d; uint64_t u64; size_t sz; void* ptr; } u;
+};
+enum {ALIGNMENT = offsetof(struct determine_alignment, u)};
+
+static size_t align_round_up(size_t sz) {
+  return ((sz + (ALIGNMENT - 1)) / ALIGNMENT) * ALIGNMENT;
+}
+
+typedef int stack_ptr;
+
+struct stack {
+  char* mem_end; // one-past-the-end of allocated region
+  stack_ptr bound;
+  stack_ptr limit; // 0 - stack is empty
+};
+
+static void stack_init(struct stack* s) {
+  s->mem_end = 0;
+  s->bound = ALIGNMENT;
+  s->limit = 0;
+}
+
+static void stack_reset(struct stack* s) {
+  assert(s->limit == 0 && "stack freed while not empty");
+  char* mem_start = s->mem_end - ( -s->bound + ALIGNMENT);
+  free(mem_start);
+  stack_init(s);
+}
+
+static int stack_pop_will_free(struct stack* s, stack_ptr p) {
+  return p == s->limit;
+}
+
+static void* stack_block(struct stack* s, stack_ptr p) {
+  return (void*)(s->mem_end + p);
+}
+
+static stack_ptr* stack_block_next(struct stack* s, stack_ptr p) {
+  return &((stack_ptr*)stack_block(s, p))[-1];
+}
+
+static void stack_reallocate(struct stack* s, size_t sz) {
+  int old_mem_length = -(s->bound) + ALIGNMENT;
+  char* old_mem_start = s->mem_end - old_mem_length;
+  
+  int new_mem_length = align_round_up((old_mem_length + sz + 256) * 2);
+  char* new_mem_start = realloc(old_mem_start, new_mem_length);
+  memmove(new_mem_start + (new_mem_length - old_mem_length),
+            new_mem_start, old_mem_length);
+  s->mem_end = new_mem_start + new_mem_length;
+  s->bound = -(new_mem_length - ALIGNMENT);
+}
+
+static stack_ptr stack_push_block(struct stack* s, stack_ptr p, size_t sz) {
+  int alloc_sz = align_round_up(sz) + ALIGNMENT;
+  stack_ptr r = s->limit - alloc_sz;
+  if (r < s->bound) {
+    stack_reallocate(s, alloc_sz);
+  }
+  s->limit = r;
+  *stack_block_next(s, r) = p;
+  return r;
+}
+
+static stack_ptr stack_pop_block(struct stack* s, stack_ptr p, size_t sz) {
+  stack_ptr r = *stack_block_next(s, p);
+  if (p == s->limit) {
+    int alloc_sz = align_round_up(sz) + ALIGNMENT;
+    s->limit += alloc_sz;
+  }
+  return r;
+}
+#endif
diff --git a/execute.c b/execute.c
index b5168b6c..3be4abe0 100644
--- a/execute.c
+++ b/execute.c
@@ -5,12 +5,10 @@
 
 #include "execute.h"
 
+#include "exec_stack.h"
 #include "opcode.h"
 #include "bytecode.h"
 
-#include "forkable_stack.h"
-#include "frame_layout.h"
-
 #include "jv_alloc.h"
 #include "jq_parser.h"
 #include "locfile.h"
@@ -20,64 +18,171 @@
 #include "builtin.h"
 
 struct jq_state {
-  struct forkable_stack data_stk;
-  struct forkable_stack frame_stk;
-  struct forkable_stack fork_stk;
   void (*nomem_handler)(void *);
   void *nomem_handler_data;
   struct bytecode* bc;
+
+  struct stack stk;
+  stack_ptr curr_frame;
+  stack_ptr stk_top;
+  stack_ptr fork_top;
+
   jv path;
   int subexp_nest;
   int debug_trace_enabled;
   int initial_execution;
 };
 
