* [Consult] g++: About "-Wunused-variable" for constant variable in header file
@ 2014-10-14 21:57 Chen Gang
2014-10-15 9:18 ` Jonathan Wakely
0 siblings, 1 reply; 3+ messages in thread
From: Chen Gang @ 2014-10-14 21:57 UTC (permalink / raw)
To: gcc-help; +Cc: Jeff Law, Peter Maydell
Hello All:
At present, I met one warning issue about gcc 5.0.0.
- For "const float a = 3.4 - 2.1 / 3;", if it is unused, gcc5 will not
report warning.
- "const char n() {return 1;}; const a = n();", if 'a' is unused, gcc5
will report warning.
For gcc old version (e.g. gcc4), it will not report warning. Is it the
new feature for gcc5, or just a gcc5's bug?
Welcome any ideas, suggestions or completions.
Thanks.
--
Chen Gang
Open share and attitude like air water and life which God blessed
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [Consult] g++: About "-Wunused-variable" for constant variable in header file
2014-10-14 21:57 [Consult] g++: About "-Wunused-variable" for constant variable in header file Chen Gang
@ 2014-10-15 9:18 ` Jonathan Wakely
2014-10-15 9:29 ` Chen Gang
0 siblings, 1 reply; 3+ messages in thread
From: Jonathan Wakely @ 2014-10-15 9:18 UTC (permalink / raw)
To: Chen Gang; +Cc: gcc-help, Jeff Law, Peter Maydell
On 14 October 2014 22:57, Chen Gang <gang.chen.5i5j@gmail.com> wrote:
> Hello All:
>
> At present, I met one warning issue about gcc 5.0.0.
>
> - For "const float a = 3.4 - 2.1 / 3;", if it is unused, gcc5 will not
> report warning.
Because there is no cost to initializing the variable.
> - "const char n() {return 1;}; const a = n();", if 'a' is unused, gcc5
> will report warning.
Because it requires dynamic initialization, running a function at
startup, which has a cost. If you don't use the variable then you
might not want to run the initialization code at startup, so you get a
warning.
> For gcc old version (e.g. gcc4), it will not report warning. Is it the
> new feature for gcc5, or just a gcc5's bug?
I think this behaviour is intended and is not a bug.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [Consult] g++: About "-Wunused-variable" for constant variable in header file
2014-10-15 9:18 ` Jonathan Wakely
@ 2014-10-15 9:29 ` Chen Gang
0 siblings, 0 replies; 3+ messages in thread
From: Chen Gang @ 2014-10-15 9:29 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: gcc-help, Jeff Law, Peter Maydell, Jakub Jelinek
On 10/15/14 17:18, Jonathan Wakely wrote:
> On 14 October 2014 22:57, Chen Gang <gang.chen.5i5j@gmail.com> wrote:
>> Hello All:
>>
>> At present, I met one warning issue about gcc 5.0.0.
>>
>> - For "const float a = 3.4 - 2.1 / 3;", if it is unused, gcc5 will not
>> report warning.
>
> Because there is no cost to initializing the variable.
>
>> - "const char n() {return 1;}; const a = n();", if 'a' is unused, gcc5
>> will report warning.
>
> Because it requires dynamic initialization, running a function at
> startup, which has a cost. If you don't use the variable then you
> might not want to run the initialization code at startup, so you get a
> warning.
>
>> For gcc old version (e.g. gcc4), it will not report warning. Is it the
>> new feature for gcc5, or just a gcc5's bug?
>
> I think this behaviour is intended and is not a bug.
>
Thank you very much four your details reply! And thank Jakub Jelinek for
the almost same reply in another original mailing thread, too.
I shall forward your reply to the other related mail thread which is
related with QEMU.
Thanks again. :-)
--
Chen Gang
Open, share, and attitude like air, water, and life which God blessed
^ permalink raw reply [flat|nested] 3+ messages in thread
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2014-10-14 21:57 [Consult] g++: About "-Wunused-variable" for constant variable in header file Chen Gang
2014-10-15 9:18 ` Jonathan Wakely
2014-10-15 9:29 ` Chen Gang
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