View Source Invariants on the Structure and Format of BEAM SSA

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.

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

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.

Variable Naming

A variable name in BEAM SSA is either an atom or a non-negative integer:

atom() | non_neg_integer()

In order to generate fresh unused variable names, all compiler transforms maintain a counter, the cnt-field in the b_function and opt_st records, 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. The Cnt value is guaranteed to never clash with a previously defined variable name. Therefore, value of Cnt can directly be used as a variable name in the SSA passes.

Note that the rules were more complicated before Erlang/OTP 27, because the Cnt value could clash with other variables.