-typedef struct {
-  FORKABLE_STACK_HEADER;
-  jv val;
-} data_stk_elem;
+struct closure {
+  struct bytecode* bc;
+  stack_ptr env;
+};
+
+union frame_entry {
+  struct closure closure;
+  jv localvar;
+};
+
+struct frame {
+  struct bytecode* bc;
+  stack_ptr env;
+  stack_ptr retdata;
+  uint16_t* retaddr;
+  /* bc->nclosures closures followed by bc->nlocals local variables */
+  union frame_entry entries[0]; 
+};
+
+static int frame_size(struct bytecode* bc) {
+  return sizeof(struct frame) + sizeof(union frame_entry) * (bc->nclosures + bc->nlocals);
+}
+
+static struct frame* frame_current(struct jq_state* jq) {
+  struct frame* fp = stack_block(&jq->stk, jq->curr_frame);
+
+  stack_ptr next = *stack_block_next(&jq->stk, jq->curr_frame);
+  if (next) {
+    struct frame* fpnext = stack_block(&jq->stk, next);
+    struct bytecode* bc = fpnext->bc;
+    assert(fp->retaddr >= bc->code && fp->retaddr < bc->code + bc->codelen);
+  } else {
+    assert(fp->retaddr == 0);
+  }
+  return fp;
+}
+
+static stack_ptr frame_get_level(struct jq_state* jq, int level) {
+  stack_ptr fr = jq->curr_frame;
+  for (int i=0; i<level; i++) {
+    struct frame* fp = stack_block(&jq->stk, fr);
+    fr = fp->env;
+  }
+  return fr;
+}
+
+static jv* frame_local_var(struct jq_state* jq, int var, int level) {
+  struct frame* fr = stack_block(&jq->stk, frame_get_level(jq, level));
+  assert(var >= 0);
+  assert(var < fr->bc->nlocals);
+  return &fr->entries[fr->bc->nclosures + var].localvar;
+}
+
+static struct closure make_closure(struct jq_state* jq, uint16_t* pc) {
+  uint16_t level = *pc++;
+  uint16_t idx = *pc++;
+  stack_ptr fridx = frame_get_level(jq, level);
+  struct frame* fr = stack_block(&jq->stk, fridx);
+  if (idx & ARG_NEWCLOSURE) {
+    int subfn_idx = idx & ~ARG_NEWCLOSURE;
+    assert(subfn_idx < fr->bc->nsubfunctions);
+    struct closure cl = {fr->bc->subfunctions[subfn_idx],
+                         fridx};
+    return cl;
+  } else {
+    int closure = idx;
+    assert(closure >= 0);
+    assert(closure < fr->bc->nclosures);
+    return fr->entries[closure].closure;
+  }
+}
+
+static struct frame* frame_push(struct jq_state* jq, struct closure callee, 
+                                uint16_t* argdef, int nargs) {
+  stack_ptr new_frame_idx = stack_push_block(&jq->stk, jq->curr_frame, frame_size(callee.bc));
+  struct frame* new_frame = stack_block(&jq->stk, new_frame_idx);
+  new_frame->bc = callee.bc;
+  new_frame->env = callee.env;
+  assert(nargs == new_frame->bc->nclosures);
+  union frame_entry* entries = new_frame->entries;
+  for (int i=0; i<nargs; i++) {
+    entries->closure = make_closure(jq, argdef + i * 2);
+    entries++;
+  }
+  for (int i=0; i<callee.bc->nlocals; i++) {
+    entries->localvar = jv_invalid();
+    entries++;
+  }
+  jq->curr_frame = new_frame_idx;
+  return new_frame;
+}
+
+static void frame_pop(struct jq_state* jq) {
+  assert(jq->curr_frame);
+  struct frame* fp = frame_current(jq);
+  if (stack_pop_will_free(&jq->stk, jq->curr_frame)) {
+    int nlocals = fp->bc->nlocals;
+    for (int i=0; i<nlocals; i++) {
+      jv_free(*frame_local_var(jq, i, 0));
+    }
+  }
+  jq->curr_frame = stack_pop_block(&jq->stk, jq->curr_frame, frame_size(fp->bc));
+}
 
 void stack_push(jq_state *jq, jv val) {
   assert(jv_is_valid(val));
-  data_stk_elem* s = forkable_stack_push(&jq->data_stk, sizeof(data_stk_elem));
-  s->val = val;
+  jq->stk_top = stack_push_block(&jq->stk, jq->stk_top, sizeof(jv));
+  jv* sval = stack_block(&jq->stk, jq->stk_top);
+  *sval = val;
 }
 
 jv stack_pop(jq_state *jq) {
-  data_stk_elem* s = forkable_stack_peek(&jq->data_stk);
-  jv val = s->val;
-  if (!forkable_stack_pop_will_free(&jq->data_stk)) {
+  jv* sval = stack_block(&jq->stk, jq->stk_top);
+  jv val = *sval;
+  if (!stack_pop_will_free(&jq->stk, jq->stk_top)) {
     val = jv_copy(val);
   }
-  forkable_stack_pop(&jq->data_stk);
+  jq->stk_top = stack_pop_block(&jq->stk, jq->stk_top, sizeof(jv));
   assert(jv_is_valid(val));
   return val;
 }
 
 
 struct forkpoint {
-  FORKABLE_STACK_HEADER;
-  struct forkable_stack_state saved_data_stack;
-  struct forkable_stack_state saved_call_stack;
+  stack_ptr saved_data_stack;
+  stack_ptr saved_curr_frame;
   int path_len, subexp_nest;
   uint16_t* return_address;
 };
 
