From 34c8b06de52ac81e2938e93ed7a219d7611c16f9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Fri, 6 Oct 2017 09:06:24 +0200
Subject: 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
---
 lib/compiler/src/beam_utils.erl | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

(limited to 'lib/compiler/src')

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) ->
-- 
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