https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106020
Bug ID: 106020
Summary: Spurious warnings about stringop overflows only with
LTO
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: lto
Assignee: unassigned at gcc dot gnu.org
Reporter: matt at godbolt dot org
CC: marxin at gcc dot gnu.org
Target Milestone: ---
When using Howard Hinnant's date library, and GCC 12.1 on x86, and then with
LTO enabled, there are many apparently spurious errors after the read.constprop
pass:
/opt/compiler-explorer/libs/date/v3.0.1/include/date/date.h: In function
'read.constprop':
/opt/compiler-explorer/libs/date/v3.0.1/include/date/date.h:6506:15: note:
destination object 'buf' of size 11
6506 | CharT buf[std::numeric_limits<unsigned>::digits10+2u] = {};
It has proven tricky to get a smaller case but https://godbolt.org/z/hPaqz7TY1
demonstrates it with an inline version of date.h - a smaller version that
doesn't inline the date library is https://godbolt.org/z/e75YG6ano and the code
amounts to:
std::chrono::system_clock::time_point parse_datetime(const std::string &sv) {
std::istringstream ss{sv};
std::chrono::system_clock::time_point tp;
ss >> date::parse("%FT%T%Z", tp); // ignoring errors for simplicity
return tp;
}
(though it has to be linked in a separate TU with LTO enabled to trigger the
issues).
I initially filed this as a bug in the date library itself:
https://github.com/HowardHinnant/date/issues/740 but we now believe this is a
compiler issue.
Compiling with GCC 11 or earlier, or with clang, or without LTO doesn't trigger
these warnings.
