http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59972
Bug ID: 59972
Summary: -Wunused-value emits inconsistent warnings for unused
computed values
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: chengniansun at gmail dot com
Gcc does not emit an unused-value warning for "fn2() ^ 0;", but emits a warning
if this expression is in a comma expression, e.g. "fn2() ^ 0, 0;". This is an
inconsistent behavior.
I guess the reason for the first case is "fn2() ^ 0 == fn2()". But from the
syntactical perspective, it is an expression and its intention is to output a
value.
Differently a non-void function call can not only return a value but also have
side effects, and thus the return values can be ignored sometimes.
$: cat s.c
extern int fn2(void);
int fn1() {
fn2() ^ 0;
return fn2() ^ 0, 0;
}
$: gcc-trunk -Wunused-value -c s.c
s.c: In function ‘fn1’:
s.c:4:16: warning: value computed is not used [-Wunused-value]
return fn2() ^ 0, 0;
^
$: gcc-trunk --version
gcc-trunk (GCC) 4.9.0 20140127 (experimental)
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$: clang-trunk -Wunused-value -c s.c
s.c:4:16: warning: expression result unused [-Wunused-value]
return fn2() ^ 0, 0;
~~~~~ ^ ~
s.c:3:9: warning: expression result unused [-Wunused-value]
fn2() ^ 0;
~~~~~ ^ ~
2 warnings generated.
