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From: "eggert at cs dot ucla.edu" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/111655] wrong code generated for __builtin_signbit and 0./0. on x86-64 -O2 Date: Sun, 01 Oct 2023 22:08:06 +0000 [thread overview] Message-ID: <bug-111655-4-lma50yy2O0@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-111655-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111655 --- Comment #5 from Paul Eggert <eggert at cs dot ucla.edu> --- > I am thinking this is all under specified really ... Although it is indeed unspecified whether 0.0/0.0 yields -NaN or +NaN, it is well understood that negating a floating point value flips its sign bit. The original test case demonstrates that gcc -O2 currently mishandles this, as that test case negates a floating point value but the sign bit remains unchanged. Old GCC and Clang handle this correctly, as do the other non-GCC compilers that I checked. As far as I know, only recentish gcc gets this wrong, and even then only when optimization is enabled. Here is a sharper example of the bug: int main () { double x = 0.0 / 0.0; return !__builtin_signbit (x) == !__builtin_signbit(-x); } This should return 0 no matter what X's value is, but it returns 1 with recent gcc -O2 on x86-64. > The match pattern which causes the issue: > (simplify > /* signbit(x) -> 0 if x is nonnegative. */ > (SIGNBIT tree_expr_nonnegative_p@0) > { integer_zero_node; }) I don't see anything wrong with that match pattern. I speculate that what's wrong is that GCC incorrectly thinks that 0.0/0.0 is nonnegative. Although it's tempting to say that the sign bit of a division is the exclusive OR of the sign bits of its operands, evidently this is not true on x86-64 when NaNs are involved.
next prev parent reply other threads:[~2023-10-01 22:08 UTC|newest] Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-10-01 17:22 [Bug tree-optimization/111655] New: wrong code generated for __builtin_signbit " eggert at cs dot ucla.edu 2023-10-01 17:29 ` [Bug target/111655] " pinskia at gcc dot gnu.org 2023-10-01 17:29 ` pinskia at gcc dot gnu.org 2023-10-01 17:40 ` [Bug target/111655] wrong code generated for __builtin_signbit and 0./0. " pinskia at gcc dot gnu.org 2023-10-01 17:42 ` pinskia at gcc dot gnu.org 2023-10-01 22:08 ` eggert at cs dot ucla.edu [this message] 2023-10-01 23:57 ` pinskia at gcc dot gnu.org 2023-10-01 23:59 ` pinskia at gcc dot gnu.org 2023-10-02 0:02 ` pinskia at gcc dot gnu.org 2023-10-02 11:08 ` [Bug target/111655] [11/12/13/14 Regression] " amonakov at gcc dot gnu.org 2023-10-04 9:37 ` [Bug middle-end/111655] " rguenth at gcc dot gnu.org 2023-10-04 11:41 ` amonakov at gcc dot gnu.org 2023-11-24 9:33 ` jakub at gcc dot gnu.org 2023-11-24 9:48 ` amonakov at gcc dot gnu.org 2023-11-24 9:54 ` rguenther at suse dot de 2024-03-22 13:38 ` law at gcc dot gnu.org 2024-03-22 17:09 ` jakub at gcc dot gnu.org 2024-04-19 11:38 ` bruno at clisp dot org
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