[Bug c++/52231] [missed optimization/diagnostics] address-of-reference
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52231 Pawel Sikora changed: What|Removed |Added Status|NEW |RESOLVED Known to work||6.2.1, 7.0 Resolution|--- |FIXED --- Comment #8 from Pawel Sikora --- s% ~/src/gcc-install/usr/local/bin/g++ pr52231.cpp -c -O2 -Wall -Wextra -fdump-tree-optimized -fdiagnostics-color=never pr52231.cpp: In function 'void bar()': pr52231.cpp:7:17: warning: the compiler can assume that the address of 'a' will never be NULL [-Waddress] if ( == 0 )// <== useless stmt (not diagnosed/not optimized) ~~~^~~~
[Bug c++/52231] [missed optimization/diagnostics] address-of-reference
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52231 Andrew Pinski pinskia at gcc dot gnu.org changed: What|Removed |Added CC||froydnj at gcc dot gnu.org --- Comment #4 from Andrew Pinski pinskia at gcc dot gnu.org --- *** Bug 65111 has been marked as a duplicate of this bug. ***
[Bug c++/52231] [missed optimization/diagnostics] address-of-reference
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52231
--- Comment #5 from Nathan Froyd froydnj at gcc dot gnu.org ---
FWIW, clang (= 3.5) understands how to optimize the original testcase in
comment 0; it even issues a -Wtautological-undefined-compare warning.
This also showed up in the context of trying to hint to the compiler that
placement new didn't need null checks:
#include new
void init(int p) { new (p) float(3.14f); }
which clang understands how to optimize and GCC does not.
[Bug c++/52231] [missed optimization/diagnostics] address-of-reference
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52231 --- Comment #6 from Jonathan Wakely redi at gcc dot gnu.org --- (In reply to Nathan Froyd from comment #5) This also showed up in the context of trying to hint to the compiler that placement new didn't need null checks: That's only become true quite recently: http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1748 See also PR 35878
[Bug c++/52231] [missed optimization/diagnostics] address-of-reference
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52231 --- Comment #7 from Nathan Froyd froydnj at gcc dot gnu.org --- (In reply to Jonathan Wakely from comment #6) (In reply to Nathan Froyd from comment #5) This also showed up in the context of trying to hint to the compiler that placement new didn't need null checks: That's only become true quite recently: http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1748 Ah, thanks for the pointer. I assumed that the compiler would be able to infer that |reference| would be non-null and eliminate the mandatory check regardless, but it's nice to have spec language to back this up. See also PR 35878 Again, thanks for the pointer. If we can do this in the frontend now, that might be simple enough that I could take a look at it myself (assuming it hasn't been done and that PR simply didn't get referenced).
[Bug c++/52231] [missed optimization/diagnostics] address-of-reference
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52231 --- Comment #1 from Andrew Pinski pinskia at gcc dot gnu.org 2012-02-13 17:56:32 UTC --- Why do you think this is a missed optimization and/or diagnostic? References are just like pointers, just a is really a and a is really *a.
[Bug c++/52231] [missed optimization/diagnostics] address-of-reference
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52231 Jonathan Wakely redi at gcc dot gnu.org changed: What|Removed |Added Status|UNCONFIRMED |NEW Last reconfirmed||2012-02-13 Ever Confirmed|0 |1 --- Comment #2 from Jonathan Wakely redi at gcc dot gnu.org 2012-02-13 18:13:23 UTC --- But the language guarantees that for a program without undefined behaviour a reference is always bound to a valid object. int* i = nullptr; int r = *i; // undefined if (r == 0) // cannot be true ;
[Bug c++/52231] [missed optimization/diagnostics] address-of-reference
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52231 --- Comment #3 from Jonathan Wakely redi at gcc dot gnu.org 2012-02-13 18:19:39 UTC --- (In reply to comment #1) References are just like pointers, just a is really a and a is really *a. This is wrong in so many ways. Pointers can be null, pointers can be uninitialized, pointers can be re-seated. Would you also argue this shouldn't be optimised? int i=0; int r = i; // ... if (i != r) unreachable();
