https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99386
Bug ID: 99386
Summary: std::variant overhead much larger compared to clang
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mail at milianw dot de
Target Milestone: ---
I've come across some code in an application I'm working on that makes use of
std::variant. The overhead imposed by std::variant compared to a raw type is
extremely high (700% and more). I created a little MWE to show this behavior:
https://github.com/milianw/cpp-variant-overhead
To reproduce, compile two versions in different build folders with both g++ or
clang++:
```
CXX=g++ cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo
CXX=clang++ cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo
```
Then run both versions:
```
perf stat -r 5 -d ./variant 0
perf stat -r 5 -d ./variant 1
perf stat -r 5 -d ./variant 2
```
I put the measurements on my machine into the README.md. The gist is, the
relative runtime overhead is huge when compiling with g++:
g++
uint64_t: 100%
std::variant<uint64_t>: 720%
std::variant<uint64_t, uint32_t>: 840%
clang++
uint64_t: 100%
std::variant<uint64_t>: 114%
std::variant<uint64_t, uint32_t>: 184%
The baseline for both g++/clang++ is roughly the same.