+struct stack_pos {
+  stack_ptr saved_data_stack, saved_curr_frame;
+};
 
-void stack_save(jq_state *jq, uint16_t* retaddr){
-  struct forkpoint* fork = forkable_stack_push(&jq->fork_stk, sizeof(struct forkpoint));
-  forkable_stack_save(&jq->data_stk, &fork->saved_data_stack);
-  forkable_stack_save(&jq->frame_stk, &fork->saved_call_stack);
+struct stack_pos stack_get_pos(jq_state* jq) {
+  struct stack_pos sp = {jq->stk_top, jq->curr_frame};
+  return sp;
+}
+
+void stack_save(jq_state *jq, uint16_t* retaddr, struct stack_pos sp){
+  jq->fork_top = stack_push_block(&jq->stk, jq->fork_top, sizeof(struct forkpoint));
+  struct forkpoint* fork = stack_block(&jq->stk, jq->fork_top);
+  fork->saved_data_stack = jq->stk_top;
+  fork->saved_curr_frame = jq->curr_frame;
   fork->path_len = 
     jv_get_kind(jq->path) == JV_KIND_ARRAY ? jv_array_length(jv_copy(jq->path)) : 0;
   fork->subexp_nest = jq->subexp_nest;
   fork->return_address = retaddr;
-}
-
-void stack_switch(jq_state *jq) {
-  struct forkpoint* fork = forkable_stack_peek(&jq->fork_stk);
-  forkable_stack_switch(&jq->data_stk, &fork->saved_data_stack);
-  forkable_stack_switch(&jq->frame_stk, &fork->saved_call_stack);
+  jq->stk_top = sp.saved_data_stack;
+  jq->curr_frame = sp.saved_curr_frame;
 }
 
 void path_append(jq_state* jq, jv component) {
@@ -92,23 +197,24 @@ void path_append(jq_state* jq, jv component) {
 }
 
 uint16_t* stack_restore(jq_state *jq){
-  while (!forkable_stack_empty(&jq->data_stk) && 
-         forkable_stack_pop_will_free(&jq->data_stk)) {
-    jv_free(stack_pop(jq));
-  }
-  while (!forkable_stack_empty(&jq->frame_stk) && 
-         forkable_stack_pop_will_free(&jq->frame_stk)) {
-    frame_pop(&jq->frame_stk);
+  while (!stack_pop_will_free(&jq->stk, jq->fork_top)) {
+    if (stack_pop_will_free(&jq->stk, jq->stk_top)) {
+      jv_free(stack_pop(jq));
+    } else if (stack_pop_will_free(&jq->stk, jq->curr_frame)) {
+      frame_pop(jq);
+    } else {
+      assert(0);
+    }
   }
 
-  if (forkable_stack_empty(&jq->fork_stk)) {
+  if (jq->fork_top == 0) {
     return 0;
   }
 
-  struct forkpoint* fork = forkable_stack_peek(&jq->fork_stk);
+  struct forkpoint* fork = stack_block(&jq->stk, jq->fork_top);
   uint16_t* retaddr = fork->return_address;
-  forkable_stack_restore(&jq->data_stk, &fork->saved_data_stack);
-  forkable_stack_restore(&jq->frame_stk, &fork->saved_call_stack);
+  jq->stk_top = fork->saved_data_stack;
+  jq->curr_frame = fork->saved_curr_frame;
   int path_len = fork->path_len;
   if (jv_get_kind(jq->path) == JV_KIND_ARRAY) {
     assert(path_len >= 0);
@@ -117,15 +223,17 @@ uint16_t* stack_restore(jq_state *jq){
     assert(path_len == 0);
   }
   jq->subexp_nest = fork->subexp_nest;
-  forkable_stack_pop(&jq->fork_stk);
+  jq->fork_top = stack_pop_block(&jq->stk, jq->fork_top, sizeof(struct forkpoint));
   return retaddr;
 }
 
 static void jq_reset(jq_state *jq) {
   while (stack_restore(jq)) {}
-  assert(forkable_stack_empty(&jq->fork_stk));
-  assert(forkable_stack_empty(&jq->data_stk));
-  assert(forkable_stack_empty(&jq->frame_stk));
+
+  assert(jq->stk_top == 0);
+  assert(jq->fork_top == 0);
+  assert(jq->curr_frame == 0);
+  stack_reset(&jq->stk);
 
   if (jv_get_kind(jq->path) != JV_KIND_INVALID)
     jv_free(jq->path);
@@ -133,21 +241,6 @@ static void jq_reset(jq_state *jq) {
 }
 
