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authorBjörn Gustavsson <[email protected]>2018-01-22 16:27:33 +0100
committerBjörn Gustavsson <[email protected]>2018-01-24 05:49:35 +0100
commitab6ac37e0c40331a2debd90f753ec2f4531e4701 (patch)
tree0dbe34e7630116c9874934d1f1013ba0571e1f8b
parent25fa1aadc987f4c3abb8b39ae5fa090a9a103daa (diff)
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Correct unsafe optimizations in beam_block
When attempting to eliminate the move/2 instruction in the following code: {bif,self,{f,0},[],{x,0}}. {move,{x,0},{x,1}}. . . . {put_tuple,2,{x,1}}. {put,{atom,ok}}. {put,{x,0}}. beam_block would produce the following unsafe code: {bif,self,{f,0},[],{x,1}}. . . . {put_tuple,2,{x,1}}. {put,{atom,ok}}. {put,{x,1}}. It is unsafe because the tuple is self-referential. The following code: {put_list,{y,6},nil,{x,4}}. {move,{x,4},{x,5}}. {put_list,{y,1},{x,5},{x,5}}. . . . {put_tuple,2,{x,6}}. {put,{x,4}}. {put,{x,5}}. would be incorrectly transformed to: {put_list,{y,6},nil,{x,5}}. {put_list,{y,1},{x,5},{x,5}}. . . . {put_tuple,2,{x,6}}. {put,{x,5}}. {put,{x,5}}. (Both elements in the built tuple get the same value.)
-rw-r--r--lib/compiler/src/beam_block.erl53
1 files changed, 38 insertions, 15 deletions
diff --git a/lib/compiler/src/beam_block.erl b/lib/compiler/src/beam_block.erl
index 39ae8d5347..7f6d3a75ae 100644
--- a/lib/compiler/src/beam_block.erl
+++ b/lib/compiler/src/beam_block.erl
@@ -289,7 +289,7 @@ opt_move(Dest, Is) ->
opt_move_1(R, [{set,[D],[R],move}|Is0], Acc) ->
%% Provided that the source register is killed by instructions
%% that follow, the optimization is safe.
- case eliminate_use_of_from_reg(Is0, R, D, []) of
+ case eliminate_use_of_from_reg(Is0, R, D) of
{yes,Is} -> opt_move_rev(D, Acc, Is);
no -> not_possible
end;
@@ -347,7 +347,7 @@ opt_tuple_element_1([{set,_,_,{alloc,_,_}}|_], _, _, _) ->
opt_tuple_element_1([{set,_,_,{try_catch,_,_}}|_], _, _, _) ->
no;
opt_tuple_element_1([{set,[D],[S],move}|Is0], I0, {_,S}, Acc) ->
- case eliminate_use_of_from_reg(Is0, S, D, []) of
+ case eliminate_use_of_from_reg(Is0, S, D) of
no ->
no;
{yes,Is} ->
@@ -389,6 +389,14 @@ is_killed_or_used(R, {set,Ss,Ds,_}) ->
%% that FromRegister is still used and that the optimization is not
%% possible.
+eliminate_use_of_from_reg(Is, From, To) ->
+ try
+ eliminate_use_of_from_reg(Is, From, To, [])
+ catch
+ throw:not_possible ->
+ no
+ end.
+
eliminate_use_of_from_reg([{set,_,_,{alloc,Live,_}}|_]=Is0, {x,X}, _, Acc) ->
if
X < Live ->
@@ -397,21 +405,32 @@ eliminate_use_of_from_reg([{set,_,_,{alloc,Live,_}}|_]=Is0, {x,X}, _, Acc) ->
{yes,reverse(Acc, Is0)}
end;
eliminate_use_of_from_reg([{set,Ds,Ss0,Op}=I0|Is], From, To, Acc) ->
+ ensure_safe_tuple(I0, To),
I = case member(From, Ss0) of
- true ->
- Ss = [case S of
- From -> To;
- _ -> S
- end || S <- Ss0],
- {set,Ds,Ss,Op};
- false ->
- I0
- end,
+ true ->
+ Ss = [case S of
+ From -> To;
+ _ -> S
+ end || S <- Ss0],
+ {set,Ds,Ss,Op};
+ false ->
+ I0
+ end,
case member(From, Ds) of
- true ->
- {yes,reverse(Acc, [I|Is])};
- false ->
- eliminate_use_of_from_reg(Is, From, To, [I|Acc])
+ true ->
+ {yes,reverse(Acc, [I|Is])};
+ false ->
+ case member(To, Ds) of
+ true ->
+ case beam_utils:is_killed_block(From, Is) of
+ true ->
+ {yes,reverse(Acc, [I|Is])};
+ false ->
+ no
+ end;
+ false ->
+ eliminate_use_of_from_reg(Is, From, To, [I|Acc])
+ end
end;
eliminate_use_of_from_reg([I]=Is, From, _To, Acc) ->
case beam_utils:is_killed_block(From, [I]) of
@@ -421,6 +440,10 @@ eliminate_use_of_from_reg([I]=Is, From, _To, Acc) ->
no
end.
+ensure_safe_tuple({set,[To],[],{put_tuple,_}}, To) ->
+ throw(not_possible);
+ensure_safe_tuple(_, _) -> ok.
+
%% opt_allocs(Instructions) -> Instructions. Optimize allocate
%% instructions inside blocks. If safe, replace an allocate_zero
%% instruction with the slightly cheaper allocate instruction.