Optimize try/catch that ignores the return value

If a try/catch is used to ignore the potential exception caused by some
expression with a side effect such as:

   try
       side_effect()
   catch _Class:_Reason ->
        ok
   end,
   .
   .
   .

the generated code will be wasteful:

    try YReg TryLabel
    %% call side_effect() here
    try_end Yreg
    jump GoodLabel

  TryLabel:
    try_case YReg
    %% try_case has set up registers as follows:
    %% x(0) -> error | exit | throw
    %% x(1) -> reason
    %% x(2) -> raw stack trace data

  GoodLabel:
    %% This code does not use any x registers.

There will be two code paths that both end up at GoodLabel, and
the try_case instruction will set up registers that are never used.

We can do better by replacing try_case with try_end to obtain this
code:

    try YReg TryLabel
    %% call side_effect() here
    try_end Yreg
    jump GoodLabel

  TryLabel:
    try_end YReg

  GoodLabel:

The jump optimizer (beam_jump) can further optimize this code
like this:

    try YReg TryLabel
    %% call side_effect() here

  TryLabel:
    try_end YReg

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/hipe/icode/hipe_beam_to_icode.erl b/lib/hipe/icode/hipe_beam_to_icode.erl
index 2abecf7f18..167f5c67bb 100644
--- a/lib/hipe/icode/hipe_beam_to_icode.erl
+++ b/lib/hipe/icode/hipe_beam_to_icode.erl
@@ -2296,6 +2296,12 @@ split_code([First|Code], Label, Instr) ->
 
 split_code([Instr|Code], Label, Instr, Prev, As) when Prev =:= Label ->
   split_code_final(Code, As);  % drop both label and instruction
+split_code([{icode_end_try}|_]=Code, Label, {try_case,_}, Prev, As)
+  when Prev =:= Label ->
+  %% The try_case has been replaced with try_end as an optimization.
+  %% Keep this instruction, since it might be the only try_end instruction
+  %% for this try/catch block.
+  split_code_final(Code, As);  % drop label
 split_code([Other|_Code], Label, Instr, Prev, _As) when Prev =:= Label ->
   ?EXIT({missing_instr_after_label, Label, Instr, [Other, Prev | _As]});
 split_code([Other|Code], Label, Instr, Prev, As) ->