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

* john/erts/minusminus_trapping/OTP-15371:
  Optimize operator '--' and yield on large inputs
  Inline erts_cmp
  Clarify a magical allocation size
  Fix trapping in lists:reverse/2

6 files changed, 796 insertions, 165 deletions

diff --git a/erts/emulator/beam/atom.c b/erts/emulator/beam/atom.c
index 5381611fab..59b51fd15e 100644
--- a/erts/emulator/beam/atom.c
+++ b/erts/emulator/beam/atom.c
@@ -174,7 +174,7 @@ atom_alloc(Atom* tmpl)
 
     /*
      * Precompute ordinal value of first 3 bytes + 7 bits.
-     * This is used by utils.c:erts_cmp_atoms().
+     * This is used by erl_utils.h:erts_cmp_atoms().
      * We cannot use the full 32 bits of the first 4 bytes,
      * since we use the sign of the difference between two
      * ordinal values to represent their relative order.
diff --git a/erts/emulator/beam/erl_alloc.types b/erts/emulator/beam/erl_alloc.types
index 5409b89bab..98ba0e5f07 100644
--- a/erts/emulator/beam/erl_alloc.types
+++ b/erts/emulator/beam/erl_alloc.types
@@ -274,6 +274,7 @@ type	ML_YIELD_STATE	SHORT_LIVED	SYSTEM		monitor_link_yield_state
 type	ML_DIST		STANDARD	SYSTEM		monitor_link_dist
 type	PF3_ARGS	SHORT_LIVED	PROCESSES	process_flag_3_arguments
 type	SETUP_CONN_ARG	SHORT_LIVED	PROCESSES	setup_connection_argument
+type    LIST_TRAP       SHORT_LIVED     PROCESSES       list_bif_trap_state
 
 type	ENVIRONMENT	SYSTEM		SYSTEM		environment
 
diff --git a/erts/emulator/beam/erl_bif_lists.c b/erts/emulator/beam/erl_bif_lists.c
index 395be67a90..a793b34852 100644
--- a/erts/emulator/beam/erl_bif_lists.c
+++ b/erts/emulator/beam/erl_bif_lists.c
@@ -29,12 +29,13 @@
 #include "sys.h"
 #include "erl_vm.h"
 #include "global.h"
-#include "erl_process.h"
-#include "error.h"
 #include "bif.h"
+#include "erl_binary.h"
+
 
 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;
@@ -146,103 +147,724 @@ 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 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_CMP_KEYS(KX, KY) CMP_TERM(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;
+    }
+}
 
-#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;
-    Sint i;
-    Sint n;
-    Sint m;
-    
-    if ((n = erts_list_length(A)) < 0) {
-	BIF_ERROR(p, BADARG);
+static int subtract_ctx_bin_dtor(Binary *context_bin) {
+    ErtsSubtractContext *context = ERTS_MAGIC_BIN_DATA(context_bin);
+    subtract_ctx_dtor(context);
+    return 1;
+}
+
+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 ((m = erts_list_length(B)) < 0) {
-	BIF_ERROR(p, BADARG);
+
+    if (uses_result_cdr) {
+        if (from->result_cdr == &from->result) {
+            to->result_cdr = &to->result;
+        } else {
+            to->result_cdr = from->result_cdr;
+        }
     }
-    
-    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);
+}
+
+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, ERTS_MAGIC_REF_THING_SIZE);
+
+    return erts_mk_magic_ref(&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));
     }
-    
-    /* 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);
+
+    if (!is_list(iterator) && !is_nil(iterator)) {
+        return -1;
     }
-    
-    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--;
-	}
+
+    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_ASSERT(!"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_internal_magic_ref(lhs)) {
+        ErtsSubtractContext *context;
+        int (*dtor)(Binary*);
+        Binary *magic_bin;
+
+        int res;
+
+        magic_bin = erts_magic_ref2bin(lhs);
+        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_CALL_ARGS);
+}
+
+BIF_RETTYPE subtract_2(BIF_ALIST_2) {
+    return subtract(bif_export[BIF_subtract_2], BIF_CALL_ARGS);
+}
+
+
 BIF_RETTYPE lists_member_2(BIF_ALIST_2)
 {
     Eterm term;
@@ -283,7 +905,7 @@ static BIF_RETTYPE lists_reverse_alloc(Process *c_p,
 {
     static const Uint CELLS_PER_RED = 40;
 
