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

2 files changed, 14 insertions, 2 deletions

diff --git a/lib/compiler/src/beam_utils.erl b/lib/compiler/src/beam_utils.erl
index a4c65397df..00f396c246 100644
--- a/lib/compiler/src/beam_utils.erl
+++ b/lib/compiler/src/beam_utils.erl
@@ -825,8 +825,14 @@ live_opt([{select,_,Src,Fail,List}=I|Is], Regs0, D, Acc) ->
     Regs1 = x_live([Src], Regs0),
     Regs = live_join_labels([Fail|List], D, Regs1),
     live_opt(Is, Regs, D, [I|Acc]);
-live_opt([{try_case,_}=I|Is], _, D, Acc) ->
-    live_opt(Is, live_call(1), D, [I|Acc]);
+live_opt([{try_case,Y}=I|Is], Regs0, D, Acc) ->
+    Regs = live_call(1),
+    case Regs0 of
+        0 ->
+            live_opt(Is, Regs, D, [{try_end,Y}|Acc]);
+        _ ->
+            live_opt(Is, live_call(1), D, [I|Acc])
+    end;
 live_opt([{loop_rec,_Fail,_Dst}=I|Is], _, D, Acc) ->
     live_opt(Is, 0, D, [I|Acc]);
 live_opt([timeout=I|Is], _, D, Acc) ->
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) ->