Merge branch 'john/erts/OTP-18.3.4/minusminus_trapping/OTP-15371' into maint-18

* john/erts/OTP-18.3.4/minusminus_trapping/OTP-15371:
  Optimize operator '--' and yield on large inputs

2 files changed, 726 insertions, 87 deletions

diff --git a/erts/emulator/beam/erl_alloc.types b/erts/emulator/beam/erl_alloc.types
index 7738531142..b988533d46 100644
--- a/erts/emulator/beam/erl_alloc.types
+++ b/erts/emulator/beam/erl_alloc.types
@@ -308,8 +308,11 @@ type	RUNQ_BLNS	LONG_LIVED	SYSTEM		run_queue_balancing
 type	THR_PRGR_IDATA	LONG_LIVED	SYSTEM		thr_prgr_internal_data
 type	THR_PRGR_DATA	LONG_LIVED	SYSTEM		thr_prgr_data
 type	T_THR_PRGR_DATA	SHORT_LIVED	SYSTEM		temp_thr_prgr_data
+
 +endif
 
+type    LIST_TRAP       SHORT_LIVED     PROCESSES       list_bif_trap_state
+
 #
 # Types used for special emulators
 #
diff --git a/erts/emulator/beam/erl_bif_lists.c b/erts/emulator/beam/erl_bif_lists.c
index 5583dcb371..a64828c86f 100644
--- a/erts/emulator/beam/erl_bif_lists.c
+++ b/erts/emulator/beam/erl_bif_lists.c
@@ -29,12 +29,21 @@
 #include "sys.h"
 #include "erl_vm.h"
 #include "global.h"
-#include "erl_process.h"
-#include "error.h"
 #include "bif.h"
+#include "erl_binary.h"
+
+#ifdef MAX
+#undef MAX
+#endif
+#define MAX(x,y) (((x)>(y))?(x):(y))
+#ifdef MIN
+#undef MIN
+#endif
+#define MIN(x,y) (((x)<(y))?(x):(y))
 
 static Eterm keyfind(int Bif, Process* p, Eterm Key, Eterm Pos, Eterm List);
 
+
 static BIF_RETTYPE append(Process* p, Eterm A, Eterm B)
 {
     Eterm list;
@@ -85,103 +94,730 @@ BIF_RETTYPE append_2(BIF_ALIST_2)
     return append(BIF_P, BIF_ARG_1, BIF_ARG_2);
 }
 
-/*
- * erlang:'--'/2
- */
+/* erlang:'--'/2
+ *
+ * Subtracts a list from another (LHS -- RHS), removing the first occurrence of
+ * each element in LHS from RHS. There is no type coercion so the elements must
+ * match exactly.
+ *
+ * The BIF is broken into several stages that can all trap individually, and it
+ * chooses its algorithm based on input size. If either input is small it will
+ * use a linear scan tuned to which side it's on, and if both inputs are large
+ * enough it will convert RHS into a multiset to provide good asymptotic
+ * behavior. */
 
-#define SMALL_VEC_SIZE 10
-static Eterm subtract(Process* p, Eterm A, Eterm B)
-{
-    Eterm  list;
-    Eterm* hp;
-    Uint  need;
-    Eterm  res;
-    Eterm small_vec[SMALL_VEC_SIZE];	/* Preallocated memory for small lists */
-    Eterm* vec_p;
-    Eterm* vp;
-    int     i;
-    int     n;
-    int     m;
-    
-    if ((n = erts_list_length(A)) < 0) {
-	BIF_ERROR(p, BADARG);
+#define SUBTRACT_LHS_THRESHOLD 16
+#define SUBTRACT_RHS_THRESHOLD 16
+
+typedef enum {
+    SUBTRACT_STAGE_START,
+    SUBTRACT_STAGE_LEN_LHS,
+
+    /* Naive linear scan that's efficient when
+     * LEN_LHS <= SUBTRACT_LHS_THRESHOLD. */
+    SUBTRACT_STAGE_NAIVE_LHS,
+
+    SUBTRACT_STAGE_LEN_RHS,
+
+    /* As SUBTRACT_STAGE_NAIVE_LHS but for RHS. */
+    SUBTRACT_STAGE_NAIVE_RHS,
+
