Implement support for debug information in the runtime system

This commit implements support for loading the debug information from
a BEAM file and the `code:get_debug_info/1` BIF for retrieving the
debug information.

As an example, given the following module:

    -module(example).                               %  1
    -export([foo/1]).                               %  2

    foo(A) ->                                       %  4
        case A of                                   %  5
            0 ->                                    %  6
                B = 1,                              %  7
                io:format("~p\n", [B]);             %  8
            1 ->                                    %  9
                C = [1,2,3],                        % 10
                io:format("~p\n", [C])              % 11
        end,                                        % 12
        A.                                          % 13

here is how to compile it with BEAM debug information and
display the debug information:

     1> c(example, beam_debug_info).
     {ok,example}
     2> code:get_debug_info(example).
     [{5,{none,[{<<"A">>,{x,0}}]}},
      {7,{1,[{<<"A">>,{y,0}}]}},
      {8,{1,[{<<"B">>,{value,1}},{<<"A">>,{y,0}}]}},
      {10,{1,[{<<"A">>,{y,0}}]}},
      {11,{1,[{<<"C">>,{value,[1,2,3]}},{<<"A">>,{y,0}}]}},
      {13,{1,[{<<"A">>,{y,0}}]}}]

erts/emulator/beam/atom.names

@@ -111,6 +111,7 @@ atom asynchronous
 atom atom
 atom atom_used
 atom attributes
+atom auto
 atom auto_connect
 atom await_exit
 atom await_microstate_accounting_modifications
@@ -217,6 +218,7 @@ atom debug_flags
 atom decentralized_counters
 atom decimals
 atom default
+atom debug_hash_fixed_number_of_locks
 atom delay_trap
 atom demonitor
 atom deterministic
@@ -486,6 +488,7 @@ atom new_processes
 atom new_ports
 atom new_uniq
 atom newline
+atom nifs
 atom no
 atom nomatch
 atom none
@@ -766,10 +769,9 @@ atom warning
 atom warning_msg
 atom wordsize
 atom write_concurrency
+atom x
 atom xor
 atom x86
+atom y
 atom yes
 atom yield
-atom nifs
-atom auto
-atom debug_hash_fixed_number_of_locks

erts/emulator/beam/beam_bif_load.c

@@ -1251,6 +1251,125 @@ any_heap_refs(Eterm* start, Eterm* end, char* mod_start, Uint mod_size)
     return 0;
 }
 
+BIF_RETTYPE code_get_debug_info_1(BIF_ALIST_1)
+{
+#ifdef BEAMASM
+    ErtsCodeIndex code_ix;
+    Module* modp;
+    const BeamCodeHeader* hdr;
+    const BeamCodeLineTab* lt;
+    const BeamDebugTab* debug;
+    Sint i;
+    Uint alloc_size;
+    Eterm result = NIL;
+    Eterm* hp;
+    Eterm* hend;
+
+    if (is_not_atom(BIF_ARG_1)) {
+        BIF_ERROR(BIF_P, BADARG);
+    }
+    code_ix = erts_active_code_ix();
+    modp = erts_get_module(BIF_ARG_1, code_ix);
+    if (modp == NULL) {
+        BIF_ERROR(BIF_P, BADARG);
+    }
+    hdr = modp->curr.code_hdr;
+    if (hdr == NULL) {
+        BIF_ERROR(BIF_P, BADARG);
+    }
+
+    lt = hdr->line_table;
+
+    debug = hdr->debug;
+    if (debug == NULL) {
+        return am_none;
+    }
+
+    alloc_size = 0;
+
+    for (i = 0; i < debug->item_count; i++) {
+        /* [ {Index, {FrameSize,[{Name,Value}]} ] */
+        alloc_size += 2 + 3 + 3 + debug->items[i].num_vars * (2 + 3 + 3);
+    }
+
+    hp = HAlloc(BIF_P, alloc_size);
+    hend = hp + alloc_size;
+
+    for (i = debug->item_count-1; i >= 0; i--) {
+        BeamDebugItem* items = &debug->items[i];
+        Uint32 location_index;
+        Sint frame_size = items->frame_size;
+        Uint32 location;
+        Uint num_vars = items->num_vars;
+        Eterm *tp = items->first + 2 * num_vars - 2;
+        Eterm frame_size_term;
+        Eterm var_list = NIL;
+        Eterm tmp;
+
+        location_index = items->location_index;
+
+        if (location_index == (Uint32)-1) {
+            continue;
+        }
+        if (lt->loc_size == 2) {
+            location = lt->loc_tab.p2[location_index];
+        } else {
+            ASSERT(lt->loc_size == 4);
+            location = lt->loc_tab.p4[location_index];
+        }
+
+        if (frame_size < 0) {
+            frame_size_term = am_none;
+        } else {
+            frame_size_term = make_small(frame_size);
+        }
+
+        while (num_vars-- != 0) {
+            Eterm val;
+            Eterm tag;
+
+            if (_is_loader_x_reg(tp[1])) {
+                Uint xreg = loader_x_reg_index(tp[1]);
+                tag = am_x;
+                val = make_small(xreg);
+            } else if (_is_loader_y_reg(tp[1])) {
+                Uint yreg = loader_y_reg_index(tp[1]);
+                tag = am_y;
+                val = make_small(yreg);
+            } else {
+                tag = am_value;
+                val = tp[1];
+            }
+            tmp = TUPLE2(hp, tag, val);
+            hp += 3;
+
+            tmp = TUPLE2(hp, tp[0], tmp);
+            hp += 3;
+
+            tp -= 2;
+
+            var_list = CONS(hp, tmp, var_list);
+            hp += 2;
+        }
+
+        tmp = TUPLE2(hp, frame_size_term, var_list);
+        hp += 3;
+
+        tmp = TUPLE2(hp, make_small(LOC_LINE(location)), tmp);
+        hp += 3;
+
+        result = CONS(hp, tmp, result);
+        hp += 2;
+    }
+
+    ASSERT(hp <= hend);
+    HRelease(BIF_P, hend, hp);
+    return result;
+#endif
+
+    BIF_ERROR(BIF_P, BADARG);
+}
+
 /*
  * Release of literal areas...
  *

erts/emulator/beam/beam_code.h

@@ -48,6 +48,7 @@
 #define MD5_SIZE MD5_DIGEST_LENGTH
 
 typedef struct BeamCodeLineTab_ BeamCodeLineTab;
+typedef struct BeamDebugTab_ BeamDebugTab;
 
 /*
  * Header of code chunks which contains additional information
@@ -99,6 +100,11 @@ typedef struct beam_code_header {
     Uint32 *loc_index_to_cover_id;
     Uint line_coverage_len;
 
+    /*
+     * Debug information. debug->items are indexed directly by
+     * the index in each `debug_line` instruction.
+     */
+    const BeamDebugTab *debug;
 #endif
 
     /*
@@ -137,6 +143,21 @@ struct BeamCodeLineTab_ {
     const void** func_tab[1];
 };
 
+/*
+ * Layout of the debug information.
+ */
+typedef struct {
+    Uint32 location_index;
+    Sint16 frame_size;
+    Uint16 num_vars;
+    Eterm *first;
+} BeamDebugItem;
+
+struct BeamDebugTab_ {
+    Uint32 item_count;
+    BeamDebugItem *items;
+};
+
 /* Total code size in bytes */
 extern Uint erts_total_code_size;