Point out the correct line number in stack traces

Sometimes the line number in a stack trace could be wrong,
for example for this code:

    t() ->
        Res = id(x), %<== Wrong line number.
        Res + 1.

    id(I) -> I.

The line number pointed out in the stack trace would be the
line before the line where the exception occurred.

The reason is the way the increment instruction instruction
is implemented:

    OpCase(i_increment_rWtd):
    {
      increment_reg_val = r(0);
    }
    I -= 1;
    goto increment__execute;

    OpCase(i_increment_xWtd):
    {
      increment_reg_val = xb(I[1]);
    }
    goto increment__execute;

    increment__execute:
    /* Common code for increment */
    .
    .
    .

(The implementation in OTP 20 is similar, but hand-coded directly
in beam_emu.c instead of generated.)

The instruction i_increment_rWtd decrements the instruction pointer (I)
before jumping to the common code. That means that I points *before*
the 'increment' instruction. If there is a 'line' instruction directly
before the 'increment' instruction (as there is in this example), the
instruction pointer will point before that line. Thus the previous line
will be picked up instead.

To eliminate this bug, we must never decrement the instruction pointer.
Instead, we can increment the other (longer) instructions in the
same group of combined instructions:

    OpCase(i_increment_rWtd):
    {
      increment_reg_val = r(0);
    }
    goto increment__execute;

    OpCase(i_increment_xWtd):
    {
      increment_reg_val = xb(I[1]);
    }
    I += 1;
    goto increment__execute;

    increment__execute:
    /* Common code for increment */
    .
    .
    .

Also fix a bug that was only a potential bug when ddaed7774eb0a
introduced relative jumps, but is now a real bug. See the added
comment for SET_I_REL() in macros.tab.

1 files changed, 14 insertions, 11 deletions

diff --git a/erts/emulator/utils/beam_makeops b/erts/emulator/utils/beam_makeops
index a9b2c8861c..e4c91f1685 100755
--- a/erts/emulator/utils/beam_makeops
+++ b/erts/emulator/utils/beam_makeops
@@ -1111,7 +1111,7 @@ sub combine_instruction_group {
     my %offsets;
     my @instrs;
     my %num_references;
-    my $group_size = 0;
+    my $group_size = 999;
 
     # Do basic error checking. Associate operands of instructions
     # with the correct micro instructions. Calculate offsets for micro
@@ -1140,7 +1140,7 @@ sub combine_instruction_group {
                 }
                 my $size = cg_combined_size($s, 1, @first);
                 $offsets{$s} = $offset
-                    unless defined $offsets{$s} and $offsets{$s} >= $offset;
+                    unless defined $offsets{$s} and $offsets{$s} < $offset;
                 $offset += $size - 1;
                 my $label = micro_label($s);
                 $num_references{$label} = 0;
@@ -1148,7 +1148,7 @@ sub combine_instruction_group {
                 $opcase = '';
             }
             $spec_op_info{$print_name}->{'size'} = $offset + 1;
-            $group_size = $offset if $group_size < $offset;
+            $group_size = $offset if $group_size >= $offset;
             push @instrs, [$specific_key,@new_subs];
         }
     }
@@ -1218,19 +1218,19 @@ sub combine_instruction_group {
 
             my $flags = '';
             my $transfer_to_next = '';
-            my $dec = 0;
+            my $inc = 0;
 
             unless ($i == $#slots) {
                 $flags = "-no_next";
                 my $next_offset = $label_to_offset{$next};
-                $dec = $next_offset - ($offset + $size);
-                $transfer_to_next = "I -= $dec;\n" if $dec;
+                $inc = ($offset + $size) - $next_offset;
+                $transfer_to_next = "I += $inc;\n" if $inc;
                 $transfer_to_next .= "goto $next;\n\n";
             }
 
             my($gen_code,$down,$up) =
                 cg_combined_code($s, 1, $flags, $offset,
-                                 $group_size-$offset-$dec, @first);
+                                 $group_size-$offset, $inc, @first);
             my $spec_label = "$opcase$label";
             $down{$spec_label} = $down;
             $up{$spec_label} = $up;
@@ -1280,7 +1280,7 @@ sub micro_label {
 
 sub cg_basic {
     my($name,@args) = @_;
-    my($size,$code,$pack_spec) = code_gen($name, 1, '', 0, undef, @args);
+    my($size,$code,$pack_spec) = code_gen($name, 1, '', 0, undef, undef, @args);
     $pack_spec = build_pack_spec($pack_spec);
     ($size,$code,$pack_spec);
 }
@@ -1291,7 +1291,7 @@ sub cg_basic {
 
 sub cg_combined_size {
     my($name,$pack,@args) = @_;
-    my($size) = code_gen($name, $pack, '', 0, undef, @args);
+    my($size) = code_gen($name, $pack, '', 0, undef, undef, @args);
     $size;
 }
 
@@ -1300,6 +1300,7 @@ sub cg_combined_size {
 #
 
 sub cg_combined_code {
+    my($name,$pack,$extra_comments,$offset,$comp_size,$inc,@args) = @_;
     my($size,$code,$pack_spec) = code_gen(@_);
     if ($pack_spec eq '') {
         ($code,'','');
@@ -1310,7 +1311,8 @@ sub cg_combined_code {
 }
 
 sub code_gen {
-    my($name,$pack,$extra_comments,$offset,$group_size,@args) = @_;
+    my($name,$pack,$extra_comments,$offset,$comp_size,$inc,@args) = @_;
+    my $group_size = defined $comp_size ? $comp_size + $inc : undef;
     my $size = 0;
     my $flags = '';
     my @f;
@@ -1416,6 +1418,7 @@ sub code_gen {
         $bindings{$var} = $f[$i];
     }
     $bindings{'NEXT_INSTRUCTION'} = "I+" . ($group_size+$offset+1);
+    $bindings{'IP_ADJUSTMENT'} = defined $inc ? $inc : 0;
     $c_code = eval { expand_all($c_code, \%bindings) };
     unless (defined $c_code) {
         warn $@;
@@ -1539,7 +1542,7 @@ sub expand_macro {
     my %new_bindings;
 
     # Keep the special, pre-defined bindings.
-    foreach my $key (qw(NEXT_INSTRUCTION)) {
+    foreach my $key (qw(NEXT_INSTRUCTION IP_ADJUSTMENT)) {
         $new_bindings{$key} = $bindings{$key};
     }