[Bug debug/78247] New: Unused inline function returning u32string causes u32string debuginfo to be emitted.

[toojays at toojays dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78247

            Bug ID: 78247
           Summary: Unused inline function returning u32string causes
                    u32string debuginfo to be emitted.
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: toojays at toojays dot net
  Target Milestone: ---

Consider the following source, compiled with and without MORE_BLOAT defined:

jscott@citra:~/src/debuginfo-bloat$ cat bloat-string.cpp
#include <string>

#ifdef MORE_BLOAT
inline std::u32string f(const char32_t* __str, size_t __len)
{ return std::u32string{__str, __len}; }
#endif

jscott@citra:~/src/debuginfo-bloat$ g++-6 -std=c++11 -gdwarf-4 -c
bloat-string.cpp -o bloat-string.o 
jscott@citra:~/src/debuginfo-bloat$ g++-6 -DMORE_BLOAT -std=c++11 -gdwarf-4 -c
bloat-string.cpp -o bloat-string-more.o 
jscott@citra:~/src/debuginfo-bloat$ ls -l bloat-string.o bloat-string-more.o
-rw-rw-r-- 1 jscott jscott 61256 Nov  8 13:58 bloat-string-more.o
-rw-rw-r-- 1 jscott jscott 24672 Nov  8 13:58 bloat-string.o


I.e. if I have an inline function refering to u32string it bloats my output
object by 40K, even though I never used that function. The extra space seems to
go toward debug info for u32string and its associated char_traits.

In effect this occurs whenever someone switches from -std=c++11 to -std=c++14,
because of the inline string literal operators at the end of basic_string.h.

I originally noticed this because switching from -std=c++11 to -std=c++14
increased the size of generated binaries. Discussion at
<https://gcc.gnu.org/ml/gcc-help/2016-11/msg00006.html>.

I tried to create a smaller example (i.e. not requiring <string>), but couldn't
figure out how. The only types I have seen affected by this problem so far are
u16string and u32string.


My test setup:
jscott@citra:~/src/debuginfo-bloat$ gcc-6 -v
Using built-in specs.
COLLECT_GCC=gcc-6
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
6.2.0-3ubuntu11~14.04' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-6 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=gcc4-compatible --disable-libstdcxx-dual-abi
--enable-gnu-unique-object --disable-vtable-verify --enable-libmpx
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 6.2.0 20160901 (Ubuntu 6.2.0-3ubuntu11~14.04)