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From: "simon-pfahler at gmx dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109942] New: Function template declared extern is implicitly instantiated on -O optimization level when return type is auto
Date: Tue, 23 May 2023 12:39:56 +0000 [thread overview]
Message-ID: <bug-109942-4@http.gcc.gnu.org/bugzilla/> (raw)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109942
Bug ID: 109942
Summary: Function template declared extern is implicitly
instantiated on -O optimization level when return type
is auto
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: simon-pfahler at gmx dot de
Target Milestone: ---
Created attachment 55143
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55143&action=edit
preprocessed *.ii file
Declaring a template function as extern, but not providing a definition should
lead to a linker error. This does not work on higher optimization levels, if
the template function's return type is `auto`.
The code is:
```
template<class T>
auto foo() { return; }
extern template auto foo<int>();
int main() { foo<int>(); }
```
This fails to link when the command line is `g++ test.cpp`, as it should. But
for `g++ -O test.cpp`, the compilation and linkage works. Explicitly stating
the return type (replacing `auto` by `void`) leads to correct behavior (linker
failure on any optimization level).
Similar examples can be constructed where compilation with optimizatio <=-O1
leads to expected failure, while compilation is successfull for optimization
>=-O2.
This is problematic when return types can not easily be determined and
explicitly stated.
The bug leads to unwanted instantiation of the template, which means longer
compile time if a definition would be provided in a different compilation unit.
Additional info:
gcc -v:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 12.2.0-14'
--with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-12
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-12-bTRWOB/gcc-12-12.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-bTRWOB/gcc-12-12.2.0/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (Debian 12.2.0-14)
Complete command line:
`g++ test.cpp` or `g++ -O test.cpp`
Compiler output message for `g++ test.cpp`:
/usr/bin/ld: /tmp/ccBQx0I3.o: in function `main':
test.cpp:(.text+0x5): undefined reference to `auto foo<int>()'
collect2: error: ld returned 1 exit status
Compiler output message for `g++ -O test.cpp`:
Nothing, as compilation is successful.
Preprocessed file (*.ii) is attached.
Let me know if there is information missing.
next reply other threads:[~2023-05-23 12:39 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-23 12:39 simon-pfahler at gmx dot de [this message]
2023-05-23 13:30 ` [Bug c++/109942] " redi at gcc dot gnu.org
2023-05-23 13:35 ` redi at gcc dot gnu.org
2023-05-23 13:40 ` simon-pfahler at gmx dot de
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