[Bug c++/109942] Function template declared extern is implicitly instantiated on -O optimization level when return type is auto
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109942 Simon Pfahler changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |WONTFIX --- Comment #3 from Simon Pfahler --- I see your point, and have realized that there is no way around explicitly stating the return types. That was wishful thinking on my part. Still, I feel like the behavior should be identical for any optimization level, but this is more personal opinion and of little relevance for standard-compliance. Marked this issue as resolved, since even if it is a bug (as you said, debatable), it certainly isn't a relevant one.
[Bug c++/109942] New: Function template declared extern is implicitly instantiated on -O optimization level when return type is auto
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
auto foo() { return; }
extern template auto foo();
int main() { foo(); }
```
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()'
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.
[Bug c++/109490] New: ICE when using custom OpenMP reduction in Lambda in Template Function
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109490
Bug ID: 109490
Summary: ICE when using custom OpenMP reduction in Lambda in
Template Function
Product: gcc
Version: 12.2.1
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 54844
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54844&action=edit
preprocessed file
g++ throws an ICE when trying to compile the following code:
template T add(int const &maxs) {
auto step = [&](auto const &maxs) {
T a = T(0);
#pragma omp declare reduction(MyRed:T
\
: omp_out += omp_in)
\
initializer(omp_priv(omp_orig))
#pragma omp parallel for schedule(dynamic, 10) reduction(MyRed : a)
for (int s = 0; s <= maxs; ++s) {
a += T(1);
}
return a;
};
return step(maxs);
}
int main() { auto a = add(100); }
Additional info:
gcc -v:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/12.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-bootstrap
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-build-config=bootstrap-lto --with-linker-hash-style=gnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.1 20230201 (GCC)
Complete command line:
g++ -fopenmp main.cpp
Compiler output message:
main.cpp: In instantiation of ‘T add(const int&) [with T = double]’:
main.cpp:17:34: required from here
main.cpp:4:9: internal compiler error: in instantiate_body, at cp/pt.cc:26443
4 | #pragma omp declare reduction(MyRed:T
\
| ^~~
0x19ea0c8 internal_error(char const*, ...)
???:0
0x6543f0 fancy_abort(char const*, int, char const*)
???:0
0x7f843c tsubst_lambda_expr(tree_node*, tree_node*, int, tree_node*)
???:0
0x7e258f tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool,
bool)
???:0
0x7db493 instantiate_decl(tree_node*, bool, bool)
???:0
0x7f9a73 instantiate_pending_templates(int)
???:0
0x6fe658 c_parse_final_cleanups()
???:0
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See
