https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112480
Bug ID: 112480
Summary: optional<T>::reset emits inefficient code when T is
trivially-destructible
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: gnu at kosak dot com
Target Milestone: ---
Created attachment 56556
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56556&action=edit
the .ii file from -save-temps
Hello,
The assembly output of this program shows that the generated code tests whether
a flag is zero, and if not, sets it to zero. This uses a compare and a branch,
when the optimal code could just force the flag to zero unconditionally with
one instruction. The emitted code is going to be non-optimal in this way for
any trivially-destructible T.
```
#include <optional>
void func(std::optional<int> &x) {
x.reset();
}
```
Relevant assembly on x86_64, invoked with g++ -S -O3 test.cc
```
_Z4funcRSt8optionalIiE:
.LFB266:
.cfi_startproc
endbr64
cmpb $0, 4(%rdi)
je .L1
movb $0, 4(%rdi)
.L1:
ret
.cfi_endproc
```
The non-optimal code arises because optional<T>::reset() needs to decide
whether or not to call ~T(), and this decision depends on whether or not the
optional is currently holding a value. However, if T is trivially destructible,
~T() is a no-op, so the test and branch is kind of a waste.
One way this could be addressed is via this one-line change to
/usr/include/c++/13/optional:
```
--- optional.ORIG 2023-11-10 19:01:35.843372670 +0000
+++ optional 2023-11-10 19:22:53.525894777 +0000
@@ -314,7 +314,7 @@
constexpr void
_M_reset() noexcept
{
- if (this->_M_engaged)
+ if (is_trivially_destructible_v<_Tp> || this->_M_engaged)
_M_destroy();
}
};
```
With this change in place, the emitted assembly is
```
_Z4funcRSt8optionalIiE:
.LFB266:
.cfi_startproc
endbr64
movb $0, 4(%rdi)
ret
.cfi_endproc
```
Output from g++ -v:
```
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/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='Ubuntu 13.2.0-4ubuntu3'
--with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-13
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --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-13-XYspKM/gcc-13-13.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-XYspKM/gcc-13-13.2.0/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
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.0 (Ubuntu 13.2.0-4ubuntu3)
```
