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File 0345-Do-not-consider-zero-floats-as-singleton.patch of Package erlang (Revision b4f72044e1af8cfe32130ab849a401a1)

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From 9d397879dad1fa829515f19abb07e2a3658fe2d3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Valim?= <jose.valim@dashbit.co>
Date: Thu, 3 Dec 2020 07:41:40 +0100
Subject: [PATCH] Do not consider zero floats as singleton

There is currently a bug in the compiler where 0.0
is considered a singleton. However, 0.0 can match both
-0.0 and 0.0 which makes this assumption invalid. Here
is a failing test:

float_zero(Config) when is_list(Config) ->
        <<16#0000000000000000:64>> = to_fbin(1*0.0),
        <<16#8000000000000000:64>> = to_fbin(-1*0.0),
        ok.

to_fbin(X = 0.0) -> <<X/float>>.

This commit changes beam_types:get_single_value/1 to not
consider float singletons and therefore not perform constant
propagation.

Furthermore, we also need to instruct beam_asm to encode
0.0 and -0.0 as two distinct literals.
---
 lib/compiler/src/beam_dict.erl    | 29 ++++++++++++++++++++++++-----
 lib/compiler/src/beam_types.erl   |  3 ++-
 lib/compiler/test/float_SUITE.erl | 10 ++++++++--
 3 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/lib/compiler/src/beam_dict.erl b/lib/compiler/src/beam_dict.erl
index 76d4e62f6b..8619056cbf 100644
--- a/lib/compiler/src/beam_dict.erl
+++ b/lib/compiler/src/beam_dict.erl
@@ -169,12 +169,28 @@ lambda(Lbl, NumFree, #asm{wrappers=Wrappers0,
 %%    literal(Literal, Dict) -> {Index,Dict'}
 -spec literal(term(), bdict()) -> {non_neg_integer(), bdict()}.
 
-literal(Lit, #asm{literals=Tab0,next_literal=NextIndex}=Dict) ->
+-dialyzer({no_improper_lists, literal/2}).
+
+literal(Lit, Dict) when is_float(Lit) ->
+    %% A literal 0.0 actually has two representations: 0.0 and -0.0.
+    %% While they are equal, they must be encoded differently (the bit sign).
+    if
+        %% We do not explicitly match on 0.0 because a buggy compiler
+        %% could replace Lit by 0.0, which would discard its sign.
+        Lit > 0.0; Lit < 0.0 ->
+            literal1([term|Lit], Dict);
+        true ->
+            literal1([binary|my_term_to_binary(Lit)], Dict)
+    end;
+literal(Lit, Dict) ->
+    literal1([term|Lit], Dict).
+
+literal1(Key, #asm{literals=Tab0,next_literal=NextIndex}=Dict) ->
     case Tab0 of
-        #{Lit:=Index} ->
+        #{Key:=Index} ->
 	    {Index,Dict};
         #{} ->
-	    Tab = Tab0#{Lit=>NextIndex},
+	    Tab = Tab0#{Key=>NextIndex},
 	    {NextIndex,Dict#asm{literals=Tab,next_literal=NextIndex+1}}
     end.
 
@@ -267,8 +283,11 @@ lambda_table(#asm{locals=Loc0,lambdas={NumLambdas,Lambdas0}}) ->
 -spec literal_table(bdict()) -> {non_neg_integer(), [[binary(),...]]}.
 
 literal_table(#asm{literals=Tab,next_literal=NumLiterals}) ->
-    L0 = maps:fold(fun(Lit, Num, Acc) ->
-			   [{Num,my_term_to_binary(Lit)}|Acc]
+    L0 = maps:fold(fun
+			([term|Lit], Num, Acc) ->
+			   [{Num,my_term_to_binary(Lit)}|Acc];
+			([binary|Lit], Num, Acc) ->
+			   [{Num,Lit}|Acc]
 		   end, [], Tab),
     L1 = lists:sort(L0),
     L = [[<<(byte_size(Term)):32>>,Term] || {_,Term} <- L1],
diff --git a/lib/compiler/src/beam_types.erl b/lib/compiler/src/beam_types.erl
index 48e720edd7..7559673a94 100644
--- a/lib/compiler/src/beam_types.erl
+++ b/lib/compiler/src/beam_types.erl
@@ -423,7 +423,8 @@ is_bs_matchable_type(Type) ->
       Result :: {ok, term()} | error.
 get_singleton_value(#t_atom{elements=[Atom]}) ->
     {ok, Atom};
-get_singleton_value(#t_float{elements={Float,Float}}) ->
+get_singleton_value(#t_float{elements={Float,Float}}) when Float =/= 0.0 ->
+    %% 0.0 is not actually a singleton as it has two encodings: 0.0 and -0.0
     {ok, Float};
 get_singleton_value(#t_integer{elements={Int,Int}}) ->
     {ok, Int};
diff --git a/lib/compiler/test/float_SUITE.erl b/lib/compiler/test/float_SUITE.erl
index 586dfe8102..c9c0beb70f 100644
--- a/lib/compiler/test/float_SUITE.erl
+++ b/lib/compiler/test/float_SUITE.erl
@@ -22,14 +22,14 @@
 	 init_per_group/2,end_per_group/2,
 	 pending/1,bif_calls/1,math_functions/1,mixed_float_and_int/1,
          subtract_number_type/1,float_followed_by_guard/1,
-         fconv_line_numbers/1,exception_signals/1]).
+         fconv_line_numbers/1,float_zero/1,exception_signals/1]).
 
 -include_lib("common_test/include/ct.hrl").
 
 suite() -> [{ct_hooks,[ts_install_cth]}].
 
 all() ->
-    [pending, bif_calls, math_functions,
+    [pending, bif_calls, math_functions, float_zero,
      mixed_float_and_int, subtract_number_type,
      float_followed_by_guard,fconv_line_numbers,
      exception_signals].
@@ -50,6 +50,12 @@ init_per_group(_GroupName, Config) ->
 end_per_group(_GroupName, Config) ->
     Config.
 
+float_zero(Config) when is_list(Config) ->
+    <<16#0000000000000000:64>> = match_on_zero_and_to_binary(1*0.0),
+    <<16#8000000000000000:64>> = match_on_zero_and_to_binary(-1*0.0),
+    ok.
+
+match_on_zero_and_to_binary(0.0 = X) -> <<X/float>>.
 
 %% Thanks to Tobias Lindahl <tobias.lindahl@it.uu.se>
 %% Shows the effect of pending exceptions on the x86.
-- 
2.26.2

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File 0345-Do-not-consider-zero-floats-as-singleton.patch of Package erlang (Revision b4f72044e1af8cfe32130ab849a401a1)

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