 
-static struct closure make_closure(struct forkable_stack* stk, frame_ptr fr, uint16_t* pc) {
-  uint16_t level = *pc++;
-  uint16_t idx = *pc++;
-  fr = frame_get_level(stk, fr, level);
-  if (idx & ARG_NEWCLOSURE) {
-    int subfn_idx = idx & ~ARG_NEWCLOSURE;
-    assert(subfn_idx < frame_self(fr)->bc->nsubfunctions);
-    struct closure cl = {frame_self(fr)->bc->subfunctions[subfn_idx],
-                         forkable_stack_to_idx(stk, fr)};
-    return cl;
-  } else {
-    return *frame_closure_arg(fr, idx);
-  }
-}
-
 void print_error(jv value) {
   assert(!jv_is_valid(value));
   jv msg = jv_invalid_get_msg(value);
@@ -172,30 +265,33 @@ jv jq_next(jq_state *jq) {
     uint16_t opcode = *pc;
 
     if (jq->debug_trace_enabled) {
-      dump_operation(frame_current_bytecode(&jq->frame_stk), pc);
+      dump_operation(frame_current(jq)->bc, pc);
       printf("\t");
       const struct opcode_description* opdesc = opcode_describe(opcode);
-      data_stk_elem* param = 0;
-      int stack_in = opdesc->stack_in;
-      if (stack_in == -1) stack_in = pc[1];
-      for (int i=0; i<stack_in; i++) {
-        if (i == 0) {
-          param = forkable_stack_peek(&jq->data_stk);
-        } else {
-          printf(" | ");
-          param = forkable_stack_peek_next(&jq->data_stk, param);
+      stack_ptr param = 0;
+      if (!backtracking) {
+        int stack_in = opdesc->stack_in;
+        if (stack_in == -1) stack_in = pc[1];
+        for (int i=0; i<stack_in; i++) {
+          if (i == 0) {
+            param = jq->stk_top;
+          } else {
+            printf(" | ");
+            param = *stack_block_next(&jq->stk, param);
+          }
+          if (!param) break;
+          jv_dump(jv_copy(*(jv*)stack_block(&jq->stk, param)), 0);
+          //printf("<%d>", jv_get_refcnt(param->val));
+          //printf(" -- ");
+          //jv_dump(jv_copy(jq->path), 0);
         }
-        if (!param) break;
-        jv_dump(jv_copy(param->val), 0);
-        //printf("<%d>", jv_get_refcnt(param->val));
-        //printf(" -- ");
-        //jv_dump(jv_copy(jq->path), 0);
+      } else {
+        printf("\t<backtracking>");
       }
 
-      if (backtracking) printf("\t<backtracking>");
-
       printf("\n");
     }
+
     if (backtracking) {
       opcode = ON_BACKTRACK(opcode);
       backtracking = 0;
@@ -206,7 +302,7 @@ jv jq_next(jq_state *jq) {
     default: assert(0 && "invalid instruction");
 
     case LOADK: {
-      jv v = jv_array_get(jv_copy(frame_current_bytecode(&jq->frame_stk)->constants), *pc++);
+      jv v = jv_array_get(jv_copy(frame_current(jq)->bc->constants), *pc++);
       assert(jv_is_valid(v));
       jv_free(stack_pop(jq));
       stack_push(jq, v);
@@ -256,8 +352,7 @@ jv jq_next(jq_state *jq) {
       jv v = stack_pop(jq);
       uint16_t level = *pc++;
       uint16_t vidx = *pc++;
-      frame_ptr fp = frame_get_level(&jq->frame_stk, frame_current(&jq->frame_stk), level);
-      jv* var = frame_local_var(fp, vidx);
+      jv* var = frame_local_var(jq, vidx, level);
       assert(jv_get_kind(*var) == JV_KIND_ARRAY);
       *var = jv_array_append(*var, v);
       break;
@@ -288,8 +383,7 @@ jv jq_next(jq_state *jq) {
     case RANGE: {
       uint16_t level = *pc++;
       uint16_t v = *pc++;
-      frame_ptr fp = frame_get_level(&jq->frame_stk, frame_current(&jq->frame_stk), level);
-      jv* var = frame_local_var(fp, v);
+      jv* var = frame_local_var(jq, v, level);
       jv max = stack_pop(jq);
       if (jv_get_kind(*var) != JV_KIND_NUMBER ||
           jv_get_kind(max) != JV_KIND_NUMBER) {
@@ -303,9 +397,10 @@ jv jq_next(jq_state *jq) {
         jv curr = jv_copy(*var);
         *var = jv_number(jv_number_value(*var) + 1);
 