+    /* Creates a multiset from RHS for faster lookups before sweeping through
+     * LHS. The set is implemented as a red-black tree and duplicate elements
+     * are handled by a counter on each node. */
+    SUBTRACT_STAGE_SET_BUILD,
+    SUBTRACT_STAGE_SET_FINISH
+} ErtsSubtractCtxStage;
+
+typedef struct subtract_node__ {
+    struct subtract_node__ *parent;
+    struct subtract_node__ *left;
+    struct subtract_node__ *right;
+    int is_red;
+
+    Eterm key;
+    Uint count;
+} subtract_tree_t;
+
+typedef struct {
+    ErtsSubtractCtxStage stage;
+
+    Eterm lhs_original;
+    Eterm rhs_original;
+
+    Uint lhs_remaining;
+    Uint rhs_remaining;
+
+    Eterm iterator;
+
+    Eterm *result_cdr;
+    Eterm result;
+
+    union {
+        Eterm lhs_elements[SUBTRACT_LHS_THRESHOLD];
+        Eterm rhs_elements[SUBTRACT_RHS_THRESHOLD];
+
+        struct {
+            subtract_tree_t *tree;
+
+            /* A memory area for the tree's nodes, saving us the need to have
+             * one allocation per node. */
+            subtract_tree_t *alloc_start;
+            subtract_tree_t *alloc;
+        } rhs_set;
+    } u;
+} ErtsSubtractContext;
+
+#define ERTS_RBT_PREFIX subtract
+#define ERTS_RBT_T subtract_tree_t
+#define ERTS_RBT_KEY_T Eterm
+#define ERTS_RBT_FLAGS_T int
+#define ERTS_RBT_INIT_EMPTY_TNODE(T) \
+    do { \
+        (T)->parent = NULL; \
+        (T)->left = NULL; \
+        (T)->right = NULL; \
+    } while(0)
+#define ERTS_RBT_IS_RED(T) ((T)->is_red)
+#define ERTS_RBT_SET_RED(T) ((T)->is_red = 1)
+#define ERTS_RBT_IS_BLACK(T) (!ERTS_RBT_IS_RED(T))
+#define ERTS_RBT_SET_BLACK(T) ((T)->is_red = 0)
+#define ERTS_RBT_GET_FLAGS(T) ((T)->is_red)
+#define ERTS_RBT_SET_FLAGS(T, F) ((T)->is_red = F)
+#define ERTS_RBT_GET_PARENT(T) ((T)->parent)
+#define ERTS_RBT_SET_PARENT(T, P) ((T)->parent = P)
+#define ERTS_RBT_GET_RIGHT(T) ((T)->right)
+#define ERTS_RBT_SET_RIGHT(T, R) ((T)->right = (R))
+#define ERTS_RBT_GET_LEFT(T) ((T)->left)
+#define ERTS_RBT_SET_LEFT(T, L) ((T)->left = (L))
+#define ERTS_RBT_GET_KEY(T) ((T)->key)
+#define ERTS_RBT_IS_LT(KX, KY) (CMP_TERM(KX, KY) < 0)
+#define ERTS_RBT_IS_EQ(KX, KY) EQ(KX, KY)
+#define ERTS_RBT_WANT_LOOKUP_INSERT
+#define ERTS_RBT_WANT_LOOKUP
+#define ERTS_RBT_WANT_DELETE
+#define ERTS_RBT_UNDEF
+
+#include "erl_rbtree.h"
+
+static int subtract_continue(Process *p, ErtsSubtractContext *context);
+
+static void subtract_ctx_dtor(ErtsSubtractContext *context) {
+    switch (context->stage) {
+        case SUBTRACT_STAGE_SET_BUILD:
+        case SUBTRACT_STAGE_SET_FINISH:
+            erts_free(ERTS_ALC_T_LIST_TRAP, context->u.rhs_set.alloc_start);
+            break;
+        default:
+            break;
     }
-    if ((m = erts_list_length(B)) < 0) {
-	BIF_ERROR(p, BADARG);
+}
+
+static void subtract_ctx_bin_dtor(Binary *context_bin) {
+    ErtsSubtractContext *context = ERTS_MAGIC_BIN_DATA(context_bin);
+    subtract_ctx_dtor(context);
+    return;
+}
+
