From 92c1920cb69fe7f3be202c6f77395c2cdbcb771e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Sun, 10 Feb 2019 13:59:54 +0100
Subject: beam_jump: Improve elimination of redundant moves

---
 lib/compiler/src/beam_jump.erl | 43 +++++++++++++++++++++++-------------------
 1 file changed, 24 insertions(+), 19 deletions(-)

(limited to 'lib/compiler/src/beam_jump.erl')

diff --git a/lib/compiler/src/beam_jump.erl b/lib/compiler/src/beam_jump.erl
index 8b0e3e32f8..6f50bfdb9c 100644
--- a/lib/compiler/src/beam_jump.erl
+++ b/lib/compiler/src/beam_jump.erl
@@ -182,18 +182,20 @@ eliminate_moves(Is) ->
 eliminate_moves([{select,select_val,Reg,_,List}=I|Is], D0, Acc) ->
     D = update_value_dict(List, Reg, D0),
     eliminate_moves(Is, D, [I|Acc]);
-eliminate_moves([{label,Lbl},{block,[{set,[Dst],[Lit],move}|BlkIs]}=Blk0|Is],
-        D, Acc0) ->
+eliminate_moves([{test,is_eq_exact,_,[Reg,Val]}=I,
+                 {block,BlkIs0}|Is], D0, Acc) ->
+    D = update_unsafe_labels(I, D0),
+    RegVal = {Reg,Val},
+    BlkIs = eliminate_moves_blk(BlkIs0, RegVal),
+    eliminate_moves([{block,BlkIs}|Is], D, [I|Acc]);
+eliminate_moves([{label,Lbl},{block,BlkIs0}=Blk|Is], D, Acc0) ->
     Acc = [{label,Lbl}|Acc0],
-    case already_has_value(Lit, Lbl, Dst, D) andalso
-        no_fallthrough(Acc0) of
-        true ->
-            %% Remove redundant 'move' instruction.
-            Blk = {block,BlkIs},
-            eliminate_moves([Blk|Is], D, Acc);
-        false ->
-            %% Keep 'move' instruction.
-            eliminate_moves([Blk0|Is], D, Acc)
+    case {no_fallthrough(Acc0),D} of
+        {true,#{Lbl:={_,_}=RegVal}} ->
+            BlkIs = eliminate_moves_blk(BlkIs0, RegVal),
+            eliminate_moves([{block,BlkIs}|Is], D, Acc);
+        {_,_} ->
+            eliminate_moves([Blk|Is], D, Acc)
     end;
 eliminate_moves([{block,[]}|Is], D, Acc) ->
     %% Empty blocks can prevent further jump optimizations.
@@ -203,17 +205,20 @@ eliminate_moves([I|Is], D0, Acc) ->
     eliminate_moves(Is, D, [I|Acc]);
 eliminate_moves([], _, Acc) -> reverse(Acc).
 
+eliminate_moves_blk([{set,[Dst],[_],move}|_]=Is, {_,Dst}) ->
+    Is;
+eliminate_moves_blk([{set,[Dst],[Lit],move}|Is], {Dst,Lit}) ->
+    %% Remove redundant 'move' instruction.
+    Is;
+eliminate_moves_blk([{set,[Dst],[_],move}|_]=Is, {Dst,_}) ->
+    Is;
+eliminate_moves_blk([{set,[_],[_],move}=I|Is], {_,_}=RegVal) ->
+    [I|eliminate_moves_blk(Is, RegVal)];
+eliminate_moves_blk(Is, _) -> Is.
+
 no_fallthrough([I|_]) ->
     is_unreachable_after(I).
 
-already_has_value(Lit, Lbl, Reg, D) ->
-    case D of
-        #{Lbl:={Reg,Lit}} ->
-            true;
-        #{} ->
-            false
-    end.
-
 update_value_dict([Lit,{f,Lbl}|T], Reg, D0) ->
     D = case D0 of
             #{Lbl:=unsafe} -> D0;
-- 
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