-        stack_save(jq, pc - 3);
+        struct stack_pos spos = stack_get_pos(jq);
         stack_push(jq, jv_copy(max));
-        stack_switch(jq);
+        stack_save(jq, pc - 3, spos);
+
         stack_push(jq, curr);
       }
       break;
@@ -315,8 +410,7 @@ jv jq_next(jq_state *jq) {
     case LOADV: {
       uint16_t level = *pc++;
       uint16_t v = *pc++;
-      frame_ptr fp = frame_get_level(&jq->frame_stk, frame_current(&jq->frame_stk), level);
-      jv* var = frame_local_var(fp, v);
+      jv* var = frame_local_var(jq, v, level);
       if (jq->debug_trace_enabled) {
         printf("V%d = ", v);
         jv_dump(jv_copy(*var), 0);
@@ -331,8 +425,7 @@ jv jq_next(jq_state *jq) {
     case LOADVN: {
       uint16_t level = *pc++;
       uint16_t v = *pc++;
-      frame_ptr fp = frame_get_level(&jq->frame_stk, frame_current(&jq->frame_stk), level);
-      jv* var = frame_local_var(fp, v);
+      jv* var = frame_local_var(jq, v, level);
       if (jq->debug_trace_enabled) {
         printf("V%d = ", v);
         jv_dump(jv_copy(*var), 0);
@@ -347,8 +440,7 @@ jv jq_next(jq_state *jq) {
     case STOREV: {
       uint16_t level = *pc++;
       uint16_t v = *pc++;
-      frame_ptr fp = frame_get_level(&jq->frame_stk, frame_current(&jq->frame_stk), level);
-      jv* var = frame_local_var(fp, v);
+      jv* var = frame_local_var(jq, v, level);
       jv val = stack_pop(jq);
       if (jq->debug_trace_enabled) {
         printf("V%d = ", v);
@@ -364,8 +456,7 @@ jv jq_next(jq_state *jq) {
       jv v = stack_pop(jq);
       stack_push(jq, jq->path);
 
-      stack_save(jq, pc - 1);
-      stack_switch(jq);
+      stack_save(jq, pc - 1, stack_get_pos(jq));
 
       stack_push(jq, jv_number(jq->subexp_nest));
       stack_push(jq, v);
@@ -384,9 +475,9 @@ jv jq_next(jq_state *jq) {
       jv path = jq->path;
       jq->path = stack_pop(jq);
 
-      stack_save(jq, pc - 1);
+      struct stack_pos spos = stack_get_pos(jq);
       stack_push(jq, jv_copy(path));
-      stack_switch(jq);
+      stack_save(jq, pc - 1, spos);
 
       stack_push(jq, path);
       jq->subexp_nest = old_subexp_nest;
@@ -475,10 +566,10 @@ jv jq_next(jq_state *jq) {
         path_append(jq, key);
         stack_push(jq, value);
       } else {
-        stack_save(jq, pc - 1);
+        struct stack_pos spos = stack_get_pos(jq);
         stack_push(jq, container);
         stack_push(jq, jv_number(idx));
-        stack_switch(jq);
+        stack_save(jq, pc - 1, spos);
         path_append(jq, key);
         stack_push(jq, value);
       }
@@ -496,8 +587,7 @@ jv jq_next(jq_state *jq) {
     }
 
     case FORK: {
-      stack_save(jq, pc - 1);
-      stack_switch(jq);
+      stack_save(jq, pc - 1, stack_get_pos(jq));
       pc++; // skip offset this time
       break;
     }
@@ -515,7 +605,7 @@ jv jq_next(jq_state *jq) {
       for (int i = 1; i < nargs; i++) {
         cfunc_input[i] = stack_pop(jq);
       }
-      struct cfunction* func = &frame_current_bytecode(&jq->frame_stk)->globals->cfunctions[*pc++];
+      struct cfunction* func = &frame_current(jq)->bc->globals->cfunctions[*pc++];
       top = cfunction_invoke(func, cfunc_input);
       if (jv_is_valid(top)) {
         stack_push(jq, top);
@@ -527,37 +617,34 @@ jv jq_next(jq_state *jq) {
     }
 
     case CALL_JQ: {
+      jv input = stack_pop(jq);
       uint16_t nclosures = *pc++;
       uint16_t* retaddr = pc + 2 + nclosures*2;
-      frame_ptr new_frame = frame_push(&jq->frame_stk, 
-                                       make_closure(&jq->frame_stk, frame_current(&jq->frame_stk), pc),
-                                       retaddr);
-      pc += 2;
-      frame_ptr old_frame = forkable_stack_peek_next(&jq->frame_stk, new_frame);
-      assert(nclosures == frame_self(new_frame)->bc->nclosures);
-      for (int i=0; i<nclosures; i++) {
-        *frame_closure_arg(new_frame, i) = make_closure(&jq->frame_stk, old_frame, pc);
-        pc += 2;
-      }
-
-      pc = frame_current_bytecode(&jq->frame_stk)->code;
+      struct frame* new_frame = frame_push(jq, make_closure(jq, pc),
+                                           pc + 2, nclosures);
+      new_frame->retdata = jq->stk_top;
+      new_frame->retaddr = retaddr;
+      pc = new_frame->bc->code;
+      stack_push(jq, input);
       break;
     }
 
