Erlang -- Invariants on the Structure and Format of BEAM SSA

# 1 Invariants on the Structure and Format of BEAM SSA

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1.1 Exception Handling

The translation of a try-catch expression into BEAM SSA has the following structure:

    @tag = new_try_tag `try`
    br @tag, ^protected_block0, ^landing_pad_block

    protected_block0:
      @success0 = ... % Something that could raise an exception
      br @success0, ^protected_block1, ^landing_pad_block

    ...

    protected_blockN:
      % The end of the protected code
      @ignored0 = kill_try_tag @tag
      br ^after_try_catch

    landing_pad_block:
      @aggregate = landingpad try, @tag
      @class  = extract @aggregate, `0` % The error class
      @reason = extract @aggregate, `1` % The reason
      @stk    = extract @aggregate, `2` % The stack trace
      @ignored1 = kill_try_tag @tag
      %% Pattern matching on @class, @reason, and @stk is done here
      %% to send control to the appropriate catch clause
      br ^after_try_catch

    after_try_catch:
      % Normal execution continues

The following invariants must hold for the SSA:

- All code that can cause an exception in one of the protected blocks must have explicit control flow edges to the landing pad block. If there are no edges to the landing pad block except from the block containing the new_try_tag, the compiler will remove the redundant exception handler.
- The extraction of the class, reason and stack trace from the result of the landingpad instruction must be done in that order. Omitting the extraction of elements which are unused is allowed.
- Both the landing pad block and the final protected block must end with a kill_try_tag instruction. Trying to share the kill_try_tag epilogue between the last protected block and the landing pad is unlikely to work.

The translation of an old-style catch expression into BEAM SSA has the following structure:

    @tag = new_try_tag `try`
    br @tag, ^protected_block0, ^landing_pad_block

    protected_block0:
      @success0 = ... % Something that could raise an exception
      br @success0, ^protected_block1, ^landing_pad_block

    ...

    protected_blockN:
      % The end of the protected code
      @successful_result = .... % The result of a successful computation
      br ^common_end_of_catch

    landing_pad_block:
       @aggregate = landingpad catch, @tag
       @catched_val = extract @ssa_agg, `0`
       br ^common_end_of_catch

    common_end_of_catch:
      @tmp = phi { @catched_val, ^landing_pad_block },
                 { @successful_result, ^protected_blockN }
      @result_of_catch_expr = catch_end @tag, @tmp

Just as for a try-catch expression all code that can cause an exception in one of the protected blocks must have explicit control flow edges to the landing pad block.

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1.2 Exception Re-issuing

A typical user-written try-catch expression will catch a subset of all possible exception classes and reasons and leave unhandled exceptions to a handler further up the call stack. Re-issuing an exception is done with the resume instruction. The resume must come after the kill_try_tag instruction in the program flow. For example, if the example in the Exception Handling Section was to only handle user throws, the relevant blocks would look like this:

    landing_pad_block:
      @aggregate = landingpad `try`, @tag
      @class  = extract @aggregate, `0` % The error class
      @reason = extract @aggregate, `1` % The reason
      @stk    = extract @aggregate, `2` % The stack trace
      @ignored1 = kill_try_tag @tag
      @is_throw = bif:'=:=' @class, `throw`
      br @is_throw ^first_block_of_throw_handler, ^reissue

    first_block_of_throw_handler:
      %% Handle the user-defined throw

    reissue:
      @tmp = resume @stk, @reason
      ret @tmp

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1.3 Function Calls

All function calls not in a tail call position must be followed by a succeeded:body-instruction unless one of the following exceptions apply:

- The function call can statically be proven to always fail.

- The function call is to the erlang-module and can statically be proven to always succeed or fail.

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1.4 Variable Naming

A variable name in BEAM SSA is either an atom, a non-negative integer or a tuple: atom() | non_neg_integer() | {atom() | non_neg_integer(), non_neg_integer()}. In order to generate fresh unused variable names, all compiler transforms maintain a counter, the cnt-field in the opt_st-record, which is incremented each time a new variable or label is created. In the following description the value of the cnt-field is called Cnt.

Due to peculiarities in the BEAM SSA code generator, a compiler transformation unfortunately cannot just use the cnt-value directly as a fresh name. There are three basic strategies for creating fresh variable names which can by used by a compiler pass:

1) A name can be derived from an existing name of the form V :: atom() | non_neg_integer() by selecting an atom, which is unique to the compiler pass, to form a new name {A, V}. The same A cannot be used by strategy 3) below.

2) A name can be derived from an existing name of the form V :: non_neg_integer() by combining it with the cnt-field into {V, Cnt}.

3) A fresh name can be created by selecting an atom A, which is unique to the compiler pass, to form the new name {A, Cnt}. The same A cannot be used by strategy 1) above.