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Somewhat simplified, beam_block would rewrite the target for
the first instruction in this code sequence:
move x(0) => y(1)
gc_bif '+' 1 x(0) => y(0)
move y(1) => x(1)
move nil => x(0)
call 2 local_function/2
The resulting code would be:
move x(0) => x(1) %% Changed target.
gc_bif '+' 1 x(0) => y(0)
move x(1) => y(1) %% Operands swapped (see 02d6135813).
move nil => x(0)
call 2 local_function/2
The resulting code is not safe because the x(1) will be killed
by the gc_bif instruction.
7a47b20c3a cleaned up move optimizations and would reject the
optimization if the target was an X register and an allocating
instruction was found. To avoid this bug, the optimization must be
rejected even if the target is a Y register.
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Two test cases were not actually run. Even if their main purpose is to
ensure that the compiler doesn't crash, we always try to also run the
test case (when practial) to also ensure that the generated code is
correct.
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Consider this code:
%% Start of block
get_tuple_element Tuple 0 Element
get_map_elements Fail Map [Key => Dest]
.
.
.
move Element UltimateDest
%% End of block
Fail:
%% Code that uses Element.
beam_block (more precisely, otp_tuple_element/1) would
incorrectly transform the code to this:
%% Start of block
get_map_elements Fail Map [Key => Dest]
.
.
.
get_tuple_element Tuple 0 UltimateDest
%% End of block
Fail:
%% Code that uses Element.
That is, the code at label Fail would use register Element,
which is either uninitalized or contains the wrong value.
We could fix this problem by always keeping label information
at hand when optimizing blocks so that we could check the code
at the failure label for get_map_elements. That would require
changes to beam_block and beam_utils. We might consider doing
that in the future if it turns out be worth it.
For now, I have decided that I want to keep the simplicity of blocks
(allowing them to be optimized without keeping label information).
That could be achieved by not including get_map_elements in
blocks. Another way, which I have chosen, is to only allow
get_map_elements as the first instruction in the block.
For background on the bug: c288ab8 introduced the beam_reorder pass
and 5f431276 introduced opt_tuple_element() in beam_block.
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