File 1571-Eliminate-redundant-branches.patch of Package erlang

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File 1571-Eliminate-redundant-branches.patch of Package erlang (Revision ab0a80da32d4196c3293bfd6d0ac4245)

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From 3e4b86a954a24aed31f1bc17d22a1f4e1e05e8aa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Mon, 28 Jun 2021 15:23:22 +0200
Subject: [PATCH 1/7] Eliminate redundant branches

`sys_core_fold` has a weak optimization that rewrites:

case BoolExpr of
        true -> true
        false -> false
    end

to simply:

BoolExpr

That optimization is weak because it will only handle a boolean
expression in a Core Erlang `case` expression, not other boolean
expressions resulting from, for example, guards.

Implement the optimization in a more effective way in the
`beam_ssa_opt` pass.
---
 lib/compiler/src/beam_ssa_opt.erl    | 124 ++++++++++++++++++++++++++-
 lib/compiler/src/sys_core_fold.erl   |  47 +++-------
 lib/compiler/test/beam_ssa_SUITE.erl |  16 +++-
 3 files changed, 149 insertions(+), 38 deletions(-)

diff --git a/lib/compiler/src/beam_ssa_opt.erl b/lib/compiler/src/beam_ssa_opt.erl
index 3ffa8ecc0a..72e320749e 100644
--- a/lib/compiler/src/beam_ssa_opt.erl
+++ b/lib/compiler/src/beam_ssa_opt.erl
@@ -39,7 +39,7 @@
 
 -include("beam_ssa_opt.hrl").
 
--import(lists, [all/2,append/1,duplicate/2,flatten/1,foldl/3,
+-import(lists, [all/2,append/1,droplast/1,duplicate/2,flatten/1,foldl/3,
                 keyfind/3,last/1,mapfoldl/3,member/2,
                 partition/2,reverse/1,reverse/2,
                 splitwith/2,sort/1,takewhile/2,unzip/1]).
@@ -301,6 +301,7 @@ epilogue_passes(Opts) ->
           ?PASS(ssa_opt_bsm_shortcut),
           ?PASS(ssa_opt_sink),
           ?PASS(ssa_opt_blockify),
+          ?PASS(ssa_opt_redundant_br),
           ?PASS(ssa_opt_merge_blocks),
           ?PASS(ssa_opt_get_tuple_element),
           ?PASS(ssa_opt_tail_calls),
@@ -3101,6 +3102,127 @@ is_tail_call_is([I|Is], Bool, Ret, Acc) ->
     is_tail_call_is(Is, Bool, Ret, [I|Acc]);
 is_tail_call_is([], _Bool, _Ret, _Acc) -> no.
 
