From 52abce861178bbce0dc3acb22cc8935f9a434981 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Fri, 24 Aug 2018 14:14:09 +0200
Subject: Move two optimizations from beam_dead to beam_a

The 'move' instruction can be eliminated in code such as:

    {test,is_eq_exact,{f,42},[{x,0},{atom,value}]}.
    {move,{atom,value},{x,0}}.

Move that optimization from beam_dead to beam_a. The optimization
will be simpler because the 'move' instruction has not yet
been moved into a block. Getting rid of 'move' earlier will
also save work for later passes.

Also move the optimization that eliminates instructions such
as from beam_dead to beam_a:

    {test_is_eq_exact,{f,42},[{x,0},{x,0}]}.
---
 lib/compiler/src/beam_a.erl    |  7 +++++++
 lib/compiler/src/beam_dead.erl | 15 ---------------
 2 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/lib/compiler/src/beam_a.erl b/lib/compiler/src/beam_a.erl
index 266e8f46c8..0abc845310 100644
--- a/lib/compiler/src/beam_a.erl
+++ b/lib/compiler/src/beam_a.erl
@@ -52,6 +52,13 @@ function({function,Name,Arity,CLabel,Is0}) ->
 	    erlang:raise(Class, Error, Stack)
     end.
 
+rename_instrs([{test,is_eq_exact,_,[Dst,Src]}=Test,
+               {move,Src,Dst}|Is]) ->
+    %% The move instruction is not needed.
+    rename_instrs([Test|Is]);
+rename_instrs([{test,is_eq_exact,_,[Same,Same]}|Is]) ->
+    %% Same literal or same register. Will always succeed.
+    rename_instrs(Is);
 rename_instrs([{apply_last,A,N}|Is]) ->
     [{apply,A},{deallocate,N},return|rename_instrs(Is)];
 rename_instrs([{call_last,A,F,N}|Is]) ->
diff --git a/lib/compiler/src/beam_dead.erl b/lib/compiler/src/beam_dead.erl
index 546f0461b9..f57bed0658 100644
--- a/lib/compiler/src/beam_dead.erl
+++ b/lib/compiler/src/beam_dead.erl
@@ -80,11 +80,6 @@ move_move_into_block([], Acc) -> reverse(Acc).
 %%% the register already contains the value being assigned, as in the
 %%% following code:
 %%%
-%%%           test is_eq_exact SomeLabel Src Dst
-%%%           move Src Dst
-%%%
-%%% or in:
-%%%
 %%%           select_val Register FailLabel [... Literal => L1...]
 %%%                      .
 %%%                      .
@@ -103,9 +98,6 @@ forward([{move,_,_}=Move|[{label,L}|_]=Is], D, Lc, Acc) ->
 forward([{bif,_,_,_,_}=Bif|[{label,L}|_]=Is], D, Lc, Acc) ->
     %% bif/4 followed by jump/1 is optimized by backward/3.
     forward([Bif,{jump,{f,L}}|Is], D, Lc, Acc);
-forward([{block,[]}|Is], D, Lc, Acc) ->
-    %% Empty blocks can prevent optimizations.
-    forward(Is, D, Lc, Acc);
 forward([{select,select_val,Reg,_,List}=I|Is], D0, Lc, Acc) ->
     D = update_value_dict(List, Reg, D0),
     forward(Is, D, Lc, [I|Acc]);
@@ -130,13 +122,6 @@ forward([{label,Lbl}=LblI|[{move,Lit,Dst}|Is1]=Is0], D, Lc, Acc) ->
 	     _ -> Is0		  %Keep move instruction.
 	 end,
     forward(Is, D, Lc, [LblI|Acc]);
-forward([{test,is_eq_exact,_,[Same,Same]}|Is], D, Lc, Acc) ->
-    forward(Is, D, Lc, Acc);
-forward([{test,is_eq_exact,_,[Dst,Src]}=I,
-	 {block,[{set,[Dst],[Src],move}|Bl]}|Is], D, Lc, Acc) ->
-    forward([I,{block,Bl}|Is], D, Lc, Acc);
-forward([{test,is_eq_exact,_,[Dst,Src]}=I,{move,Src,Dst}|Is], D, Lc, Acc) ->
-    forward([I|Is], D, Lc, Acc);
 forward([{test,_,_,_}=I|Is]=Is0, D, Lc, Acc) ->
     %% Help the second, backward pass to by inserting labels after
     %% relational operators so that they can be skipped if they are
-- 
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