     case RET: {
-      uint16_t* retaddr = *frame_current_retaddr(&jq->frame_stk);
+      jv value = stack_pop(jq);
+      assert(jq->stk_top == frame_current(jq)->retdata);
+      uint16_t* retaddr = frame_current(jq)->retaddr;
       if (retaddr) {
         // function return
         pc = retaddr;
-        frame_pop(&jq->frame_stk);
+        frame_pop(jq);
       } else {
         // top-level return, yielding value
-        jv value = stack_pop(jq);
-        stack_save(jq, pc - 1);
+        struct stack_pos spos = stack_get_pos(jq);
         stack_push(jq, jv_null());
-        stack_switch(jq);
+        stack_save(jq, pc - 1, spos);
         return value;
       }
+      stack_push(jq, value);
       break;
     }
     case ON_BACKTRACK(RET): {
@@ -576,9 +663,12 @@ jq_state *jq_init(void) {
   memset(jq, 0, sizeof(*jq));
   jq->debug_trace_enabled = 0;
   jq->initial_execution = 1;
-  forkable_stack_init(&jq->data_stk, sizeof(data_stk_elem) * 100);
-  forkable_stack_init(&jq->frame_stk, 1024);
-  forkable_stack_init(&jq->fork_stk, 1024);
+
+  stack_init(&jq->stk);
+  jq->stk_top = 0;
+  jq->fork_top = 0;
+  jq->curr_frame = 0;
+
   jq->path = jv_null();
   return jq;
 }
@@ -589,14 +679,18 @@ void jq_set_nomem_handler(jq_state *jq, void (*nomem_handler)(void *), void *dat
   jq->nomem_handler_data = data;
 }
 
+
 void jq_start(jq_state *jq, jv input, int flags) {
   jv_nomem_handler(jq->nomem_handler, jq->nomem_handler_data);
   jq_reset(jq);
-  stack_push(jq, input);
+  
   struct closure top = {jq->bc, -1};
-  frame_push(&jq->frame_stk, top, 0);
-  stack_save(jq, jq->bc->code);
-  stack_switch(jq);
+  struct frame* top_frame = frame_push(jq, top, 0, 0);
+  top_frame->retdata = 0;
+  top_frame->retaddr = 0;
+
+  stack_push(jq, input);
+  stack_save(jq, jq->bc->code, stack_get_pos(jq));
   if (flags & JQ_DEBUG_TRACE) {
     jq->debug_trace_enabled = 1;
   } else {
@@ -612,11 +706,9 @@ void jq_teardown(jq_state **jq) {
   *jq = NULL;
 
   jq_reset(old_jq);
-  forkable_stack_free(&old_jq->fork_stk);
-  forkable_stack_free(&old_jq->data_stk);
-  forkable_stack_free(&old_jq->frame_stk);
   bytecode_free(old_jq->bc);
   old_jq->bc = 0;
+
   jv_mem_free(old_jq);
 }
 