-    Eterm *heap_top, *heap_end;
+    Eterm *alloc_top, *alloc_end;
     Uint cells_left, max_cells;
     Eterm list, tail;
     Eterm lookahead;
@@ -305,18 +927,18 @@ static BIF_RETTYPE lists_reverse_alloc(Process *c_p,
         BIF_ERROR(c_p, BADARG);
     }
 
-    heap_top = HAlloc(c_p, 2 * (max_cells - cells_left));
-    heap_end = heap_top + 2 * (max_cells - cells_left);
+    alloc_top = HAlloc(c_p, 2 * (max_cells - cells_left));
+    alloc_end = alloc_top + 2 * (max_cells - cells_left);
 
-    while (heap_top < heap_end) {
+    while (alloc_top < alloc_end) {
         Eterm *pair = list_val(list);
 
-        tail = CONS(heap_top, CAR(pair), tail);
+        tail = CONS(alloc_top, CAR(pair), tail);
         list = CDR(pair);
 
         ASSERT(is_list(list) || is_nil(list));
 
-        heap_top += 2;
+        alloc_top += 2;
     }
 
     if (is_nil(list)) {
@@ -333,7 +955,7 @@ static BIF_RETTYPE lists_reverse_onheap(Process *c_p,
 {
     static const Uint CELLS_PER_RED = 60;
 
-    Eterm *heap_top, *heap_end;
+    Eterm *alloc_start, *alloc_top, *alloc_end;
     Uint cells_left, max_cells;
     Eterm list, tail;
 
@@ -343,21 +965,27 @@ static BIF_RETTYPE lists_reverse_onheap(Process *c_p,
     cells_left = max_cells = CELLS_PER_RED * (1 + ERTS_BIF_REDS_LEFT(c_p));
 
     ASSERT(HEAP_LIMIT(c_p) >= HEAP_TOP(c_p) + 2);
-    heap_end = HEAP_LIMIT(c_p) - 2;
-    heap_top = HEAP_TOP(c_p);
+    alloc_start = HEAP_TOP(c_p);
+    alloc_end = HEAP_LIMIT(c_p) - 2;
+    alloc_top = alloc_start;
+
+    /* Don't process more cells than we have reductions for. */
+    alloc_end = MIN(alloc_top + (cells_left * 2), alloc_end);
 
-    while (heap_top < heap_end && is_list(list)) {
+    while (alloc_top < alloc_end && is_list(list)) {
         Eterm *pair = list_val(list);
 
-        tail = CONS(heap_top, CAR(pair), tail);
+        tail = CONS(alloc_top, CAR(pair), tail);
         list = CDR(pair);
 
-        heap_top += 2;
+        alloc_top += 2;
     }
 
-    cells_left -= (heap_top - heap_end) / 2;
+    cells_left -= (alloc_top - alloc_start) / 2;
+    HEAP_TOP(c_p) = alloc_top;
+
+    ASSERT(cells_left >= 0 && cells_left <= max_cells);
     BUMP_REDS(c_p, (max_cells - cells_left) / CELLS_PER_RED);
-    HEAP_TOP(c_p) = heap_top;
 
     if (is_nil(list)) {
         BIF_RET(tail);
diff --git a/erts/emulator/beam/erl_utils.h b/erts/emulator/beam/erl_utils.h
index b3bfa69052..880febba8b 100644
--- a/erts/emulator/beam/erl_utils.h
+++ b/erts/emulator/beam/erl_utils.h
@@ -22,6 +22,7 @@
 #define ERL_UTILS_H__
 
 #include "sys.h"
+#include "atom.h"
 #include "erl_printf.h"
 
 struct process;
@@ -112,10 +113,12 @@ int eq(Eterm, Eterm);
 
 #define EQ(x,y) (((x) == (y)) || (is_not_both_immed((x),(y)) && eq((x),(y))))
 
-int erts_cmp_atoms(Eterm a, Eterm b);
-Sint erts_cmp(Eterm, Eterm, int, int);
-Sint erts_cmp_compound(Eterm, Eterm, int, int);
+ERTS_GLB_INLINE Sint erts_cmp(Eterm, Eterm, int, int);
+ERTS_GLB_INLINE int erts_cmp_atoms(Eterm a, Eterm b);
+
 Sint cmp(Eterm a, Eterm b);
+Sint erts_cmp_compound(Eterm, Eterm, int, int);
+
 #define CMP(A,B)                         erts_cmp(A,B,0,0)
 #define CMP_TERM(A,B)                    erts_cmp(A,B,1,0)
 #define CMP_EQ_ONLY(A,B)                 erts_cmp(A,B,0,1)
@@ -150,4 +153,56 @@ Sint cmp(Eterm a, Eterm b);
 	if (erts_cmp_compound(X,Y,0,EqOnly) Op 0) { Action; };	\
     }
 