+%%%
+%%% Eliminate redundant branches.
+%%%
+%%% Redundant `br` instructions following calls to guard BIFs such as:
+%%%
+%%%     @bif_result = bif:Bif ...
+%%%     br @bif_result, ^100, ^200
+%%%
+%%%   100:
+%%%      ret `true`
+%%%
+%%%   200:
+%%%      ret `false`
+%%%
+%%% can can be rewritten to:
+%%%
+%%%     @bif_result = bif:Bif ...
+%%%     ret @bif_result
+%%%
+%%% A similar rewriting is possible if the true and false branches end
+%%% up at a phi node.
+%%%
+%%% A code sequence such as:
+%%%
+%%%   @ssa_bool = bif:'=:=' Var, Other
+%%%   br @ssa_bool, ^100, ^200
+%%%
+%%% 100:
+%%%   ret Other
+%%%
+%%% 200:
+%%%   ret Var
+%%%
+%%% can be rewritten to:
+%%%
+%%%   ret Var
+%%%
+
+ssa_opt_redundant_br({#opt_st{ssa=Blocks0}=St, FuncDb}) ->
+    Blocks = redundant_br(beam_ssa:rpo(Blocks0), Blocks0),
+    {St#opt_st{ssa=Blocks}, FuncDb}.
+
+redundant_br([L|Ls], Blocks0) ->
+    Blk0 = map_get(L, Blocks0),
+    case Blk0 of
+        #b_blk{is=Is,
+               last=#b_br{bool=#b_var{}=Bool,
+                          succ=Succ,
+                          fail=Fail}} ->
+            case Blocks0 of
+                #{Succ := #b_blk{is=[],last=#b_ret{arg=#b_literal{val=true}}},
+                  Fail := #b_blk{is=[],last=#b_ret{arg=#b_literal{val=false}}}} ->
+                    case redundant_br_safe_bool(Is, Bool) of
+                        true ->
+                            Blk = Blk0#b_blk{last=#b_ret{arg=Bool}},
+                            Blocks = Blocks0#{L => Blk},
+                            redundant_br(Ls, Blocks);
+                        false ->
+                            redundant_br(Ls, Blocks0)
+                    end;
+                #{Succ := #b_blk{is=[],last=#b_br{succ=PhiL,fail=PhiL}},
+                  Fail := #b_blk{is=[],last=#b_br{succ=PhiL,fail=PhiL}}} ->
+                    case redundant_br_safe_bool(Is, Bool) of
+                        true ->
+                            Blocks = redundant_br_phi(L, Blk0, PhiL, Blocks0),
+                            redundant_br(Ls, Blocks);
+                        false ->
+                            redundant_br(Ls, Blocks0)
+                    end;
+                #{Succ := #b_blk{is=[],last=#b_ret{arg=Other}},
+                  Fail := #b_blk{is=[],last=#b_ret{arg=Var}}} when Is =/= [] ->
+                    case last(Is) of
+                        #b_set{op={bif,'=:='},args=[Var,Other]} ->
+                            Blk = Blk0#b_blk{is=droplast(Is),
+                                             last=#b_ret{arg=Var}},
+                            Blocks = Blocks0#{L => Blk},
+                            redundant_br(Ls, Blocks);
+                        #b_set{} ->
+                            redundant_br(Ls, Blocks0)
+                    end;
+                #{} ->
+                    redundant_br(Ls, Blocks0)
+            end;
+        _ ->
+            redundant_br(Ls, Blocks0)
+    end;
+redundant_br([], Blocks) -> Blocks.
+
+redundant_br_phi(L, Blk0, PhiL, Blocks) ->
+    #b_blk{is=Is0} = PhiBlk0 = map_get(PhiL, Blocks),
+    case Is0 of
+        [#b_set{op=phi},#b_set{op=phi}|_] ->
+            Blocks;
+        [#b_set{op=phi,args=PhiArgs0}=I0|Is] ->
+            #b_blk{last=#b_br{succ=Succ,fail=Fail}} = Blk0,
+            BoolPhiArgs = [{#b_literal{val=false},Fail},
+                           {#b_literal{val=true},Succ}],
+            PhiArgs1 = ordsets:from_list(PhiArgs0),
+            case ordsets:is_subset(BoolPhiArgs, PhiArgs1) of
+                true ->
+                    #b_blk{last=#b_br{bool=Bool}} = Blk0,
+                    PhiArgs = ordsets:add_element({Bool,L}, PhiArgs1),
+                    I = I0#b_set{args=PhiArgs},
+                    PhiBlk = PhiBlk0#b_blk{is=[I|Is]},
+                    Br = #b_br{bool=#b_literal{val=true},succ=PhiL,fail=PhiL},
+                    Blk = Blk0#b_blk{last=Br},
+                    Blocks#{L := Blk, PhiL := PhiBlk};
+                false ->
+                    Blocks
+            end
+    end.
+
+redundant_br_safe_bool([], _Bool) ->
+    true;
+redundant_br_safe_bool(Is, Bool) ->
+    case last(Is) of
+        #b_set{op={bif,_}} -> true;
+        #b_set{op=has_map_field} -> true;
+        #b_set{dst=Dst} -> Dst =/= Bool
+    end.
+
 %%%
 %%% Common utilities.
 %%%
diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 01e9987196..3653d5e647 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -324,19 +324,16 @@ expr(#c_case{}=Case0, Ctxt, Sub) ->
     %%   (in addition to any warnings that may have been emitted
     %%   according to the rules above).
     %%
-    case opt_bool_case(Case0, Sub) of
-	#c_case{anno=Anno,arg=Arg0,clauses=Cs0}=Case1 ->
-	    Arg1 = body(Arg0, value, Sub),
-	    LitExpr = cerl:is_literal(Arg1),
-	    {Arg2,Cs1} = case_opt(Arg1, Cs0, Sub),
-	    Cs2 = clauses(Arg2, Cs1, Ctxt, Sub, LitExpr, Anno),
-	    Case = Case1#c_case{arg=Arg2,clauses=Cs2},
-	    warn_no_clause_match(Case1, Case),
-	    Expr = eval_case(Case, Sub),
-            move_case_into_arg(Expr, Sub);
-	Other ->
-	    expr(Other, Ctxt, Sub)
-    end;
+    Case1 = opt_bool_case(Case0, Sub),
+    #c_case{anno=Anno,arg=Arg0,clauses=Cs0} = Case1,
+    Arg1 = body(Arg0, value, Sub),
+    LitExpr = cerl:is_literal(Arg1),
+    {Arg2,Cs1} = case_opt(Arg1, Cs0, Sub),
+    Cs2 = clauses(Arg2, Cs1, Ctxt, Sub, LitExpr, Anno),
+    Case = Case1#c_case{arg=Arg2,clauses=Cs2},
+    warn_no_clause_match(Case1, Case),
+    Expr = eval_case(Case, Sub),
+    move_case_into_arg(Expr, Sub);
 expr(#c_apply{anno=Anno,op=Op0,args=As0}=Apply0, _, Sub) ->
     Op1 = expr(Op0, value, Sub),
     As1 = expr_list(As0, value, Sub),
@@ -1593,34 +1590,12 @@ opt_bool_not(#c_case{arg=Arg,clauses=Cs0}=Case0) ->
 	    Case = Case0#c_case{arg=Expr,clauses=Cs},
 	    opt_bool_not(Case);
 	_ ->
-	    opt_bool_case_redundant(Case0)
+            Case0
     end.
 
