https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92798
Bug ID: 92798
Summary: -fshort-enums can break iterators of std::map
Product: gcc
Version: 9.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: fc9vqetne38 at temp dot mailbox.org
Target Milestone: ---
Hi,
I'm having problems with iterators of std::map when -fshort-enums is enabled.
It somehow depends on the combination of compiler, stdlib and optimizations
(tested g++ and clang++, libstdc++ and libc++). The size of key (int vs. long)
and map also seem to matter. My layman guess is, that it's somehow related to
operator--.
Short example that shows the (to me unexpected) behavior, used versions of g++
and clang++ at the end:
#include <map>
#include <iostream>
int main() {
auto m = std::map<long, long>{{0,0}, {1,1}, {2,2}};
std::cout << std::boolalpha;
std::cout << (&(*m.begin()) == &(*m.rbegin())) << ' ';
std::cout << (m.begin() == --m.end()) << ' ';
std::cout << (++m.rbegin() == m.rend()) << ' ';
std::cout << (++m.begin() == m.end()) << ' ';
std::cout << (++(--m.end()) == m.end()) << '\n';
}
Compiling with g++ -fshort-enums main.cpp gives
true true true false false
while g++ main.cpp gives
false false false false true
I also tried compiling with clang++, libc++ and optimizations enabled in
various combinations.
1) g++ -nostdinc++ -I/usr/include/c++/v1 main.cpp -nodefaultlibs -lc++ -lc++abi
-lm -lc -lgcc_s -lgcc -fshort-enums
2) g++ -nostdinc++ -I/usr/include/c++/v1 main.cpp -nodefaultlibs -lc++ -lc++abi
-lm -lc -lgcc_s -lgcc -fshort-enums -O
3) clang++ -stdlib=libstdc++ -fshort-enums main.cpp
4) clang++ -O -stdlib=libstdc++ -fshort-enums main.cpp
5) clang++ -stdlib=libc++ -fshort-enums main.cpp
6) clang++ -O -stdlib=libc++ -fshort-enums main.cpp
All 1)-6) give
false false false false true
though 4) sometimes produces
true true true false false
e.g. when doing a range-based loop over the map before the comparisons.
Hope the report is legit and not too messy.
Cheers,
Christian
PS:
opensuse tumbleweed
g++ (SUSE Linux) 9.2.1 20190903 [gcc-9-branch revision 275330]
clang version 9.0.0 (tags/RELEASE_900/final 372316)