+#define erts_float_comp(x,y) (((x)<(y)) ? -1 : (((x)==(y)) ? 0 : 1))
+
+#if ERTS_GLB_INLINE_INCL_FUNC_DEF
+
+ERTS_GLB_INLINE int erts_cmp_atoms(Eterm a, Eterm b) {
+    Atom *aa = atom_tab(atom_val(a));
+    Atom *bb = atom_tab(atom_val(b));
+
+    byte *name_a, *name_b;
+    int len_a, len_b, diff;
+
+    diff = aa->ord0 - bb->ord0;
+
+    if (diff != 0) {
+        return diff;
+    }
+
+    name_a = &aa->name[3];
+    name_b = &bb->name[3];
+    len_a = aa->len-3;
+    len_b = bb->len-3;
+
+    if (len_a > 0 && len_b > 0) {
+        diff = sys_memcmp(name_a, name_b, MIN(len_a, len_b));
+
+        if (diff != 0) {
+            return diff;
+        }
+    }
+
+    return len_a - len_b;
+}
+
+ERTS_GLB_INLINE Sint erts_cmp(Eterm a, Eterm b, int exact, int eq_only) {
+    if (is_atom(a) && is_atom(b)) {
+        return erts_cmp_atoms(a, b);
+    } else if (is_both_small(a, b)) {
+        return (signed_val(a) - signed_val(b));
+    } else if (is_float(a) && is_float(b)) {
+        FloatDef af, bf;
+
+        GET_DOUBLE(a, af);
+        GET_DOUBLE(b, bf);
+
+        return erts_float_comp(af.fd, bf.fd);
+    }
+
+    return erts_cmp_compound(a,b,exact,eq_only);
+}
+
+#endif /* ERTS_GLB_INLINE_INCL_FUNC_DEF */
+
 #endif
diff --git a/erts/emulator/beam/external.c b/erts/emulator/beam/external.c
index 621ba108ba..9a66e491f3 100644
--- a/erts/emulator/beam/external.c
+++ b/erts/emulator/beam/external.c
@@ -1953,7 +1953,8 @@ static Eterm erts_term_to_binary_int(Process* p, Eterm Term, int level, Uint fla
 
 #define RETURN_STATE()							\
     do {								\
-	hp = HAlloc(p, ERTS_MAGIC_REF_THING_SIZE+3);                    \
+	static const int TUPLE2_SIZE = 2 + 1;				\
+	hp = HAlloc(p, ERTS_MAGIC_REF_THING_SIZE + TUPLE2_SIZE);        \
 	c_term = erts_mk_magic_ref(&hp, &MSO(p), context_b);            \
 	res = TUPLE2(hp, Term, c_term);					\
 	BUMP_ALL_REDS(p);                                               \
diff --git a/erts/emulator/beam/utils.c b/erts/emulator/beam/utils.c
index 08f8ca9788..d81bd89a48 100644
--- a/erts/emulator/beam/utils.c
+++ b/erts/emulator/beam/utils.c
@@ -2615,27 +2615,6 @@ not_equal:
 }
 
 
-/* 
- * Lexically compare two strings of bytes (string s1 length l1 and s2 l2).
- *
- *	s1 < s2	return -1
- *	s1 = s2	return  0
- *	s1 > s2 return +1
- */
-static int cmpbytes(byte *s1, int l1, byte *s2, int l2)
-{
-    int i;
-    i = 0;
-    while((i < l1) && (i < l2)) {
-	if (s1[i] < s2[i]) return(-1);
-	if (s1[i] > s2[i]) return(1);
-	i++;
-    }
-    if (l1 < l2) return(-1);
-    if (l1 > l2) return(1);
-    return(0);
-}
-
 
 /*
  * Compare objects.
@@ -2649,20 +2628,6 @@ static int cmpbytes(byte *s1, int l1, byte *s2, int l2)
  *
  */
 