 opt_bool_not_invert(#c_clause{pats=[#c_literal{val=Bool}]}=C) ->
     C#c_clause{pats=[#c_literal{val=not Bool}]}.
 
-%% opt_bool_case_redundant(Core) -> Core'.
-%%  If the sole purpose of the case is to verify that the case
-%%  expression is indeed boolean, we do not need the case
-%%  (since we have already verified that the case expression is
-%%  boolean).
-%%
-%%    case BoolExpr of
-%%      true -> true   	       	       ==>      BoolExpr
-%%      false -> false
-%%    end.
-%%
-opt_bool_case_redundant(#c_case{arg=Arg,clauses=Cs}=Case) ->
-    case all(fun opt_bool_case_redundant_1/1, Cs) of
-	true -> Arg;
-	false -> Case
-    end.
-
-opt_bool_case_redundant_1(#c_clause{pats=[#c_literal{val=B}],
-				    body=#c_literal{val=B}}) ->
-    true;
-opt_bool_case_redundant_1(_) -> false.
-
 %% eval_case(Case) -> #c_case{} | #c_let{}.
 %%  If possible, evaluate a case at compile time.  We know that the
 %%  last clause is guaranteed to match so if there is only one clause
diff --git a/lib/compiler/test/beam_ssa_SUITE.erl b/lib/compiler/test/beam_ssa_SUITE.erl
index 590ee231ff..6bada57a2a 100644
--- a/lib/compiler/test/beam_ssa_SUITE.erl
+++ b/lib/compiler/test/beam_ssa_SUITE.erl
@@ -25,7 +25,8 @@
          cover_ssa_dead/1,combine_sw/1,share_opt/1,
          beam_ssa_dead_crash/1,stack_init/1,
          mapfoldl/0,mapfoldl/1,
-         grab_bag/1,coverage/1]).
+         grab_bag/1,redundant_br/1,
+         coverage/1]).
 
 suite() -> [{ct_hooks,[ts_install_cth]}].
 
@@ -45,6 +46,7 @@ groups() ->
        beam_ssa_dead_crash,
        stack_init,
        grab_bag,
+       redundant_br,
        coverage
       ]}].
 
@@ -1084,6 +1086,18 @@ grab_bag_17() ->
             []
     end.
 
+redundant_br(_Config) ->
+    {false,{x,y,z}} = redundant_br_1(id({x,y,z})),
+    {true,[[a,b,c]]} = redundant_br_1(id([[[a,b,c]]])),
+    ok.
+
+redundant_br_1(Specs0) ->
+    {Join,Specs} =
+        if
+            is_list(hd(hd(Specs0))) -> {true,hd(Specs0)};
+            true -> {false,Specs0}
+        end,
+    id({Join,Specs}).
 
 coverage(_Config) ->
 
-- 
2.31.1

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File 1571-Eliminate-redundant-branches.patch of Package erlang (Revision ab0a80da32d4196c3293bfd6d0ac4245)

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