+static void subtract_ctx_move(ErtsSubtractContext *from,
+                              ErtsSubtractContext *to) {
+    int uses_result_cdr = 0;
+
+    to->stage = from->stage;
+
+    to->lhs_original = from->lhs_original;
+    to->rhs_original = from->rhs_original;
+
+    to->lhs_remaining = from->lhs_remaining;
+    to->rhs_remaining = from->rhs_remaining;
+
+    to->iterator = from->iterator;
+    to->result = from->result;
+
+    switch (to->stage) {
+        case SUBTRACT_STAGE_NAIVE_LHS:
+            sys_memcpy(to->u.lhs_elements,
+                       from->u.lhs_elements,
+                       sizeof(Eterm) * to->lhs_remaining);
+            break;
+        case SUBTRACT_STAGE_NAIVE_RHS:
+            sys_memcpy(to->u.rhs_elements,
+                       from->u.rhs_elements,
+                       sizeof(Eterm) * to->rhs_remaining);
+
+            uses_result_cdr = 1;
+            break;
+        case SUBTRACT_STAGE_SET_FINISH:
+            uses_result_cdr = 1;
+            /* FALL THROUGH */
+        case SUBTRACT_STAGE_SET_BUILD:
+            to->u.rhs_set.alloc_start = from->u.rhs_set.alloc_start;
+            to->u.rhs_set.alloc = from->u.rhs_set.alloc;
+            to->u.rhs_set.tree = from->u.rhs_set.tree;
+            break;
+        default:
+            break;
     }
-    
-    if (n == 0)
-	BIF_RET(NIL);
-    if (m == 0)
-	BIF_RET(A);
-    
-    /* allocate element vector */
-    if (n <= SMALL_VEC_SIZE)
-	vec_p = small_vec;
-    else
-	vec_p = (Eterm*) erts_alloc(ERTS_ALC_T_TMP, n * sizeof(Eterm));
-    
-    /* PUT ALL ELEMENTS IN VP */
-    vp = vec_p;
-    list = A;
-    i = n;
-    while(i--) {
-	Eterm* listp = list_val(list);
-	*vp++ = CAR(listp);
-	list = CDR(listp);
+
+    if (uses_result_cdr) {
+        if (from->result_cdr == &from->result) {
+            to->result_cdr = &to->result;
+        } else {
+            to->result_cdr = from->result_cdr;
+        }
     }
-    
-    /* UNMARK ALL DELETED CELLS */
-    list = B;
-    m = 0;  /* number of deleted elements */
-    while(is_list(list)) {
-	Eterm* listp = list_val(list);
-	Eterm  elem = CAR(listp);
-	i = n;
-	vp = vec_p;
-	while(i--) {
-	    if (is_value(*vp) && eq(*vp, elem)) {
-		*vp = THE_NON_VALUE;
-		m++;
-		break;
-	    }
-	    vp++;
-	}
-	list = CDR(listp);
+}
+
+static Eterm subtract_create_trap_state(Process *p,
+                                        ErtsSubtractContext *context) {
+    Binary *state_bin;
+    Eterm *hp;
+
+    state_bin = erts_create_magic_binary(sizeof(ErtsSubtractContext),
+                                         subtract_ctx_bin_dtor);
+
+    subtract_ctx_move(context, ERTS_MAGIC_BIN_DATA(state_bin));
+
+    hp = HAlloc(p, PROC_BIN_SIZE);
+
+    return erts_mk_magic_binary_term(&hp, &MSO(p), state_bin);
+}
+
+static int subtract_enter_len_lhs(Process *p, ErtsSubtractContext *context) {
+    context->stage = SUBTRACT_STAGE_LEN_LHS;
+
+    context->iterator = context->lhs_original;
