From 5d1471fae699fed9318ea6cad939156c2775d8be Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= Date: Tue, 4 Feb 2014 09:19:39 +0100 Subject: sys_core_fold: Prevent case expressions from being evaluated twice MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit In e12b7d5331c58b41db06cadfa4af75b78b62a2b1, a bug was introduced that would cause case expressions to be evaluated more than once if there were aliases in the pattern. Example: X = Y = io:put_chars("some chars"), {X,Y} That would be rewritten to code similar to (but in Core Erlang): X = io:put_chars("some chars"), X = io:put_chars("some chars"), {X,Y} Make sure that we only evalute the expression once by doing a transformation similar to (but in Core Erlang): NewVar = io:put_chars("some chars"), X = NewVar, Y = NewVar, {X,Y} Reported-by: José Valim Reported-by: Anthony Ramine --- lib/compiler/src/sys_core_fold.erl | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) (limited to 'lib/compiler/src/sys_core_fold.erl') diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl index 1cdbac5693..b7bbf40b4a 100644 --- a/lib/compiler/src/sys_core_fold.erl +++ b/lib/compiler/src/sys_core_fold.erl @@ -1925,9 +1925,30 @@ eval_case(#c_case{arg=E,clauses=[#c_clause{pats=Ps0,body=B}]}, Sub) -> true -> cerl:values_es(E); false -> [E] end, - {true,Bs} = cerl_clauses:match_list(Ps0, Es), + %% Consider: + %% + %% case SomeSideEffect() of + %% X=Y -> ... + %% end + %% + %% We must not rewrite it to: + %% + %% let = in ... + %% + %% because SomeSideEffect() would be called evaluated twice. + %% + %% Instead we must evaluate the case expression in an outer let + %% like this: + %% + %% let NewVar = SomeSideEffect() in + %% let = in ... + %% + Vs = make_vars([], length(Es)), + {true,Bs} = cerl_clauses:match_list(Ps0, Vs), {Ps,As} = unzip(Bs), - expr(#c_let{vars=Ps,arg=core_lib:make_values(As),body=B}, sub_new(Sub)); + InnerLet = cerl:c_let(Ps, core_lib:make_values(As), B), + Let = cerl:c_let(Vs, E, InnerLet), + expr(Let, sub_new(Sub)); eval_case(Case, _) -> Case. %% case_opt(CaseArg, [Clause]) -> {CaseArg,[Clause]}. -- cgit v1.2.3