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9a50a5d5fc1 changed the update of I, but forgot to update
the preceding assertion.
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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.
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* bjorn/erts/relative-jumps:
Pack failure labels in i_select_val2 and i_select_tuple_arity2
Optimize i_select_tuple_arity2 and is_select_lins
Rewrite select_val_bins so that its labels can be packed
Pack sequences of trailing 'f' operands
Implement packing of 'f' and 'j'
Make sure that mask literals are 64 bits
Use relative failure labels
Add information about offset to common group start position
Remove JUMP_OFFSET
Refactor instructions to support relative jumps
Introduce a new trace_jump/1 instruction for tracing
Avoid using $Src more than once
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Relative failure in itself is not an optimization, but we plan to
pack failure labels in the future to save memory.
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It has served its purpose.
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Introduce new macros that can be used for relative jumps and
use them consistently.
Test that everything works by using a non-zero constant JUMP_OFFSET.
The loader subtracts JUMP_OFFSET from loaded labels, and all
instructions that use 'f' operands add it back.
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In a correct Erlang programs, we can expect that:
* A GC test instruction (such as test_heap) is more likely
not to do the GC.
* A BIF is more likely to succeed than to fail.
* A BIF is more likely to fail in a guard than in a body.
* An apply or fun call is likely to succeed.
Annotate conditions accordingly.
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The instruction put_map_assoc/5 (used for updating a map) has a
failure operand, but it can't actually fail provided that its "map"
argument is a map.
The following code:
M#{key=>value}.
will be compiled to:
{test,is_map,{f,3},[{x,0}]}.
{line,[...]}.
{put_map_assoc,{f,0},{x,0},{x,0},1,{list,[{atom,key},{atom,value}]}}.
return.
{label,3}.
%% Code that produces a 'badmap' exception follows.
Because of the is_map instruction, {x,0} always contains a map when
the put_map_assoc instruction is executed. Therefore we can remove
the failure operand. That will save one word, and also eliminate
two tests at run-time.
The only problem is that the compiler in OTP 17 did not emit a
is_map instruction before the put_map_assoc instruction. Therefore,
we must add an instruction that tests for a map if the code was
compiled with the OTP 17 compiler.
Unfortunately, there is no safe and relatively easy way to known that
the OTP 17 compiler was used, so we will check whether a compiler
before OTP 20 was used. OTP 20 introduced a new chunk type for atoms,
which is trivial to check.
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