+    context->lhs_remaining = 0;
+
+    return subtract_continue(p, context);
+}
+
+static int subtract_enter_len_rhs(Process *p, ErtsSubtractContext *context) {
+    context->stage = SUBTRACT_STAGE_LEN_RHS;
+
+    context->iterator = context->rhs_original;
+    context->rhs_remaining = 0;
+
+    return subtract_continue(p, context);
+}
+
+static int subtract_get_length(Process *p, Eterm *iterator_p, Uint *count_p) {
+    static const Sint ELEMENTS_PER_RED = 32;
+
+    Sint budget, count;
+    Eterm iterator;
+
+    budget = ELEMENTS_PER_RED * ERTS_BIF_REDS_LEFT(p);
+    iterator = *iterator_p;
+
+#ifdef DEBUG
+    budget = budget / 10 + 1;
+#endif
+
+    for (count = 0; count < budget && is_list(iterator); count++) {
+        iterator = CDR(list_val(iterator));
     }
-    
-    if (m == n)      /* All deleted ? */
-	res = NIL;
-    else if (m == 0)  /* None deleted ? */
-	res = A;
-    else {			/* REBUILD LIST */
-	res = NIL;
-	need = 2*(n - m);
-	hp = HAlloc(p, need);
-	vp = vec_p + n - 1;
-	while(vp >= vec_p) {
-	    if (is_value(*vp)) {
-		res = CONS(hp, *vp, res);
-		hp += 2;
-	    }
-	    vp--;
-	}
+
+    if (!is_list(iterator) && !is_nil(iterator)) {
+        return -1;
+    }
+
+    BUMP_REDS(p, count / ELEMENTS_PER_RED);
+
+    *iterator_p = iterator;
+    *count_p += count;
+
+    if (is_nil(iterator)) {
+        return 1;
     }
-    if (vec_p != small_vec)
-	erts_free(ERTS_ALC_T_TMP, (void *) vec_p);
-    BIF_RET(res);
+
+    return 0;
 }
 
-BIF_RETTYPE ebif_minusminus_2(BIF_ALIST_2)
-{
-    return subtract(BIF_P, BIF_ARG_1, BIF_ARG_2);
+static int subtract_enter_naive_lhs(Process *p, ErtsSubtractContext *context) {
+    Eterm iterator;
+    int i = 0;
+
+    context->stage = SUBTRACT_STAGE_NAIVE_LHS;
+
+    context->iterator = context->rhs_original;
+    context->result = NIL;
+
+    iterator = context->lhs_original;
+
+    while (is_list(iterator)) {
+        const Eterm *cell = list_val(iterator);
+
+        ASSERT(i < SUBTRACT_LHS_THRESHOLD);
+
+        context->u.lhs_elements[i++] = CAR(cell);
+        iterator = CDR(cell);
+    }
+
+    ASSERT(i == context->lhs_remaining);
+
+    return subtract_continue(p, context);
 }
 
-BIF_RETTYPE subtract_2(BIF_ALIST_2)
-{
-    return subtract(BIF_P, BIF_ARG_1, BIF_ARG_2);
+static int subtract_naive_lhs(Process *p, ErtsSubtractContext *context) {
+    const Sint CHECKS_PER_RED = 16;
+    Sint checks, budget;
+
+    budget = CHECKS_PER_RED * ERTS_BIF_REDS_LEFT(p);
+    checks = 0;
+
+    while (checks < budget && is_list(context->iterator)) {
+        const Eterm *cell;
+        Eterm value, next;
+        int found_at;
+
+        cell = list_val(context->iterator);
+
+        value = CAR(cell);
+        next = CDR(cell);
+
+        for (found_at = 0; found_at < context->lhs_remaining; found_at++) {
+            if (EQ(value, context->u.lhs_elements[found_at])) {
+                /* We shift the array one step down as we have to preserve
+                 * order.