diff --git a/forkable_stack.h b/forkable_stack.h
deleted file mode 100644
index 74fcf433..00000000
--- a/forkable_stack.h
+++ /dev/null
@@ -1,172 +0,0 @@
-#ifndef FORKABLE_STACK_H
-#define FORKABLE_STACK_H
-#include <stddef.h>
-#include <assert.h>
-#include <string.h>
-#include "jv_alloc.h"
-
-struct forkable_stack_header {
-  /* distance in bytes between this header and the header
-     of the next object on the stack */
-  int next_delta;
-};
-
-#define FORKABLE_STACK_HEADER struct forkable_stack_header fk_header_
-
-struct forkable_stack {
-  // Stack grows down from stk+length
-  char* stk;
-
-  // stk+length is just past end of allocated area
-  // stk = jv_mem_alloc(length)
-  int length;
-
-  // stk+pos is header of top object, or stk+length if empty
-  int pos;
-
-  // everything after-or-including stk+savedlimit must be preserved
-  int savedlimit;
-};
-
-static void forkable_stack_check(struct forkable_stack* s) {
-  assert(s->stk);
-  assert(s->length > 0);
-  assert(s->pos >= 0 && s->pos <= s->length);
-  assert(s->savedlimit >= 0 && s->savedlimit <= s->length);
-}
-
-static int forkable_stack_empty(struct forkable_stack* s) {
-  return s->pos == s->length;
-}
-
-static void forkable_stack_init(struct forkable_stack* s, size_t sz) {
-  s->stk = jv_mem_alloc(sz);
-  s->length = sz;
-  s->pos = s->length;
-  s->savedlimit = s->length;
-  forkable_stack_check(s);
-}
-
-static void forkable_stack_free(struct forkable_stack* s) {
-  assert(forkable_stack_empty(s));
-  assert(s->savedlimit == s->length);
-  jv_mem_free(s->stk);
-  s->stk = 0;
-}
-
-static void* forkable_stack_push(struct forkable_stack* s, size_t sz_size) {
-  assert(sz_size > 0 && sz_size % sizeof(struct forkable_stack_header) == 0);
-  int size = (int)sz_size;
-  forkable_stack_check(s);
-  int curr = s->pos < s->savedlimit ? s->pos : s->savedlimit;
-  if (curr - size < 0) {
-    int oldlen = s->length;
-    s->length = (size + oldlen + 1024) * 2;
-    s->stk = jv_mem_realloc(s->stk, s->length);
-    int shift = s->length - oldlen;
-    memmove(s->stk + shift, s->stk, oldlen);
-    s->pos += shift;
-    s->savedlimit += shift;
-    curr += shift;
-  }
-  int newpos = curr - size;
-  assert(newpos < s->pos);
-  void* ret = (void*)(s->stk + newpos);
-  ((struct forkable_stack_header*)ret)->next_delta = s->pos - newpos;
-  s->pos = newpos;
-  forkable_stack_check(s);
-  return ret;
-}
-
-static void* forkable_stack_peek(struct forkable_stack* s) {
-  assert(!forkable_stack_empty(s));
-  forkable_stack_check(s);
-  return (void*)(s->stk + s->pos);
-}
-
-static void* forkable_stack_peek_next(struct forkable_stack* s, void* top) {
-  forkable_stack_check(s);
-  struct forkable_stack_header* elem = top;
-  int elempos = (char*)top - s->stk;
-  assert(elempos >= 0 && elempos < s->length);
-  assert(elempos + elem->next_delta <= s->length);
-  if (elempos + elem->next_delta < s->length) {
-    return (void*)(s->stk + elempos + elem->next_delta);
-  } else {
-    return 0;
-  }
-}
-
-// Returns 1 if the next forkable_stack_pop will permanently remove an
-// object from the stack (i.e. the top object was not saved with a fork)
-static int forkable_stack_pop_will_free(struct forkable_stack* s) {
-  return s->pos < s->savedlimit;
-}
-
-static void forkable_stack_pop(struct forkable_stack* s) {
-  forkable_stack_check(s);
-  struct forkable_stack_header* elem = forkable_stack_peek(s);
-  int reclaim_upto = s->pos + elem->next_delta < s->savedlimit ? 
-    s->pos + elem->next_delta : s->savedlimit;
-  assert(reclaim_upto <= s->length);
-  int reclaimed = reclaim_upto > s->pos ? reclaim_upto - s->pos : 0;
-  assert(0 <= reclaimed && s->pos + reclaimed <= s->length);
-  // s->pos <= i < reclaim_upto means that position i is to be freed
-  assert((reclaimed > 0) == forkable_stack_pop_will_free(s));
-  int new_pos = s->pos + elem->next_delta;
-  jv_mem_invalidate(elem, reclaimed);
-  s->pos = new_pos;
-}
-
-
-
-struct forkable_stack_state {
-  // We save the previous pos, savedlimit as
-  // length-pos, length-savedlimit since these
-  // values are stable across stack reallocations.
-  int prev_pos_delta, prev_limit_delta;
-};
-
-static void forkable_stack_save(struct forkable_stack* s, struct forkable_stack_state* state) {
-  forkable_stack_check(s);
-  state->prev_pos_delta = s->length - s->pos;
