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authorBjörn Gustavsson <[email protected]>2010-04-15 10:18:53 +0200
committerBjörn Gustavsson <[email protected]>2010-04-15 16:41:50 +0200
commitc34ad2d537e41c9b31e240aa1e6fadd994115a16 (patch)
tree2cbe3141d13a1ac40fdfa7486eef89be053f3100 /lib/compiler/src
parentab47252a5f7d540d4119d38dffe69acca86d2a41 (diff)
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Silence warnings for expressions that are assigned to "_"
There is currently no zero-cost way to silence the warning "the result of the expression is ignored", which is issued for code such as: list_to_integer(S), ok Such code can be useful for assertions or input validation. Teach the compiler to silence the warning for expressions that are explicitly assigned to to the "_" variable, such as: _ = list_to_integer(S), ok Implement it by having the v3_core pass annotate calls in Core Erlang like this: let <_> = ( call 'erlang':'list_to_integer'(S) -| ['result_not_wanted'] ) in 'ok' and modifiy sys_core_fold to suppress the warning for any call having the annotation. We deliberately do not make it possible to silence the warnings for expressions like: {build,an,unnecessary,term}, ok or is_list(L), ok because we don't know of any real-world scenarios in which that would be useful.
Diffstat (limited to 'lib/compiler/src')
-rw-r--r--lib/compiler/src/sys_core_fold.erl17
-rw-r--r--lib/compiler/src/v3_core.erl26
2 files changed, 31 insertions, 12 deletions
diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 068478496b..3b374be015 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -602,15 +602,23 @@ count_bits_1(Int, Bits) -> count_bits_1(Int bsr 64, Bits+64).
%% a rewritten expression consisting of a sequence of
%% the arguments only is returned.
-useless_call(effect, #c_call{module=#c_literal{val=Mod},
+useless_call(effect, #c_call{anno=Anno,
+ module=#c_literal{val=Mod},
name=#c_literal{val=Name},
args=Args}=Call) ->
A = length(Args),
case erl_bifs:is_safe(Mod, Name, A) of
false ->
case erl_bifs:is_pure(Mod, Name, A) of
- true -> add_warning(Call, result_ignored);
- false -> ok
+ true ->
+ case member(result_not_wanted, Anno) of
+ false ->
+ add_warning(Call, result_ignored);
+ true ->
+ ok
+ end;
+ false ->
+ ok
end,
no;
true ->
@@ -2806,7 +2814,8 @@ format_error({no_effect,{erlang,F,A}}) ->
end,
flatten(io_lib:format(Fmt, Args));
format_error(result_ignored) ->
- "the result of the expression is ignored";
+ "the result of the expression is ignored "
+ "(suppress the warning by assigning the expression to the _ variable)";
format_error(useless_building) ->
"a term is constructed, but never used";
format_error(bin_opt_alias) ->
diff --git a/lib/compiler/src/v3_core.erl b/lib/compiler/src/v3_core.erl
index 4ac2196d79..b2f0ac75c7 100644
--- a/lib/compiler/src/v3_core.erl
+++ b/lib/compiler/src/v3_core.erl
@@ -128,6 +128,7 @@
-record(core, {vcount=0 :: non_neg_integer(), %Variable counter
fcount=0 :: non_neg_integer(), %Function counter
in_guard=false :: boolean(), %In guard or not.
+ wanted=true :: boolean(), %Result wanted or not.
opts :: [compile:option()], %Options.
es=[] :: [error()], %Errors.
ws=[] :: [warning()], %Warnings.
@@ -580,10 +581,14 @@ expr({'fun',L,{function,F,A},{_,_,_}=Id}, St) ->
{#c_var{anno=Lanno++[{id,Id}],name={F,A}},[],St};
expr({'fun',L,{clauses,Cs},Id}, St) ->
fun_tq(Id, Cs, L, St);
-expr({call,L,{remote,_,M,F},As0}, St0) ->
+expr({call,L,{remote,_,M,F},As0}, #core{wanted=Wanted}=St0) ->
{[M1,F1|As1],Aps,St1} = safe_list([M,F|As0], St0),
Lanno = lineno_anno(L, St1),
- {#icall{anno=#a{anno=Lanno},module=M1,name=F1,args=As1},Aps,St1};
+ Anno = case Wanted of
+ false -> [result_not_wanted|Lanno];
+ true -> Lanno
+ end,
+ {#icall{anno=#a{anno=Anno},module=M1,name=F1,args=As1},Aps,St1};
expr({call,Lc,{atom,Lf,F},As0}, St0) ->
{As1,Aps,St1} = safe_list(As0, St0),
Op = #c_var{anno=lineno_anno(Lf, St1),name={F,length(As1)}},
@@ -596,23 +601,28 @@ expr({call,L,FunExp,As0}, St0) ->
expr({match,L,P0,E0}, St0) ->
%% First fold matches together to create aliases.
{P1,E1} = fold_match(E0, P0),
- {E2,Eps,St1} = novars(E1, St0),
+ St1 = case P1 of
+ {var,_,'_'} -> St0#core{wanted=false};
+ _ -> St0
+ end,
+ {E2,Eps,St2} = novars(E1, St1),
+ St3 = St2#core{wanted=St0#core.wanted},
P2 = try
- pattern(P1, St1)
+ pattern(P1, St3)
catch
throw:Thrown ->
Thrown
end,
- {Fpat,St2} = new_var(St1),
+ {Fpat,St4} = new_var(St3),
Fc = fail_clause([Fpat], c_tuple([#c_literal{val=badmatch},Fpat])),
- Lanno = lineno_anno(L, St2),
+ Lanno = lineno_anno(L, St4),
case P2 of
nomatch ->
- St = add_warning(L, nomatch, St2),
+ St = add_warning(L, nomatch, St4),
{#icase{anno=#a{anno=Lanno},
args=[E2],clauses=[],fc=Fc},Eps,St};
Other when not is_atom(Other) ->
- {#imatch{anno=#a{anno=Lanno},pat=P2,arg=E2,fc=Fc},Eps,St2}
+ {#imatch{anno=#a{anno=Lanno},pat=P2,arg=E2,fc=Fc},Eps,St4}
end;
expr({op,_,'++',{lc,Llc,E,Qs},More}, St0) ->
%% Optimise '++' here because of the list comprehension algorithm.