+                 *
+                 * Note that we can't exit early as that would suppress errors
+                 * in the right-hand side (this runs prior to determining the
+                 * length of RHS). */
+
+                context->lhs_remaining--;
+                sys_memmove(&context->u.lhs_elements[found_at],
+                            &context->u.lhs_elements[found_at + 1],
+                            (context->lhs_remaining - found_at) * sizeof(Eterm));
+                break;
+            }
+        }
+
+        checks += MAX(1, context->lhs_remaining);
+        context->iterator = next;
+    }
+
+    BUMP_REDS(p, MIN(checks, budget) / CHECKS_PER_RED);
+
+    if (is_list(context->iterator)) {
+        return 0;
+    } else if (!is_nil(context->iterator)) {
+        return -1;
+    }
+
+    if (context->lhs_remaining > 0) {
+        Eterm *hp;
+        int i;
+
+        hp = HAlloc(p, context->lhs_remaining * 2);
+
+        for (i = context->lhs_remaining - 1; i >= 0; i--) {
+            Eterm value = context->u.lhs_elements[i];
+
+            context->result = CONS(hp, value, context->result);
+            hp += 2;
+        }
+    }
+
+    ASSERT(context->lhs_remaining > 0 || context->result == NIL);
+
+    return 1;
+}
+
+static int subtract_enter_naive_rhs(Process *p, ErtsSubtractContext *context) {
+    Eterm iterator;
+    int i = 0;
+
+    context->stage = SUBTRACT_STAGE_NAIVE_RHS;
+
+    context->iterator = context->lhs_original;
+    context->result_cdr = &context->result;
+    context->result = NIL;
+
+    iterator = context->rhs_original;
+
+    while (is_list(iterator)) {
+        const Eterm *cell = list_val(iterator);
+
+        ASSERT(i < SUBTRACT_RHS_THRESHOLD);
+
+        context->u.rhs_elements[i++] = CAR(cell);
+        iterator = CDR(cell);
+    }
+
+    ASSERT(i == context->rhs_remaining);
+
+    return subtract_continue(p, context);
+}
+
+static int subtract_naive_rhs(Process *p, ErtsSubtractContext *context) {
+    const Sint CHECKS_PER_RED = 16;
+    Sint checks, budget;
+
+    budget = CHECKS_PER_RED * ERTS_BIF_REDS_LEFT(p);
+    checks = 0;
+
+#ifdef DEBUG
+    budget = budget / 10 + 1;
+#endif
+
+    while (checks < budget && is_list(context->iterator)) {
+        const Eterm *cell;
+        Eterm value, next;
+        int found_at;
+
+        cell = list_val(context->iterator);
+        value = CAR(cell);
+        next = CDR(cell);
+
+        for (found_at = context->rhs_remaining - 1; found_at >= 0; found_at--) {
+            if (EQ(value, context->u.rhs_elements[found_at])) {
+                break;
+            }
+        }
+
+        if (found_at < 0) {
+            /* Destructively add the value to the result. This is safe
+             * since the GC is disabled and the unfinished term is never
+             * leaked to the outside world. */
+            Eterm *hp = HAllocX(p, 2, context->lhs_remaining * 2);
+
+            *context->result_cdr = make_list(hp);
+            context->result_cdr = &CDR(hp);
+
+            CAR(hp) = value;
+        } else if (found_at >= 0) {
+            Eterm swap;
+
+            if (context->rhs_remaining-- == 1) {
+                /* We've run out of items to remove, so the rest of the
+                 * result will be equal to the remainder of the input. We know
+                 * that LHS is well-formed as any errors would've been reported
+                 * during length determination. */
+                *context->result_cdr = next;
+
+                BUMP_REDS(p, MIN(budget, checks) / CHECKS_PER_RED);
+
+                return 1;
+            }
+
+            swap = context->u.rhs_elements[context->rhs_remaining];
+            context->u.rhs_elements[found_at] = swap;
+        }
+
+        checks += context->rhs_remaining;
+        context->iterator = next;
+        context->lhs_remaining--;
+    }
+
+    /* The result only has to be terminated when returning it to the user, but
+     * we're doing it when trapping as well to prevent headaches when
+     * debugging. */
+    *context->result_cdr = NIL;
+
+    BUMP_REDS(p, MIN(budget, checks) / CHECKS_PER_RED);
+
+    if (is_list(context->iterator)) {
