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, 21 insertions, 0 deletions

diff --git a/erts/emulator/test/exception_SUITE.erl b/erts/emulator/test/exception_SUITE.erl
index e473a10be7..0f27251fcb 100644
--- a/erts/emulator/test/exception_SUITE.erl
+++ b/erts/emulator/test/exception_SUITE.erl
@@ -662,6 +662,15 @@ line_numbers(Config) when is_list(Config) ->
               {?MODULE,line_numbers,1,_}|_]}} =
     (catch applied_bif_2()),
 
+    {'EXIT',{badarith,
+             [{?MODULE,increment1,1,[{file,"increment.erl"},{line,45}]},
+              {?MODULE,line_numbers,1,_}|_]}} =
+        (catch increment1(x)),
+    {'EXIT',{badarith,
+             [{?MODULE,increment2,1,[{file,"increment.erl"},{line,48}]},
+              {?MODULE,line_numbers,1,_}|_]}} =
+        (catch increment2(x)),
+
     ok.
 
 id(I) -> I.
@@ -762,3 +771,15 @@ applied_bif_2() ->				%Line 8
     R = process_info(self(), current_location),	%Line 9
     fail = R,					%Line 10
     ok.						%Line 11
+
+%% The increment instruction used to decrement the instruction
+%% pointer, which would cause the line number in a stack trace to
+%% be the previous line number.
+
+-file("increment.erl", 42).
+increment1(Arg) ->                              %Line 43
+    Res = id(Arg),                              %Line 44
+    Res + 1.                                    %Line 45
+increment2(Arg) ->                              %Line 46
+    _ = id(Arg),                                %Line 47
+    Arg + 1.                                    %Line 48