-
-#define float_comp(x,y)    (((x)<(y)) ? -1 : (((x)==(y)) ? 0 : 1))
-
-int erts_cmp_atoms(Eterm a, Eterm b)
-{
-    Atom *aa = atom_tab(atom_val(a));
-    Atom *bb = atom_tab(atom_val(b));
-    int diff = aa->ord0 - bb->ord0;
-    if (diff)
-	return diff;
-    return cmpbytes(aa->name+3, aa->len-3,
-		    bb->name+3, bb->len-3);
-}
-
 /* cmp(Eterm a, Eterm b)
  *  For compatibility with HiPE - arith-based compare.
  */
@@ -2673,22 +2638,6 @@ Sint cmp(Eterm a, Eterm b)
 
 Sint erts_cmp_compound(Eterm a, Eterm b, int exact, int eq_only);
 
-Sint erts_cmp(Eterm a, Eterm b, int exact, int eq_only)
-{
-    if (is_atom(a) && is_atom(b)) {
-        return erts_cmp_atoms(a, b);
-    } else if (is_both_small(a, b)) {
-        return (signed_val(a) - signed_val(b));
-    } else if (is_float(a) && is_float(b)) {
-        FloatDef af, bf;
-        GET_DOUBLE(a, af);
-        GET_DOUBLE(b, bf);
-        return float_comp(af.fd, bf.fd);
-    }
-    return erts_cmp_compound(a,b,exact,eq_only);
-}
-
-
 /* erts_cmp(Eterm a, Eterm b, int exact)
  * exact = 1 -> term-based compare
  * exact = 0 -> arith-based compare
@@ -2985,7 +2934,7 @@ tailrecur_ne:
 
 		    GET_DOUBLE(a, af);
 		    GET_DOUBLE(b, bf);
-		    ON_CMP_GOTO(float_comp(af.fd, bf.fd));
+		    ON_CMP_GOTO(erts_float_comp(af.fd, bf.fd));
 		}
 	    case (_TAG_HEADER_POS_BIG >> _TAG_PRIMARY_SIZE):
 	    case (_TAG_HEADER_NEG_BIG >> _TAG_PRIMARY_SIZE):
@@ -3022,10 +2971,7 @@ tailrecur_ne:
 		    ErlFunThing* f2 = (ErlFunThing *) fun_val(b);
 		    Sint diff;
 
-		    diff = cmpbytes(atom_tab(atom_val(f1->fe->module))->name,
-				    atom_tab(atom_val(f1->fe->module))->len,
-				    atom_tab(atom_val(f2->fe->module))->name,
-				    atom_tab(atom_val(f2->fe->module))->len);
+                    diff = erts_cmp_atoms((f1->fe)->module, (f2->fe)->module);
 		    if (diff != 0) {
 			RETURN_NEQ(diff);
 		    }
@@ -3219,7 +3165,7 @@ tailrecur_ne:
 	    if (f2.fd < MAX_LOSSLESS_FLOAT && f2.fd > MIN_LOSSLESS_FLOAT) {
 		/* Float is within the no loss limit */
 		f1.fd = signed_val(aw);
-		j = float_comp(f1.fd, f2.fd);
+		j = erts_float_comp(f1.fd, f2.fd);
 	    }
 #if ERTS_SIZEOF_ETERM == 8
 	    else if (f2.fd > (double) (MAX_SMALL + 1)) {
@@ -3266,7 +3212,7 @@ tailrecur_ne:
 		if (big_to_double(aw, &f1.fd) < 0) {
 		    j = big_sign(aw) ? -1 : 1;
 		} else {
-		    j = float_comp(f1.fd, f2.fd);
+		    j = erts_float_comp(f1.fd, f2.fd);
 		}
 	    } else {
 		big = double_to_big(f2.fd, big_buf, sizeof(big_buf)/sizeof(Eterm));
@@ -3282,7 +3228,7 @@ tailrecur_ne:
 	    if (f1.fd < MAX_LOSSLESS_FLOAT && f1.fd > MIN_LOSSLESS_FLOAT) {
 		/* Float is within the no loss limit */
 		f2.fd = signed_val(bw);
-		j = float_comp(f1.fd, f2.fd);
+		j = erts_float_comp(f1.fd, f2.fd);
 	    }
 #if ERTS_SIZEOF_ETERM == 8
 	    else if (f1.fd > (double) (MAX_SMALL + 1)) {