+        ASSERT(context->lhs_remaining > 0 && context->rhs_remaining > 0);
+        return 0;
+    }
+
+    return 1;
+}
+
+static int subtract_enter_set_build(Process *p, ErtsSubtractContext *context) {
+    context->stage = SUBTRACT_STAGE_SET_BUILD;
+
+    context->u.rhs_set.alloc_start =
+        erts_alloc(ERTS_ALC_T_LIST_TRAP,
+                   context->rhs_remaining * sizeof(subtract_tree_t));
+
+    context->u.rhs_set.alloc = context->u.rhs_set.alloc_start;
+    context->u.rhs_set.tree = NULL;
+
+    context->iterator = context->rhs_original;
+
+    return subtract_continue(p, context);
+}
+
+static int subtract_set_build(Process *p, ErtsSubtractContext *context) {
+    const static Sint INSERTIONS_PER_RED = 16;
+    Sint budget, insertions;
+
+    budget = INSERTIONS_PER_RED * ERTS_BIF_REDS_LEFT(p);
+    insertions = 0;
+
+#ifdef DEBUG
+    budget = budget / 10 + 1;
+#endif
+
+    while (insertions < budget && is_list(context->iterator)) {
+        subtract_tree_t *existing_node, *new_node;
+        const Eterm *cell;
+        Eterm value, next;
+
+        cell = list_val(context->iterator);
+        value = CAR(cell);
+        next = CDR(cell);
+
+        new_node = context->u.rhs_set.alloc;
+        new_node->key = value;
+        new_node->count = 1;
+
+        existing_node = subtract_rbt_lookup_insert(&context->u.rhs_set.tree,
+                                                   new_node);
+
+        if (existing_node != NULL) {
+            existing_node->count++;
+        } else {
+            context->u.rhs_set.alloc++;
+        }
+
+        context->iterator = next;
+        insertions++;
+    }
+
+    BUMP_REDS(p, insertions / INSERTIONS_PER_RED);
+
+    ASSERT(is_list(context->iterator) || is_nil(context->iterator));
+    ASSERT(context->u.rhs_set.tree != NULL);
+
+    return is_nil(context->iterator);
+}
+
+static int subtract_enter_set_finish(Process *p, ErtsSubtractContext *context) {
+    context->stage = SUBTRACT_STAGE_SET_FINISH;
+
+    context->result_cdr = &context->result;
+    context->result = NIL;
+
+    context->iterator = context->lhs_original;
+
+    return subtract_continue(p, context);
+}
+
+static int subtract_set_finish(Process *p, ErtsSubtractContext *context) {
+    const Sint CHECKS_PER_RED = 8;
+    Sint checks, budget;
+
+    budget = CHECKS_PER_RED * ERTS_BIF_REDS_LEFT(p);
+    checks = 0;
+
+#ifdef DEBUG
+    budget = budget / 10 + 1;
+#endif
+
+    while (checks < budget && is_list(context->iterator)) {
+        subtract_tree_t *node;
+        const Eterm *cell;
+        Eterm value, next;
+
+        cell = list_val(context->iterator);
+        value = CAR(cell);
+        next = CDR(cell);
+
+        ASSERT(context->rhs_remaining > 0);
+
+        node = subtract_rbt_lookup(context->u.rhs_set.tree, value);
+
+        if (node == NULL) {
+            Eterm *hp = HAllocX(p, 2, context->lhs_remaining * 2);
+
+            *context->result_cdr = make_list(hp);
+            context->result_cdr = &CDR(hp);
+
+            CAR(hp) = value;
+        } else {
+            if (context->rhs_remaining-- == 1) {
+                *context->result_cdr = next;
+
+                BUMP_REDS(p, checks / CHECKS_PER_RED);
+
+                return 1;
+            }
+
+            if (node->count-- == 1) {
+                subtract_rbt_delete(&context->u.rhs_set.tree, node);
+            }
+        }
+
+        context->iterator = next;
+        context->lhs_remaining--;
+        checks++;
+    }
+
+    *context->result_cdr = NIL;
+
+    BUMP_REDS(p, checks / CHECKS_PER_RED);
+
+    if (is_list(context->iterator)) {
+        ASSERT(context->lhs_remaining > 0 && context->rhs_remaining > 0);
+        return 0;
+    }
+
+    return 1;
+}
+
+static int subtract_continue(Process *p, ErtsSubtractContext *context) {
+    switch (context->stage) {
+        case SUBTRACT_STAGE_START: {
+            return subtract_enter_len_lhs(p, context);
+        }
+
+        case SUBTRACT_STAGE_LEN_LHS: {
+            int res = subtract_get_length(p,
+                                          &context->iterator,
+                                          &context->lhs_remaining);
+
