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* [Bug c++/97435] New: Lifetime of temporaries not correctly extending when optiimzation are enabled @ 2020-10-15 6:44 chris at chrissavoie dot com 2020-10-15 7:33 ` [Bug c++/97435] " rguenth at gcc dot gnu.org 2020-10-15 8:51 ` redi at gcc dot gnu.org 0 siblings, 2 replies; 3+ messages in thread From: chris at chrissavoie dot com @ 2020-10-15 6:44 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97435 Bug ID: 97435 Summary: Lifetime of temporaries not correctly extending when optiimzation are enabled Product: gcc Version: 10.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: chris at chrissavoie dot com Target Milestone: --- Godbolt example: https://godbolt.org/z/fssed1 The following code: struct A { __attribute__((noinline)) A(int i) : data(i) {} A& operator+=(int i) { data += i; return *this; } int data; }; int main() { A const& b = A(5) += 5; A c(6); return b.data + c.data; // 16 expected } Will return 16 when compiled without optimizations, 11 when compiled with optimizations (-O1 or greater) and 6 with noinline disabled and optimizations on. Browsing versions on godbolt it looks like the inline version of the bug appeared between versions 4.9.3 and 5.1. The noinline version appeared between versions 4.6.4 and 4.7.3. ^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/97435] Lifetime of temporaries not correctly extending when optiimzation are enabled 2020-10-15 6:44 [Bug c++/97435] New: Lifetime of temporaries not correctly extending when optiimzation are enabled chris at chrissavoie dot com @ 2020-10-15 7:33 ` rguenth at gcc dot gnu.org 2020-10-15 8:51 ` redi at gcc dot gnu.org 1 sibling, 0 replies; 3+ messages in thread From: rguenth at gcc dot gnu.org @ 2020-10-15 7:33 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97435 --- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- I believe the A(5) object doesn't live beyond the stmts lifetime and thus this is invalid. ^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/97435] Lifetime of temporaries not correctly extending when optiimzation are enabled 2020-10-15 6:44 [Bug c++/97435] New: Lifetime of temporaries not correctly extending when optiimzation are enabled chris at chrissavoie dot com 2020-10-15 7:33 ` [Bug c++/97435] " rguenth at gcc dot gnu.org @ 2020-10-15 8:51 ` redi at gcc dot gnu.org 1 sibling, 0 replies; 3+ messages in thread From: redi at gcc dot gnu.org @ 2020-10-15 8:51 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97435 Jonathan Wakely <redi at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- Yes, this is undefined behaviour. It's equivalent to: const int& f(const int& i) { return i; } const int& i = f(5); int j = i; // undefined Lifetime is only extended if the reference is bound directly to the temporary object, not to another reference that happens to be bound to it inside some function. Consider the case where the body of A::operator+= is not visible to the compiler. How is it supposed to know whether the returned value refers to *this or not, and therefore how is it supposed to know whether to destroy the temporary A(5) immediately or not? ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2020-10-15 8:51 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2020-10-15 6:44 [Bug c++/97435] New: Lifetime of temporaries not correctly extending when optiimzation are enabled chris at chrissavoie dot com 2020-10-15 7:33 ` [Bug c++/97435] " rguenth at gcc dot gnu.org 2020-10-15 8:51 ` redi at gcc dot gnu.org
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