Re: std=c++14 greatly increases debuginfo size. why?

[John Steele Scott <toojays at toojays dot net>]

On 08/11/16 11:46, Jonathan Wakely wrote:
> On 7 November 2016 at 23:36, John Steele Scott <toojays@toojays.net> wrote:
>> On 04/11/16 22:32, John Steele Scott wrote:
>>> Hi,
>>>
>>> A colleague of mine noticed that switching from -std=c++11 to -std=c++14
>>> dramatically increased the size of our binary.
>>>
>>> After some investigation, this seems to be due to debuginfo.
>>>
>>> Consider the following simple example, compiled four different ways:
>>>
>>> jscott@citra:~/src/debuginfo-bloat$ cat simple.cpp
>>> #include <string>
>>>
>>> size_t length (const std::string &data)
>>> {
>>>   size_t len = 0;
>>>   for (auto iter = data.cbegin(); iter != data.cend(); iter++)
>>>   {
>>>     len++;
>>>   }
>>>
>>>   return len;
>>> }
>>> jscott@citra:~/src/debuginfo-bloat$ g++-6 -gdwarf-4 -std=c++11 -c simple.cpp -o simple-11.o
>>> jscott@citra:~/src/debuginfo-bloat$ g++-6 -gdwarf-4 -std=c++14 -c simple.cpp -o simple-14.o
>>> jscott@citra:~/src/debuginfo-bloat$ g++-6 -gdwarf-4 -std=c++11 -c simple.cpp -o simple-11-g1.o -g1
>>> jscott@citra:~/src/debuginfo-bloat$ g++-6 -gdwarf-4 -std=c++14 -c simple.cpp -o simple-14-g1.o -g1
>>> jscott@citra:~/src/debuginfo-bloat$ ls -l simple*o
>>> -rw-rw-r-- 1 jscott jscott   6904 Nov  4 22:25 simple-11-g1.o
>>> -rw-rw-r-- 1 jscott jscott  60192 Nov  4 22:25 simple-11.o
>>> -rw-rw-r-- 1 jscott jscott   6904 Nov  4 22:25 simple-14-g1.o
>>> -rw-rw-r-- 1 jscott jscott 129376 Nov  4 22:25 simple-14.o
>>>
>>>
>>> So with the default -g2, the C++14 output is almost double that of C++11. But
>>> with -g1 they are the same.
>>>
>>> Can someone tell me why this is so? Am I getting much better debuginfo for those
>>> bytes?
>>
>> I did some further digging with dwarfdump, and the increase in this case is due
>> to std::literals::string_literals. If I ifdef those operators out (at the end of
>> basic_string.h), the C++11 and C++14 sizes are the same.
> 
> I was going to suggest that this is almost certainly due to additional
> declarations in the headers with C++14 is used, not a difference in
> dwarf output caused by the -std options.
> 
>> I guess these operators cause GCC to think that the wchar_t, char16_t, char32_t
>> versions of basic_string are used, and so it must emit debuginfo for them. But
>> I'm not using the operators, therefore these classes are not used.
>>
>> Sounds like a GCC bug to me - it shouldn't emit debuginfo for types referenced
>> by an inline function if that function is never used.
> 
> There might be some reason the debuginfo is useful in that case, but I
> suggest opening a bug report so it can be assessed properly. Thanks.
> 

Thanks Jonathan. I filed <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78247>.

Cheers,

John