-  state->prev_limit_delta = s->length - s->savedlimit;
-  if (s->pos < s->savedlimit) s->savedlimit = s->pos;
-}
-
-static void forkable_stack_switch(struct forkable_stack* s, struct forkable_stack_state* state) {
-  forkable_stack_check(s);
-  int curr_pos = s->pos;
-  s->pos = s->length - state->prev_pos_delta;
-  state->prev_pos_delta = s->length - curr_pos;
-
-  int curr_limit = s->savedlimit;
-  if (curr_pos < curr_limit) s->savedlimit = curr_pos;
-  //state->prevlimit = curr_limit; FIXME clean up
-  forkable_stack_check(s);
-}
-
-static void forkable_stack_restore(struct forkable_stack* s, struct forkable_stack_state* state) {
-  forkable_stack_check(s);
-  assert(s->savedlimit <= s->length - state->prev_pos_delta);
-  assert(s->savedlimit <= s->length - state->prev_limit_delta);
-  s->pos = s->length - state->prev_pos_delta;
-  s->savedlimit = s->length - state->prev_limit_delta;
-  forkable_stack_check(s);
-}
-
-typedef int stack_idx;
-
-static stack_idx forkable_stack_to_idx(struct forkable_stack* s, void* ptr) {
-  char* item = ptr;
-  int pos = item - s->stk;
-  assert(pos >= 0 && pos < s->length);
-  return s->length - pos;
-}
-
-static void* forkable_stack_from_idx(struct forkable_stack* s, stack_idx idx) {
-  assert(idx >= 1 && idx <= s->length);
-  return &s->stk[s->length - idx];
-}
-
-#endif
diff --git a/frame_layout.h b/frame_layout.h
deleted file mode 100644
index dbbe3000..00000000
--- a/frame_layout.h
+++ /dev/null
@@ -1,104 +0,0 @@
-#ifndef FRAME_LAYOUT_H
-#include "forkable_stack.h"
-#include "bytecode.h"
-
-struct closure {
-  struct bytecode* bc;
-  stack_idx env;
-};
-
-struct continuation {
-  struct bytecode* bc;
-  stack_idx env;
-  uint16_t* retaddr;
-};
-
-typedef union frame_elem {
-  FORKABLE_STACK_HEADER;
-  struct continuation cont;
-  struct closure closure;
-  jv jsonval;
-} *frame_ptr;
-
-/*
- * Frame layout
- * fr[0] - FORKABLE_STACK_HEADER (next pointer)
- * fr[1] - self (used to store return addresses, etc)
- * fr[2...nclosures+2] - closure params
- * fr[nclosures+2..nclosures+nlocals+2] - local variables
- */
-
-static int frame_size(struct bytecode* bc) {
-  return sizeof(union frame_elem) * (bc->nclosures + bc->nlocals + 2);
-}
-
-static struct continuation* frame_self(frame_ptr fr) {
-  return &fr[1].cont;
-}
-
-static struct closure* frame_closure_arg(frame_ptr fr, int closure) {
-  assert(closure >= 0);
-  assert(closure < frame_self(fr)->bc->nclosures);
-  return &fr[2+closure].closure;
-}
-
-static jv* frame_local_var(frame_ptr fr, int var) {
-  assert(var >= 0);
-  assert(var < frame_self(fr)->bc->nlocals);
-  return &fr[2 + frame_self(fr)->bc->nclosures + var].jsonval;
-}
-
-
-static frame_ptr frame_current(struct forkable_stack* stk) {
-  frame_ptr fp = forkable_stack_peek(stk);
-  if (forkable_stack_peek_next(stk, fp)) {
-    struct bytecode* bc = frame_self(forkable_stack_peek_next(stk, fp))->bc;
-    assert(frame_self(fp)->retaddr >= bc->code && frame_self(fp)->retaddr < bc->code + bc->codelen);
-  } else {
-    assert(frame_self(fp)->retaddr == 0);
-  }
-  return fp;
-}
-
-static struct bytecode* frame_current_bytecode(struct forkable_stack* stk) {
-  return frame_self(frame_current(stk))->bc;
-}
-static uint16_t** frame_current_retaddr(struct forkable_stack* stk) {
-  return &frame_self(frame_current(stk))->retaddr;
-}
-
-static frame_ptr frame_get_parent(struct forkable_stack* stk, frame_ptr fr) {
-  return forkable_stack_from_idx(stk, frame_self(fr)->env);
-}
-
-static frame_ptr frame_get_level(struct forkable_stack* stk, frame_ptr fr, int level) {
-  for (int i=0; i<level; i++) {
-    fr = frame_get_parent(stk, fr);
-  }
-  return fr;
-}
-
-static frame_ptr frame_push(struct forkable_stack* stk, struct closure cl, uint16_t* retaddr) {
-  frame_ptr fp = forkable_stack_push(stk, frame_size(cl.bc));
-  struct continuation* cc = frame_self(fp);
-  cc->bc = cl.bc;
-  cc->env = cl.env;
-  cc->retaddr = retaddr;
-  for (int i=0; i<cl.bc->nlocals; i++) {
-    *frame_local_var(fp, i) = jv_invalid();
-  }
-  return fp;
-}
-
-static void frame_pop(struct forkable_stack* stk) {
-  frame_ptr fp = frame_current(stk);
-  if (forkable_stack_pop_will_free(stk)) {
-    int nlocals = frame_self(fp)->bc->nlocals;
-    for (int i=0; i<nlocals; i++) {
-      jv_free(*frame_local_var(fp, i));
-    }
-  }
-  forkable_stack_pop(stk);
-}
-
-#endif