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-rw-r--r--lib/compiler/src/beam_dead.erl22
1 files changed, 20 insertions, 2 deletions
diff --git a/lib/compiler/src/beam_dead.erl b/lib/compiler/src/beam_dead.erl
index 3606af9d75..8ea949425e 100644
--- a/lib/compiler/src/beam_dead.erl
+++ b/lib/compiler/src/beam_dead.erl
@@ -266,12 +266,30 @@ backward([{jump,{f,To0}},{move,Src,Reg}=Move|Is], D, Acc) ->
false -> backward([Move|Is], D, [Jump|Acc]);
true -> backward([Jump|Is], D, Acc)
end;
-backward([{jump,{f,To}}=J|[{bif,Op,_,Ops,Reg}|Is]=Is0], D, Acc) ->
+backward([{jump,{f,To}}=J|[{bif,Op,{f,BifFail},Ops,Reg}|Is]=Is0], D, Acc) ->
try replace_comp_op(To, Reg, Op, Ops, D) of
I -> backward(Is, D, I++Acc)
catch
- throw:not_possible -> backward(Is0, D, [J|Acc])
+ throw:not_possible ->
+ case To =:= BifFail of
+ true ->
+ %% The bif instruction is redundant. See the comment
+ %% in the next clause for why there is no need to
+ %% test for liveness of Reg at label To.
+ backward([J|Is], D, Acc);
+ false ->
+ backward(Is0, D, [J|Acc])
+ end
end;
+backward([{jump,{f,To}}=J|[{gc_bif,_,{f,To},_,_,_Dst}|Is]], D, Acc) ->
+ %% The gc_bif instruction is redundant, since either the gc_bif
+ %% instruction itself or the jump instruction will transfer control
+ %% to label To. Note that a gc_bif instruction does not assign its
+ %% destination register if the failure branch is taken; therefore,
+ %% the code at label To is not allowed to assume that the destination
+ %% register is initialized, and it is therefore no need to test
+ %% for liveness of the destination register at label To.
+ backward([J|Is], D, Acc);
backward([{test,bs_start_match2,F,Live,[R,_]=Args,Ctxt}|Is], D,
[{test,bs_match_string,F,[Ctxt,Bs]},
{test,bs_test_tail2,F,[Ctxt,0]}|Acc0]=Acc) ->