Eliminate crash in beam_bool

beam_bool would crash when attempting to optimize BEAM code similar
to this code:

  bif '=:=' Reg1 SomeValue => y(0)
  bif '=:=' Reg2 {atom,true} => x(2)
  bif '=:=' Reg3 {atom,true} => x(3)
  bif 'or' x(2) x(3) => x(2)
  is_eq_exact Fail x(2) {atom,true}

The problem is that the first instruction that assigns a value to a Y
register. beam_bool:ssa_assign/2 will not accept a Y register
argument.

We could change ssa_assign/2 to accept a Y register, but that would
only cause the entire optimization to be rejected later because the Y
register is alive in the code that follows. Therefore, a better
solution is to modify extend_block/3 so that the instruction that
assign to Y registers are not added to the block. That is, the
optimizer will only operate on the following code:

  bif '=:=' Reg2 {atom,true} => x(2)
  bif '=:=' Reg3 {atom,true} => x(3)
  bif 'or' x(2) x(3) => x(2)
  is_eq_exact Fail x(2) {atom,true}

Usually the optimization will succeed, rewriting the four instructions
to a select_val instruction.

Assembly code such as the above can be produced by code similar to:

  Y = Something == SomethingElse,
  case Y of
     Condition; OtherCondition ->
       . . .
  end,
  . . .,
  Y.

Reported-by: http://bugs.erlang.org/browse/ERL-143
Reported-by: José Valim

0 files changed, 0 insertions, 0 deletions