New GCC 7 -Wformat-truncation suppressed by (and only by) -Og?

[Martin Sebor]

On 02/23/2017 06:51 AM, Stephan Bergmann wrote:
> At least with a recent GCC 7 trunk build ("gcc (GCC) 7.0.1 20170221
> (experimental)"), I noticed that -Wformat-truncation warnings happen to
> not be emitted if and only if -Og is given:

That's unfortunately a bug.  The warning runs at different stages
depending on whether or not optimization is enabled and it looks
like the global GCC variable it uses to determine this is not set
for -Og.  Can you please open a bug in Bugzilla?

Thanks
Martin

>
>> $ cat test.c
>> #include <stdio.h>
>> int main() {
>>     char buf[3];
>>     snprintf(buf, sizeof buf, "%s", "foo");
>>     return 0;
>> }
>> $ gcc -Wformat-truncation -Og ~/test.c
>> $ gcc -Wformat-truncation -O ~/test.c
>> test.c: In function ‘main’:
>> test.c:4:34: warning: ‘snprintf’ output truncated before the last
>> format character [-Wformat-truncation=]
>>      snprintf(buf, sizeof buf, "%s", "foo");
>>                                   ^
>> test.c:4:5: note: ‘snprintf’ output 4 bytes into a destination of size 3
>>      snprintf(buf, sizeof buf, "%s", "foo");
>>      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Any other optimization level (-O1..3/s/fast) does emit the warning.  The
> documentation makes it clear that the behavior of that warning may be
> coupled to the optimization level, but this difference between -Og and
> everything else still looks somewhat odd.  Could it be by mistake?