+            if (res != 1) {
+                return res;
+            }
+
+            if (context->lhs_remaining <= SUBTRACT_LHS_THRESHOLD) {
+                return subtract_enter_naive_lhs(p, context);
+            }
+
+            return subtract_enter_len_rhs(p, context);
+        }
+
+        case SUBTRACT_STAGE_NAIVE_LHS: {
+            return subtract_naive_lhs(p, context);
+        }
+
+        case SUBTRACT_STAGE_LEN_RHS: {
+            int res = subtract_get_length(p,
+                                          &context->iterator,
+                                          &context->rhs_remaining);
+
+            if (res != 1) {
+                return res;
+            }
+
+            /* We've walked through both lists fully now so we no longer need
+             * to check for errors past this point. */
+
+            if (context->rhs_remaining <= SUBTRACT_RHS_THRESHOLD) {
+                return subtract_enter_naive_rhs(p, context);
+            }
+
+            return subtract_enter_set_build(p, context);
+        }
+
+        case SUBTRACT_STAGE_NAIVE_RHS: {
+            return subtract_naive_rhs(p, context);
+        }
+
+        case SUBTRACT_STAGE_SET_BUILD: {
+            int res = subtract_set_build(p, context);
+
+            if (res != 1) {
+                return res;
+            }
+
+            return subtract_enter_set_finish(p, context);
+        }
+
+        case SUBTRACT_STAGE_SET_FINISH: {
+            return subtract_set_finish(p, context);
+        }
+
+        default:
+            erts_exit(ERTS_ABORT_EXIT, "unreachable");
+    }
+}
+
+static int subtract_start(Process *p, Eterm lhs, Eterm rhs,
+                          ErtsSubtractContext *context) {
+    context->stage = SUBTRACT_STAGE_START;
+
+    context->lhs_original = lhs;
+    context->rhs_original = rhs;
+
+    return subtract_continue(p, context);
 }
 
+/* erlang:'--'/2 */
+static Eterm subtract(Export *bif_entry, BIF_ALIST_2) {
+    Eterm lhs = BIF_ARG_1, rhs = BIF_ARG_2;
+
+    if ((is_list(lhs) || is_nil(lhs)) && (is_list(rhs) || is_nil(rhs))) {
+        /* We start with the context on the stack in the hopes that we won't
+         * have to trap. */
+        ErtsSubtractContext context;
+        int res;
+
+        res = subtract_start(BIF_P, lhs, rhs, &context);
+
+        if (res == 0) {
+            Eterm state_mref;
+
+            state_mref = subtract_create_trap_state(BIF_P, &context);
+            erts_set_gc_state(BIF_P, 0);
+
+            BIF_TRAP2(bif_entry, BIF_P, state_mref, NIL);
+        }
+
+        subtract_ctx_dtor(&context);
+
+        if (res < 0) {
+            BIF_ERROR(BIF_P, BADARG);
+        }
+
+        BIF_RET(context.result);
+    } else if (is_binary(lhs)) {
+        ErtsSubtractContext *context;
+        void (*dtor)(Binary*);
+        Binary *magic_bin;
+        Eterm *bin = (Eterm*)binary_val(BIF_ARG_1);
+
+        int res;
+
+        if (*bin != HEADER_PROC_BIN) {
+            BIF_ERROR(BIF_P, BADARG);
+        }
+
+        magic_bin = ((ProcBin *) binary_val(BIF_ARG_1))->val;
+        dtor = ERTS_MAGIC_BIN_DESTRUCTOR(magic_bin);
+
+        if (dtor != subtract_ctx_bin_dtor) {
+            BIF_ERROR(BIF_P, BADARG);
+        }
+
+        ASSERT(BIF_P->flags & F_DISABLE_GC);
+        ASSERT(rhs == NIL);
+
+        context = ERTS_MAGIC_BIN_DATA(magic_bin);
+        res = subtract_continue(BIF_P, context);
+
+        if (res == 0) {
+            BIF_TRAP2(bif_entry, BIF_P, lhs, NIL);
+        }
+
+        erts_set_gc_state(BIF_P, 1);
+
+        if (res < 0) {
+            ERTS_BIF_ERROR_TRAPPED2(BIF_P, BADARG, bif_entry,
+                                    context->lhs_original,
+                                    context->rhs_original);
+        }
+
+        BIF_RET(context->result);
+    }
+
+    ASSERT(!(BIF_P->flags & F_DISABLE_GC));
+
+    BIF_ERROR(BIF_P, BADARG);
+}
+
+BIF_RETTYPE ebif_minusminus_2(BIF_ALIST_2) {
+    return subtract(bif_export[BIF_ebif_minusminus_2], BIF_P, BIF__ARGS);
+}
+
+BIF_RETTYPE subtract_2(BIF_ALIST_2) {
+    return subtract(bif_export[BIF_subtract_2], BIF_P, BIF__ARGS);
+}
+
+
 BIF_RETTYPE lists_member_2(BIF_ALIST_2)
 {
     Eterm term;