https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97553
Bug ID: 97553
Summary: [missed optimization] constexprness not noticed when
UBsan enabled
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: eyalroz at technion dot ac.il
Target Milestone: ---
(GodBolt example: https://godbolt.org/z/Kvan5c)
Consider the following code:
#include <string_view>
constexpr std::string_view f() { return "hello"; }
static constexpr std::string_view g() {
auto x { f() };
return x.substr(1, 3);
}
int foo() { return g().length(); }
if you compile it with flags `--std=c++17 -O3`, it results in a pleasant:
foo():
mov eax, 3
ret
but if you also enabled undefined-behavior sanitization, i.e. `--std=c++17
-fsanitize=undefined -O3`, then you get a much longer program with UB-related
instrumentation - which is never used.
I'm not sure if it's because some optimizations are disabled with UBsan, in
which case this might be a "misfeature", or whether they're enabled but the
optimization is just missed.
